LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION OF

RANDOM WALKS ON THE HEISENBERG GROUP

E.BREUILLARD

To the memory of Martine Babillot

Abstract.We prove local limit theorems for products of independent random

variables on the Heisenberg group which are identically distributed with respect

to an arbitrary centered and compactly supported probability measure .We also

provide uniform estimates for translates of a bounded set by comparing

n

to the

associated heat kernel.This,in turn,enables us to show the equidistribution of

Heisenberg-unipotent random walks on nite volume homogeneous spaces G=.

The goal of this paper is to show the local limit theorem for centered probability

measures on the Heisenberg group along with some of its renements and applica-

tions.The local limit problem on non-commutative Lie groups has been studied by

many authors in the last thirty or forty years (Ito-Kawada,Arnold-Krylov,Kazhdan,

Bougerol,Le Page,Guivarc'h,Varopoulos,etc.).In the classical commutative case or

in the compact group case the local limit theorem is available under the weakest as-

sumptions on the probability measure (see [Sto] and [ItK]).In [Bou],Bougerol solves

the local limit problem for absolutely continuous probability measures on semisimple

Lie groups.In a recent work,Alexopoulos [Ale] obtains a very precise local limit

theorem and estimates a la Berry-Essen for probability measures with a continu-

ous density of compact support on an arbitrary connected Lie group of polynomial

growth.

However,in this problem,an assumption of absolute continuity of the probability

measure with respect to the Haar measure is often made,while the case of a possibly

singular (e.g.nitely supported) measure remains generally open.Such cases include

nitely supported probability measures on the group of isometries of the Euclidean

3-space (see [Kaz],[Gui2]) or the case of semisimple groups [Bou].Similarly the

speed of convergence to equidistribution is not well understood and seems to depend

on dicult arithmetic questions (cf.the spectral gap conjecture [Sar] about the

equidistribution on the sphere).In this paper however,we treat the case of an

arbitrary,possibly singular,measure.We will focus on the simplest nilpotent Lie

group:the rst Heisenberg group.

Comparing the convolution powers of the measure with the associated heat kernel,

we also obtain a uniform version of the local limit theorem yielding a fairly precise

estimate on the asymptotic behavior of centered random walks on the Heisenberg

1

2 E.BREUILLARD

group.This generalizes a result of Stone [Sto] in the commutative case.This estimate

allows to show further equidistribution results for random walks on homogeneous

spaces.Following this strategy,and making use of Ratner's theorem on orbits of

unipotent ows (see [Rat],[Sha],[Sta]),we show at the end of the paper that centered

Heisenberg-unipotent randomwalks on homogeneous spaces G=;where is discrete

of nite co-volume in a Lie group G;are equidistributed in the closure of the orbit

on which they live.

In the non-centered case,an interesting phenomenon can occur:non-centered

unipotent random walks may not converge to any probability measure on G=.For

instance if is not co-compact,they may stay outside an arbitrary compact set with

high probability at arbitrary large times.Hence the hypothesis that the walk should

be centered is crucial (unless of course the Haar measure is uniquely ergodic for the

unipotent subgroup).

Finally let us remark that the results of this paper should extend to the case of

an arbitrary simply connected nilpotent Lie group,but the technical diculty of the

forthcoming proofs forced us to restrict our attention to the Heisenberg group.

1.Statement of the results

Let G be the group of 33 upper-triangular unipotent matrices and e the identity

in G.Let us x the Haar measure on G,dg = dxdydz where

g =

0@

1 x z

0 1 y

0 0 1

1A

is simply denoted by g = (x;y;z).We also denote by jAj the Haar measure of a

Borel set A and we x a homogeneous norm kgk = maxfjxj;jyj;jzj

1=2

g on G:

We consider a probability measure on G with the following properties:

compactly supported.

centered:

R

p(x)d(x) = 0 where p:G!G=[G;G] is the canonical map.

aperiodic:for any proper closed subgroup H G and any x 2 G,(xH) < 1.

In particular,we make no assumption of smoothness for ,which can be for in-

stance nitely supported.

The convolution product of measures is denoted by and convolution powers

simply denoted by

n

.

The central limit theorem for G is well known (see the work of Wehn [Weh],

as well as Tutubalin [Tut] and Crepel-Raugi [Rau]).It states that if (d

t

)

t

is the

semigroup of dilations given by d

t

(x;y;z) = (tx;ty;t

2

z) then the sequence d 1

p

n

(

n

)

converges to some gaussian measure on G (in the sense of probability measures,

i.e.

R

f d 1

p

n

d

n

!

R

fd for every bounded continuous function on G).The

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 3

measure lies inside a gaussian semigroup of probability measures (

t

)

t>0

dened by

its generating distribution

(1) Af =

d

dt

t=0

Z

f(x)d

t

(x) =

z@

z

f(e) +

xy@

2

xy

f(e) +

1

2

x

2

@

2

x

f(e) +

1

2

y

2

@

2

y

f(e)

where

z =

R

zd(x;y;z) and

xy =

R

xyd(x;y;z).Then =

1

and

t

= p

t

(g)dg

where p

t

(g) is the heat kernel associated to the operator dened by (1),hence is a

strictly positive fastly decreasing smooth function on G (see [VSC]).It is the density

of the Brownian Motion corresponding to A on G.

Let c() = p

1

(e) > 0.For general references about gaussian semigroups and the

Levy-Khintchin-Hunt formula,see [Gre],[Neu] and the original article of Hunt [Hun],

as well as the survey article [Breu2] where a self-contained proof of Wehn's central

limit theorem can be found.

We say that satises Cramer's condition if

sup

t

2

+s

2

1

Z

e

i(tx+sy)

d(x;y;z)

< 1

We obtain the theorems below without this assumption,but at some point we get a

slightly stronger result if this assumption is made (apparently for technical reasons,

but we have no guess whether it is necessary).

Let us state the local limit theorem for G together with a uniform version for

translates of a bounded set.These results generalize to the Heisenberg group the

well known theorems of classical probability theory on R

n

(see [Bre] and [Sto]).

The method of proof makes use of the unitary representations of G to obtain a

crucial spectral bound (Proposition 3.2).The main point after that is the study of

an auxiliary quadratic dynamical system (see equations (30)),in order to obtain a

key estimate similar to the domination condition appearing in the classical proof of

the local limit theorem on the real line (see [Bre]).

Theorem 1.1.(Local limit theorem) Let be a compactly supported aperiodic cen-

tered probability measure on G.Let f be a compactly supported continuous function

on G.Then the following convergence holds uniformly when z varies in compact

subsets of G

lim

n!+1

n

2

Z

G

f(gz)d

n

(g) = c()

Z

G

f(g)dg

Theorem 1.2.(Uniform local limit theorem) Let be a compactly supported aperi-

odic centered probability measure on G and (v

t

)

t>0

the corresponding limit gaussian

semigroup.Then for any bounded Borel subset B G with j@Bj = 0,

lim

n!+1

sup

x2G

n

2

j

n

(xB)

n

(xB)j = 0

4 E.BREUILLARD

If we assume additionally Cramer's condition,then

lim

n!+1

sup

x;y2G

n

2

j

n

(xBy)

n

(xBy)j = 0

Let us remark that the choice of (

t

)

t>0

depends on the choice we made of a semi-

group of dilations (d

t

)

t>0

.Any other choice (d

0t

)

t>0

for the semi-group of dilations

is of the form d

t

1

for some automorphism of G.The associated gaussian

semi-group (

0

t

)

t>0

is obtained from (

t

)

t>0

by composing by some automorphism of

G and Theorem 1.2 remains valid if we take (

0

t

)

t>0

instead of (

t

)

t>0

.

Theorem 1.3.(Concentration function) Under the assumptions above for ,for

every bounded set K G,there is a constant C

K

such that for all integers n

sup

x2G

n

(xK)

C

K

n

2

If we suppose additionally Cramer's condition,then

sup

x;y2G

n

(xKy)

C

K

n

2

This uniform version of the local limit theorem for translates enables to show the

following corollary.The point of this result is that the function f is not assumed to

tend to 0 at innity,and in particular,can have a periodic type of behavior.For the

classical commutative case see [Breu].

Corollary 1.4.Let f be an arbitrary left-uniformly continuous function on G such

that the following limit exists

(2) lim

T

x

!1;T

y

!1;T

z

!1

1

T

x

T

y

T

z

Z

T

x

0

Z

T

y

0

Z

T

z

0

f(x;y;z)dxdydz =`

where`2 C.Then

lim

n!+1

Z

G

f(g)d

n

(g) =`

This corollary enables to prove further equidistribution results on homogeneous

spaces.In the last section we show how to derive the equidistribution of Heisenberg-

unipotent random walks on homogeneous spaces,such as horospheric random walks

on complex hyperbolic manifolds,namely,

Theorem1.5.Let G be a connected Lie group and a lattice in G.Let H be a closed

subgroup of G consisting of unipotent elements and isomorphic to the Heisenberg

group.Let be a centered compactly supported aperiodic probability measure on H.

Then for an arbitrary x 2 G= and for any bounded and continuous function f on

G=,

lim

n!+1

Z

H

f(hx)d

n

(h) =

Z

G

f(g)dm

x

(g)

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 5

where m

x

is the unique H-invariant ergodic probability measure on G= whose support

is the closure of the orbit

Hx.

The existence of the measure m

x

is given by Ratner's theorem (see [Rat1 to 3],

[Sta]).The corresponding deterministic result was proved for one parameter sub-

groups by Ratner [Rat] elaborating on a weaker qualitative recurrence result due to

Margulis [Mar] and subsequently generalized by Dani [Dan],and for general unipo-

tent groups by Shah [Sha].

Note that this also implies that the only -stationary probability measures on G=

(i.e.the measures such that = ) are the H-invariant ones.This means,in

the terminology of Furstenberg (cf.[Fus]),that the action of H on G= is sti with

respect to .But this also follows from the fact that (H;) has the Choquet-Deny

property (i.e.the absence of bounded -harmonic functions) as follows fromthe work

of Guivarc'h [Gui].

Also note that according to a general random ergodic theorem of Oseledec (cf.

[Ose]),which is proved in the case when is symmetric (i.e.(A) = (A

1

)),

the convergence of Theorem 1.5 above holds for m-almost every x in G= for any

H-ergodic probability measure m on G=.We underline that,in Theorem 1.5,we

capture the behavior of the random walk for every starting point x.

In the above theorem,the assumption that is centered cannot be removed.As

will be shown in x10.2.1,a simple use of the central limit theorem shows that for

certain (in fact almost all) lattices in R

2

;any non centered unipotent random walk

starting at that point in the space of lattices SL

2

(R)=SL

2

(Z) will diverge,i.e.may

remain very far with high probability at some arbitrary large time.

When H is uniquely ergodic on G= (e.g.the horocycle owon a compact Riemann

surface),then Theorem 1.5 follows easily from an equidistribution theorem due to

Guivarc'h (Theoreme V.5.in [Gui]) and holds even when is not centered.

This application was originally motivated by the work of Eskin and Margulis [EsM],

where they studied the case of random walks on G= obtained by a measure whose

support is Zariski dense in a semisimple group.Their main result is that the sequence

n

x

is relatively compact in the space of probability measures on G=.

2.Notations and outline of the paper

We keep the notations and terminology introduced in the last section.Let be the

regular representation of G on the functions on G:(g)f(x) = f(g

1

x).We denote

by h;i the scalar product on L

2

(G).

G is identied with its Lie algebra g by writing g = (x;y;z) with the help of the

dieomorphism

g!G(3)

xX +yY +zZ 7!e

yY

e

xX

e

zZ

where X;Y and Z are the upper triangular elementary matrices,with [X;Y ] = Z.

6 E.BREUILLARD

The product in G is given by

gg

0

= (x +x

0

;y +y

0

;z +z

0

+xy

0

)

In the sequel,we will need to look at possible other parametrizations of G,in

particular at those of the form

:R

3

!G

(x;y;z) 7!(x;y;z +(x;y))

where (x;y) is a quadratic form in x and y.The Fourier transform of a function

on G can be dened in dierent ways depending on the choice of a parametrization.

Given a function f:G!C,we shall denote by F

(f):(R

3

)

_

!C the Fourier

transform taken in the parametrization dened by

.When = 0,we simply write

F

0

(f) =

b

f.The variable in the dual space (R

3

)

_

will be denoted by = (t;s;).The

formula reads:

F

(f):(R

3

)

_

!C(4)

= (t;s;) 7!

1

(2)

3=2

Z

f(

(x;y;z))e

i(tx+sy+z)

dxdydz

Let C

c

(G) be the space of continuous and compactly supported functions on G.

Let be a probability measure satisfying the properties listed in the introduction

and (X;Y;Z) a random variable on G distributed according to .We set once and

for all

(5) =

E(XY )

2E(Y

2

)

Theorem 2.1.Let (x;y) = y

2

and F

the Fourier transform just dened.Let f

and g be two functions on G with f 2 C

c

(G),g integrable and F

(g) 2 C

c

(G).Then

lim

n!+1

n

2

h(

n

)f;gi = c()

Z

G

f

Z

G

g

and,if we suppose additionally that F

(g) is absolutely continuous,then uniformly

when z varies in compact subsets,

lim

n!+1

n

2

Z

g(z

1

x)d

n

(x) = c()

Z

G

g

Below,we derive Theorem 1.1 from Theorem 2.1.The strategy for proving Theo-

rem2.1 follows the general scheme provided by Stone's proof of the local limit theorem

on R

d

[Sto] and is as follows.Looking at the Fourier transform of the integral,we

give an explicit decomposition of the regular representation of G into a continuous

direct sum of primary representations and treat each part of the integral to show that

only the part with small 's and small s and t's gives a contribution.Then we gain

control on this part by showing a domination condition on the integrand.This is

achieved by performing a Taylor expansion in s;t; of the characteristic function of

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 7

n

.Lebesgue's dominated convergence theorem combined with the point-wise con-

vergence granted by the central limit theoremon Gcompletes the proof.The proof of

Theorem (1:2) makes use of the estimates previously obtained and goes along similar

lines.

3.Irreducible unitary representations of G

The irreducible unitary representations of G are well known.Apart from char-

acters,there is a one-parameter family of irreducible unitary representations

( 2 Rnf0g) modeled on L

2

(R) by

(g)f(t) = e

i(tx+z)

f(t +y)

The following two propositions will be crucial in the proof.The rst is quite

standard (see [Gre],and [Gui]):

Proposition 3.1.Let be an aperiodic probability measure on G.Then for any

closed interval I Rnf0g

(6) sup

2I

k

()k < 1

Proof.Let

n

! 2 I be such that

n

(

1

)f

n

(t);f

n

(t)

!1

for some sequence of vectors f

n

2 L

2

(R) of norm1.Then,up to taking a subsequence,

for

1

-almost every x,

h

n

(x)f

n

(t);f

n

(t)i!1

But = fx 2 G;h

n

(x)f

n

(t);f

n

(t)i!1g is clearly a subgroup of G,and

1

() = 1.Since is aperiodic, is dense in G,hence [;] is dense in the

center of G.In particular,we can nd (0;0;z) 2 ,such that z =2 2Z.Then

e

i

n

z

f

n

f

n

!0

which implies e

iz

= 1 and provides the desired contradiction.

The next proposition gives an estimate of the norm of the operators

().When

is taken to be the symmetric Dirac measure on (1;0;0) and (0;1;0),this operator

can be viewed as acting on`

2

(Z) and then coincide with the well-known Harper

operator (see [BVZ]) studied in mathematical physics.

Proposition 3.2.Let be a probability measure on G whose support is not contained

in a coset of an abelian subgroup of G.Then we have

(7) lim inf

!0

1 k

()k

jj

> 0

8 E.BREUILLARD

Proof.Note that if we dene

1

(B) = (B

1

) for every Borel subset B of G,then

(

1

) is the adjoint of

() and

(

1

) is self-adjoint and non-negative.

If (7) does not hold,then we can nd unit vectors f

2 L

2

(R) such that for

arbitrarily small 's

(

1

)f

;f

1 jo()j

From the assumption made on ,we can nd two non-commuting elements x

0

and

x

1

lying in the support of

1

.For each we can then nd x

0

close to x

0

and x

1

close to x

1

(i.e.

x

1

x

1

and

x

0

x

0

< kx

1

x

0

k=3) such that for i = 0;1

Re

(x

i

)f

;f

1 jo()j

The commutator (x

0

;x

1

) belongs to the center,hence is of the form (0;0;c

).From

the choice of x

i

it follows that c < c

< 1=c for some constant c 2 (0;1).From the

Stone-Von Neumann theorem (cf.[CoG]) we can then nd an isometry I

of L

2

(R)

such that,conjugating by I

,

(x

0

) is turned into the translation by c

and

(x

1

)

is turned into the multiplication by e

it

.Hence we can assume that for arbitrarily

small 's

Re hf

(t +c

);f

(t)i 1 jo()j

Re

e

it

f

(t);f

(t)

1 jo()j

Or equivalently

kf

(t +c

) f

(t)k = o(

p

jj)(8)

e

it

f

(t) f

(t)

= o(

p

jj)(9)

Let A

"

= ft 2 R;d(t;2Z) <"g.We deduce from (9) that

(10)

Z

A

c"

jf

(t)j

2

dt = o(jj="

2

)

and from (8) that for any positive integer n

kf

(t +nc

) f

(t)k = o(n

p

jj)

or(11) Re hf

(t +nc

);f

(t)i 1 o(n

2

)

We now take n = [

1

2

p

jj

] +1 and"= c

p

jj=12.Then for small enough we have

1 > jnc

j > 3".So for small and as soon as"< ( 1)=2,making use of (10)

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 9

and applying the Cauchy-Schwarz inequality we have

jhf

(t +nc

);f

(t)ij

s

Z

A

"

jf

(t +nc

)j

2

Z

A

"

jf

(t)j

2

+

s

Z

A

c"

jf

(t +nc

)j

2

Z

A

c"

jf

(t)j

2

2

s

Z

A

c"

jf

(t)j

2

= o(

p

jj=") = o(1)

which yields the desired contradiction with (11).

4.Reducing to small values of

Here we will begin the proof of Theorem (2:1).Recall that G is identied with its

Lie algebra via the choice of the basis (X;Y;Z) and the coordinates (x;y;z) dened

as in equation (3).The center of G is the one parameter subgroup H = e

RZ

.We

are going to decompose the regular representation of G into a continuous direct sum

of other unitary representations.Every character of H is determined by a number

2

b

H

=

R

_

.If f 2 L

1

(G);we dene for x 2 G

f

(x) =

1

p

2

Z

R

f(xe

zZ

)e

iz

dz

We check that f

(xe

zZ

) = e

iz

f

(x).By the Fourier isometry,if z 7!f(xe

zZ

) is in

L

2

(R),then 7!f

(x) is in L

2

(R

_

),and

Z

R

jf(xe

zZ

)j

2

dz =

Z

R

_

jf

(x)j

2

d

By Fubini's theorem,it follows that,if f also belongs to L

2

(G) then jf

(x)j is in

L

2

(G=H) for almost every .Let us write H

the Hilbert space of measurable

functions F on G such that F(xe

zZ

) = e

iz

F(x) and jF(x)j is square integrable on

G=H.Then H

is a realization of the induced representation

= Ind

GH

,where G

acts by left translations.The above Plancherel formula for f

shows that we have

the continuous sum decomposition

=

Z

d

and if f;g belong to L

2

(G)

h(

n

)f;gi =

Z

h

(

n

)f

;g

i

H

d

10 E.BREUILLARD

It is easy to see that

is a primary unitary representation of G and that the

representation

dened above is the only irreducible representation of G contained

in

:Moreover,its multiplicity is innite:

(12)

=

Z

(s)ds

with

(s)'

for all s.

Now from Proposition (3:2) and from (12) we obtain that for some"2 (0;1) there

exists some c > 0 such that for all 2 R with jj "and all n 2 N

(13) k

(

n

)k k

()k

n

e

cjjn

therefore for any integer k

0

3,whenever D > k

0

=c we have

n

2

Z

D

log n

n

jj"

h

(

n

)f

;g

i

H

d

n

2

Z

D

log n

n

jj"

e

k

0

log n

kf

k

H

kg

k

H

d

1

n

k

0

2

Z

2R

kf

k

H

kg

k

H

d

1

n

k

0

2

kfk

L

2

(G)

kgk

L

2

(G)

!0 as n!1(14)

The last step follows from the Cauchy-Schwarz inequality.Hence this part of the

integral tends to 0.Similarly if I denotes any of the intervals [A;"] or [";A]

where A is some positive number such that g

is identically zero outside [A;A],

then it follows from Proposition (3:2) and from (12) that there is some constant

2 (0;1) such that

sup

2I

k

(

n

)k

n

Hence

n

2

Z

2I

h

(

n

)f

;g

i d

n

2

n

Z

2R

kf

k

H

kg

k

H

d

n

2

n

kfk

L

2

(G)

kgk

L

2

(G)

C

n

k

0

2

kfk

L

2

(G)

kgk

L

2

(G)

(15)for some constant C > 0 depending on .The right hand side clearly tends to 0 as

n tends to innity.

Now observe that if F

(g) (dened in Theorem 2.1) has compact support,then

g

is identically zero outside some bounded set of values of .More precisely,if for

some xed ,F

(g)(t;s;) = 0 for all t and s,then g

vanishes identically.The last

argument allows then to reduce to small values of (i.e.less that D

log n

n

for some

xed D > 0).

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 11

We are now going to perform Fourier integration one step further,i.e.on G=H.

We x an arbitrary Borel section

:G=H!G (given in the above coordinates by

(x;y) 7!(x;y;(x;y)) where is some measurable function on R

2

).Then f

(

(

x))

is in L

2

(G=H) for almost every .

In those coordinates,we can write down explicitely the decomposition of into the

continuous sum of the

's.Indeed,let f and g be two functions in L

1

(G)\L

2

(G)

and y be given in G.We have on the one hand:

h(y)f;gi =

Z

R

_

(y

1

)f

;g

H

d

and on the other hand,

h(y)f;gi =

Z

G=H

Z

H

f(y

1

xe

zZ

)

g(xe

zZ

)dzd

x

=

Z

G=H

Z

R

_

f

(y

1

x)

g

(x)dd

x

=

Z

R

_

Z

G=H

f

(y

1

x)

g

(x)d

xd

=

Z

R

_

Z

R

2

f

(y

1

(x;y))

g

(

(x;y))dxdyd

For notational convenience we set

y;;

(x;y):= f

(y

1

(x;y)) and

;

(x;y):=

g

(

(x;y)):For almost all 2 R,these functions belong to L

2

(R

2

).Performing

Fourier transform on L

2

(R

2

) now we get:

h(y)f;gi =

Z

R

_

Z

R

2_

[

y;;

(t;s)

d

;

(t;s)dtdsd

Now a straightforward computation yields that

d

;

(t;s) is the Fourier transform at

(t;s;) of g

dened by

(16) g

:= (x;y;z) 7!g(x;y;(x;y) +z)

d

;

(t;s) =

b

g

(t;s;)

Similarly if y = (y

x

;y

y

;y

z

) we compute,

[

y;;

(t;s) =

1

(2)

3=2

Z

e

i(y

x

(t+y)+y

y

s+(y

z

(x+y

x

;y+y

y

)

e

i(tx+sy+z)

f(x;y;z)dxdydz

Hence,if we suppose additionally that f has compact support on G and F

(g) =

b

g

has compact support on R

3

,we obtain

h(y)f;gi =

1

(2)

3=2

Z

dxf(x)

Z

R

_

Z

R

2_

e

i[y

x

(t+y)+y

y

s+(y

z

(x+y

x

;y+y

y

))]

e

i(tx+sy+z)

b

g

(t;s;)d

12 E.BREUILLARD

in other words,

h(y)f;gi =

Z

G

dx

(2)

3=2

f(x)

e

i (;y;x)

;

b

g

(t y;s;)

=(t;s;)2R

3

where x = (x;y;z),y = (y

x

;y

y

;y

z

), = (t;s;) and (;y;x) = t(y

x

+x) +s(y

y

+

y) +(z xy +y

z

(x +y

x

;y +y

y

)).

Let S is a random variable in G with distribution

n

.The quantity we are inter-

ested in is

n

2

E(h(S)f;gi) =

Z

G

dx

(2)

3=2

f(x)

E(e

i (;S;x)

);

b

g

(t y;s;)

=(t;s;)2R

3

Let D > 0.We split it in two parts and write n

2

E(h(S)f;gi) = A

n

+B

n

where

A

n

=

Z

G

dx

(2)

3=2

f(x)J

n

(x)

B

n

=

Z

G

dx

(2)

3=2

f(x)I

n

(x)

and

J

n

(x) = n

2

Z

jjD

log n

n

E(e

i (;S;x)

);

b

g

(t y;s;)

(t;s)2R

2

d

I

n

(x) = n

2

Z

jjD

log n

n

E(e

i (;S;x)

);

b

g

(t y;s;)

(t;s)2R

2

d

The part A

n

has already been dealt with,because the above computations show that

A

n

= n

2

Z

jjD

log n

n

h

(

n

)f

;g

i

H

d

and (applying (14) and (15)) there is C 0 (depending on D, and the size of the

set f;9(t;s);F

g

(g)(s;t;) 6= 0g) such that if n 1

(17) jA

n

j

C

n

k

0

2

kfk

L

2

(G)

kgk

L

2

(G)

which tends to zero as soon as k

0

is taken such that D k

0

=c 3=c (where c was

the constant dened in (13)).

Hence in the sequel,xing x = (x;y;z) 2 G;we shall focus on the term I

n

(x).

Before going further,we shall x once and for all the section .We take it of the

form proposed in Theorem 2.1,that is (x;y) = y

2

where is dened in terms of

the moments of in equation (5):In this case,

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 13

E(e

i (;S;x)

) = E(e

i[t(S

x

+x)+s(S

y

+y)+(z+S

z

(x+S

x

;y+S

y

))]

)

= e

i(tx+sy+z)

E(e

i

[

tS

x

+sS

y

+S

z

(y+S

y

)

2

]

)

= e

i(tx+sy+(zy

2

))

E(e

i

[

tS

x

+(s2y)S

y

+(S

z

S

2

y

)

]

Hence,

I

n

(x) = n

2

Z

jjD

log n

n

E(

n

(t;s;));e

i

b

g

(t y;s +2y;)

(t;s)2R

2

)

d

where = tx +sy +(z xy +y

2

) and

(18)

n

(t;s;) = e

i

[

tS

x

+sS

y

+(S

z

S

2

y

)

]

We shall next estimate E(

n

(t;s;)) for small values of .

5.Evaluation of the integral for small

Let us x D > 0.The remainder of this section is devoted to nding a suitable

bound for the expectation E(

n

(t;s;)) when jj D

log n

n

and s and t take values

that are bounded away from 0 and innity.

Let G

n

= (X

n

;Y

n

;Z

n

) be a sequence of independent random variables identically

distributed according to the probability measure on G.We write S

n

= G

n

::: G

1

the product of these variables.The law of S

n

is

n

.Bearing in mind the form of the

product on G,we get S

n

= (S

n;x

;S

n;y

;S

n;z

) where

S

n;x

= X

1

+:::+X

n

S

n;y

= Y

1

+:::+Y

n

S

n;z

= Z

1

+:::+Z

n

+X

2

Y

1

+X

3

(Y

1

+Y

2

) +::::+X

n

(Y

1

+:::+Y

n1

)

Let

n

be the random variable dened in (18) as follows

n

=

n

(t;s;) = e

i(tS

n;x

+sS

n;y

+(S

n;z

S

2

n;y

))

(19)

= e

iU

2

n

Y

1kn

e

iX

k

(t+U

k1

)

e

i(sY

k

+Z

k

)

and

0

= 1

where U

k

= Y

1

+:::+Y

k

and U

0

= 0 as above.In the sequel we x the value of (as

in equation (5)) to be

= E(XY )=2E(Y

2

):

We will also use the following notation:

(t;s) = E(e

i(X

1

t+Y

1

s)

)

14 E.BREUILLARD

5.1.Moderate deviations.We shall need the following lemma about moderate

deviations.Since we could not nd a reference for this precise form of the estimate

we want (about the maximum of the random walk up to time n),we include a proof.

It follows the well known argument of Cramer via Laplace transforms.

Lemma 5.1.Let U

0

= 0 and U

n

= Y

1

+:::+Y

n

be a sum of independent identically

distributed real random variables Y

n

's,which are assumed of compact support and

centered.Let A

n

be the event fmax

0kn

jU

k

j

p

nlog ng and let A

cn

be the com-

plementary event.Then for every non-negative integer p;there are constants c

p

> 0

and C

p

> 0 such that for all integers n 2 N

E( max

0kn

jU

k

j

p

1

A

cn

) C

p

e

c

p

log

2

n

Proof.Let Y be some random variable distributed according to the common distri-

bution of the Y

n

's.Since Y has compact support,if D > 0 is a bound for the support,

we obviously get

jU

n

j Dn

for all n.Hence for any xed p 0

E( max

0kn

jU

k

j

p

1

A

cn

) D

p

n

p

P(A

cn

)

Therefore it is enough to show the lemma when p = 0.

We can assume that Y is not identically 0.Dene the Laplace transform () of

Y by e

()

= E(e

Y

) for a positive real > 0.Then dene the Fenchel transform

(x) = sup

>0

(x ()) for a given x > 0.Clearly,

(x) is a non-decreasing

function of x > 0.The function () is strictly convex,since its second derivative

00

() = e

2()

(E(e

Y

)E(Y

2

e

Y

) E(Y e

Y

)

2

) is > 0 from the Cauchy-Schwarz in-

equality.In particular the supremum

(x) is attained for a unique value

x

of

given by the equation

0

(

x

) = x.And if x > 0 then

x

> 0 because

0

(0) = 0 since

Y is centered:Dierentiating the relation

0

(

x

) = x,we obtain

d

x

dx

=

1

00

(

x

)

hence

d

dx

=

x

and

d

2

dx

2

=

1

00

(

x

)

> 0

Therefore

(x) is strictly convex for x > 0:

Since Y has compact support,it has moments of any order and in particular we

have the following Taylor expansion

() =

2

2

E(Y

2

) +O(

3

)

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 15

and(20)

0

() = E(Y

2

) +O(

2

)

In particular,as x tends to 0 the value

x

tends to 0 too.We set

(0) = 0:By

denition of

,we have for any x > x

0

> 0

(21)

(x)

x

0

x (

x

0

)

Now let us study the function

near 0.From (20),we get

x

=

x

E(Y

2

) +O(

x

)

and

lim

x!0

+

x

E(Y

2

)

x

= 1

Hence

(x) =

x

x (

x

)

=

x

x

2x

E(Y

2

)

2

+O(

3x

)

=

x

2

E(Y

2

)

(u

u

2

2

) +O(ux)

3

where u =

x

E(Y

2

)=x.As x tends to 0,u tends to 1,therefore

(22)

(x)

x

2

4E(Y

2

)

for any suciently small x.

Similarly

(

x

)

x

=

2x

2x

E(Y

2

) +O(

3x

x

)

=

x

u

2

+O(

2x

u)

Hence(23)

(

x

)

x

3

4

x

for all suciently small x:Fix x

0

> 0 such that both (22) and (23) hold for all

x 2 (0;x

0

].From (21) we obtain immediately for all x x

0

(24)

(x)

x

0

(x

3

4

x

0

)

Now let us now apply these estimates obtained above for

(x) to the probability

P(U

k

>

p

nlog(n)).We write for x > 0 and > 0

E(e

U

n

) e

x

P(U

n

> x)

16 E.BREUILLARD

or

P(U

n

> x) e

n(

x

n

())

Hence taking the supremum of > 0

(25) P(U

n

> x) exp(n

(

x

n

))

Take two integers k and n with k n.We have

P(U

k

>

p

nlog(n)) exp(k

(

p

nlog n

k

))

Suppose rst that

p

nlog n

k

x

0

.Then it follows from (24) that

P(U

k

>

p

nlog(n)) exp(

x

0

p

nlog n +

3

4

kx

0

x

0

)

exp(

1

4

x

0

p

nlog n) e

c(log n)

2

where we can take c =

x

0

=4.

Now assume that

p

nlog n

k

x

0

.Then it follows from (22) that

P(U

k

>

p

nlog(n)) e

c(log n)

2

where c can be taken to be 1=4E(Y

2

).

By taking the lesser of the two c above we can now write

P(U

k

>

p

nlog(n)) e

c(log n)

2

for all integers n and k with n k 1:Hence

P(A

cn

) 2ne

c(log n)

2

Therefore there is a constant c

0

> 0 smaller that c such that when n is larger than

say n

0

we have

P(A

cn

) e

c

0

(log n)

2

For C

0

we may take e

c

0

(log n

0

)

2

and we have obtained the desired inequality.

Obviously,we may,and do,assume that c

p+1

< c

p

for all p.This lemma will enable

us to reduce to the case when U

k

does not take very big values.Let us also remark

that in the above proof,the constant c

p

can be chosen to depend only on the size of

the support of Y,i.e.on M = minft;P(jY j < t) = 1g.In the sequel,we will use

freely the result of Lemma 5.1,in particular the fact that E(jU

n

j

p

) = O

p

(n

p=2

log

p

(n))

for any p 0.

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 17

5.2.Estimate for\large"values of s and t.

Proposition 5.2.Let";A;D be positive numbers with A >"> 0.Let k be an

arbitrary positive integer.Then the following estimate holds uniformly when 2

[D

log n

n

;D

log n

n

] and"< s

2

+t

2

< A:

E(

n

) = O(

1

n

k

)

where the constant in O depends on";A;k;D and .

Proof.The proof will proceed by induction.Recall that A

n

is the event fmax

0kn

jU

k

j

p

nlog ng.In fact,we will take for induction hypothesis the following statement,

which we denote by H

j;k

for j 2 N and k 2 Z:

E(

n

U

j

n

) = O(

log

2k+3j

n

n

k=2

)

holds uniformly when the parameters (s;t;) vary in the range dened in the state-

ment of the proposition,and where O depends only on ,",A,D,k and j.

Clearly H

0;k

for all k 0 implies the proposition.Also note that H

j;k

implies

H

j;k1

.

Let k + j 0.Suppose that D > 0 is given and let 2 [D

log n

n

;D

log n

n

].We

do not make an assumption on (s;t) for the moment.Making use of Lemma 5.1

and of the independence of the random variables G

i

's,we have the following Taylor

expansion.For all positive integers p n,

E(

p

U

j

p

) = E(

p

U

j

p

1

A

p

) +O

;j

(e

c

j

log

2

p

)

= E(

p1

U

j

p

e

i(2U

p1

Y

p

+Y

2

p

)

e

iX

p

(t+U

p1

)

e

i(sY

p

+Z

p

)

1

A

p

) +O

;j

(e

c

j

log

2

p

)

= E[

p1

U

j

p

e

i(X

p

t+Y

p

s)

1

A

p

k+j

X

l=0

(i)

l

l!

l

X

q=0

C

q

l

(Z

p

Y

2

p

)

lq

((X

p

2Y

p

)U

p1

)

q

]+

+

k+j+1

O

;D;k;j

((

p

plog p)

k+2j+1

) +O

;j

(e

c

j

log

2

p

)

18 E.BREUILLARD

Further expanding and using Lemma 5.1 again:

E(

p

U

j

p

) = E[

p1

j

X

r=0

C

r

j

U

r

p1

Y

jr

p

!

e

i(X

p

t+Y

p

s)

k+j

X

l=0

(i)

l

l!

l

X

q=0

C

q

l

(Z

p

Y

2

p

)

lq

((X

p

2Y

p

)U

p1

)

q

] +O

;D;k;j

(

log

2k+3j+2

p

p

(k+1)=2

)

=

k+j

X

l=0

l

X

q=0

j

X

r=0

C

r

j

C

q

l

(i)

l

l!

E(

p1

U

r+q

p1

)

E(e

i(X

1

t+Y

1

s)

Y

jr

1

(Z

1

Y

2

1

)

lq

(X

1

2Y

1

)

q

)+

+O

;D;k;j

(

log

2(k+1)+3j

p

p

(k+1)=2

)

The last expression can be written as a sum of (t;s)E(

p1

U

j

p1

) (where (t;s) =

E(e

i(X

1

t+Y

1

s)

)) and a linear combination with bounded coecients of a bounded num-

ber of terms (the bounds depend only on ,D,k and j) of the form

l

E(

p1

U

m

p1

)

with 0 m j +l and 0 l k +j and (m;l) 6= (j;0),plus a remainder term.

We now x",A and D and consider (t;s;) in the range dened in the statement

of the proposition.In the rest of the proof,when the Landau notion O is used,we

implicitly mean that the corresponding constant depends only on ,",A,D,k and

j.

Let j 0 and k + j 0.Now suppose H

m;k+12l

holds for all 0 m j + l

and 0 l k +j except when (m;l) = (j;0).We are going to show that it implies

H

j;k+1

.Since jj D

log n

n

D

log p

p

,we have for all these values of m and l:

l

E(

p1

U

m

p1

) = O(

log

l+2(k+12l)+3m

p

p

(k+1)=2

) = O(

log

2(k+1)+3j

p

p

(k+1)=2

)

Hence,

E(

p

U

j

p

) = (t;s)E(

p1

U

j

p1

) +O(

log

2(k+1)+3j

p

p

(k+1)=2

)

Therefore,recursively on p,we obtain for all n

E(

n

U

j

n

) =

n

X

p=1

(s;t)

np

O(

log

2(k+1)+3j

p

p

(k+1)=2

)(26)

= n(s;t)

n=2

O(n

jkj

) +(1 +:::+(t;s)

n=2+1

)O(

log

2(k+1)+3j

n

n

(k+1)=2

)

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 19

As above,since is aperiodic,it follows that the law of (X;Y ) on R

2

is aperiodic.

Hence

sup

"<s

2

+t

2

<A

j(t;s)j < 1

therefore (26) yields

E(

n

U

j

n

) = O(

log

2(k+1)+3j

n

n

(k+1)=2

)

Thus we obtain H

j;k+1

.

Now,we note that from Lemma 5.1,H

j;j

holds for all j 0.This also guarantees

H

j;k

when k + j 0.Then we proceed by induction on h = j + k.From the

considerations above,we obtain H

j;k

fromthe knowledge of other H

j

0

;k

0 with j

0

+k

0

<

j +k.So we are done.

Remark 5.3.If we make the following additional assumption on

sup

t

2

+s

2

1

j(t;s)j < 1

then Proposition (5:2) holds uniformly in A and

E(

n

) = O(

1

n

k

)

holds uniformly in (t;s;) when 2 [D

log n

n

;D

log n

n

] and t

2

+s

2

"and the constant

in O depends only";k;D and .

5.3.Estimate for intermediate values of s and t.Now we will treat the case

when jj D

log n

n

and D

log

8

n

p

n

j(s;t)j "for some xed (large) D > 0.

Proposition 5.4.With the notation above,there is some"="() > 0,such that

uniformly in ,jj D

log n

n

Z

D

log

8

n

p

n

j(s;t)j"

jE(

n

(t;s;))jdtds = O(

1

nlog

2

n

)

where the constant in O depends on and D.

Proof.Let us write r:= j(t;s)j =

p

t

2

+s

2

and now choose"="() > 0 so that

for j(t;s)j ",j@

1

(t;s)j = O

(r) and j@

2

(t;s)j = O

(r).Choosing"smaller if

necessary,we can assume that when j(t;s)j "

9C = C() > 0;j(t;s)j e

Cr

2

or equivalently,

1

1 j(t;s)j

= O

(

1

r

2

)

20 E.BREUILLARD

We now suppose that jj D

log n

n

.Until the end of the proof,when we use

Landau's notation O we implicitly mean that the underlying constant depends only

on and D.

Fromthe computation in the subsection above (with j = 2 and k = 1),we obtain

that E(

p

U

2

p

) is a sum of (t;s)E(

p1

U

2

p1

) and a linear combination with bounded

coecients of the terms E(

p1

U

m

p1

) with 0 m 2,E(

p1

),E(

p1

U

p1

)@

2

(t;s),

and E(

p1

U

3

p1

)(@

1

(t;s)+@

2

(t;s)),with a remainder of order O

;D

(log

6

n).Hence,

making use of Lemma 5.1,for all n and p,p n,and if D

log

2

n

p

n

r ",we have

E(

p

U

2

p

) = (t;s)E(

p1

U

2

p1

) +rO(

p

nlog

4

n)

where O is independent of p and n.For any given p,

n

2

p n,we iterate this

equation

n

4

times.We obtain:

E(

p

U

2

p

) =

n=4

(s;t)O(nlog

2

n) +(1 +:::+(t;s)

n=4

)rO(

p

nlog

4

n)

=

1

1 j(t;s)j

rO(

p

nlog

4

n)

=

1

r

O(

p

nlog

4

n)(27)

since

n=4

(s;t)O(nlog

2

n) = e

Cnr

2

=4

O(nlog

2

n) =

1

r

O(

p

nlog

4

n),because D

log n

p

n

r ".

Similarly we can express E(

p

) as above (taking j = 0 and k = 1 in the proof

of prop.5.2) as a sum of (t;s)E(

p1

) and a linear combination with bounded

coecients of the terms E(

p1

) and E(

p1

U

p1

)(@

1

+@

2

) with a rest of order

O(

log

4

n

n

):Since D

log

2

n

p

n

r "we have

E(

p

) = (t;s)E(

p1

) +rO(

log

2

n

p

n

)

Iterating as above,we obtain for

n

2

p n

(28) E(

p

) =

1

r

O(

log

2

n

p

n

)

Nowwe look at E(

p

U

p

).As above (take j = 1,k = 0) it is a sumof (t;s)E(

p1

U

p1

)

and a linear combination with bounded coecients of the terms @

2

(t;s)E(

p1

),

E(

p1

);E(

p1

U

p1

);and (@

1

+@

2

)E(

p1

U

2

p1

);with a rest of order O(

log

5

n

p

n

).

But thanks to (28) and (27) all these terms are O(

log

5

n

p

n

) when

n

2

p n.Hence,

E(

p

U

p

) = (t;s)E(

p1

U

p1

) +O(

log

5

n

p

n

)

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 21

And then,iterating this relation,for all p,

3n

4

p n,we get

(29) E(

p

U

p

) =

1

r

2

O(

log

5

n

p

n

)

Finally we again decompose E(

p

) but pushing one step further the Taylor ex-

pansion and we see by the above calculation (take j = 0,k = 2) that it is a sum

of (t;s)E(

p1

) and a linear combination with bounded coecients of the terms

E(

p1

),E(

p1

U

p1

)@

1

(t;s);

2

E(

p1

),

2

E(

p1

U

p1

) and

2

E(

p1

U

2

p1

) with

a rest of order O(

log

6

n

n

p

n

):Thanks to (27),(28),(29) and Lemma 5.1,they are all of

order at most

1

r

O(

log

6

n

n

p

n

) when

3n

4

p n.Thus

E(

p

) = (t;s)E(

p1

) +

1

r

O(

log

6

n

n

p

n

)

and consequently for large n

E(

n

) =

1

r

3

O(

log

6

n

n

p

n

)

Then,integrating over r when D

log

8

n

p

n

r "we obtain:

Z

D

log

8

n

p

n

j(s;t)j"

jE(

n

)jdtds O(

log

6

n

n

p

n

)

Z

D

log

8

n

p

n

r"

1

r

3

rdr

O(

1

nlog

2

n

)

This concludes the proof of the proposition.

6.Study of a dynamical system

In this section we study a quadratic dynamical system in the complex plane and

give precise estimates that will be crucial in the proof of the domination condition

in the next section (i.e.Proposition 7.2).

We rst x three real numbers x;y;z satisfying the following condition

:= det

0@

1 x z

x 1 y

z y 1

1A

= 1 +2xyz x

2

y

2

z

2

0

22 E.BREUILLARD

And suppose additionally that jxj 1,jyj 1 and jzj < 1.Now dene the following

three sequences recursively:

a

k+1

= a

k

+

1

2

2

2

c

2k

+2ic

k

y(30)

b

k+1

= b

k

+

1

2

2

2

b

2k

+2ib

k

z

c

k+1

= c

k

+

x

2

2

2

b

k

c

k

+ib

k

y +ic

k

z

where the initial values a

0

;b

0

and c

0

are arbitrary and is a given real number.In

the applications below, will be small (of order log(n)=n) and the above dynamical

system can be viewed as a perturbation of that given when = 0.In this section we

will study the behavior of the above three sequences depending on initial values and

also on the values of the parameters x;y;z and .

We rst note that (b

k

) is a quadratic dynamical system and is therefore conjugate

to P

:u 7!u

2

+c

for some complex number c

.A straightforward computation

shows that if we set

x

k

:=

1

2

+iz 2

2

b

k

then we have x

k+1

= P

(x

k

) where c

=

1

4

2

(1 z

2

).In the limit when tends to

0,then c

tends to

1

4

,which is on the boundary of the Mandelbrot set.As long as

is small enough and non-zero,then the Fatou set corresponding to P

has exactly one

bounded connected component.Moreover,as soon as jj < 1,for every starting point

x

0

lying inside this component,the resulting sequence of iterates (x

k

) will converge

to the attracting xed point x

given by

x

=

1

2

jj

p

1 z

2

Thus for 6= 0 and jj < 1,the sequence (b

k

) converges to

b

=

iz +sgn()

p

1 z

2

2

Now let us dene

v

k

:= c

k

+

yz x

1 z

2

b

k

Then it is easy to check directly from the equations (30) that v

k

satises

(31) v

k+1

v = (x

k

+

1

2

)(v

k

v)

where(32) v =

i

2

y xz

1 z

2

Let us write y

k

=

1

2

+x

k

,then we obtain

(33) v

k

v = (v

0

v)y

0

::: y

k1

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 23

Similarly,if we let

f

k

:= a

k

+

yz x

1 z

2

c

k

then we nd that

f

k+1

f

k

=

2(1 z

2

)

+(v

k

v)(1 (

1

2

+x

k

))

yz x

1 z

2

2

2

(v

k

v)

2

Making use of (31),it follows that for k 1

a

k

= f

0

+

yz x

1 z

2

2

b

k

+

yz x

1 z

2

(v

0

2v

k

) +(34)

k

2(1 z

2

)

2

2

(v

0

v)

2

k1

X

p=0

(y

0

::: y

p1

)

2

We are now going to study the dynamical system (30) in the particular case when

the initial values are dened by

a

0

= 0(35)

b

0

=

i

2

z

c

0

=

i

2

w

where w is a xed real number.Then x

0

=

1

2

belongs to the lled Julia set of P

,

and the sequence (x

k

) (hence (y

k

) too) stays on the real line and satises

1

2

jj

p

1 z

2

= x

x

k

1

2

Additionally,

v

0

=

i

2

(w +z

yz x

1 z

2

)

Together with (31) and (32) this shows that v

k

belongs to iR for all k.With these

initial values,(34) takes the form

a

k

=

1

2

yz x

1 z

2

2

(iz +

1

(1 y

k

)) +

i

2

yz x

1 z

2

(2w

2y

1 z

2

+z

yz x

1 z

2

)

i

yz x

1 z

2

(w y)y

0

::: y

k1

+

+k

2(1 z

2

)

+

1

2

(w y)

2

k1

X

p=0

(y

0

::: y

p1

)

2

24 E.BREUILLARD

taking the real part we get

Re(a

k

) =

1

2jj

yz x

1 z

2

2

1 y

k

jj

+k

2(1 z

2

)

+(36)

1

2

(w y)

2

k1

X

p=0

(y

0

::: y

p1

)

2

The following lemma summarizes the computations above and encloses the infor-

mation that will be relevant to the sequel:

Lemma 6.1.In the dynamical system dened by (30),with initial values given by

(35),the following holds.

Take jj 1=2.Then for all k 0

(i) jkj 1=

p

1 z

2

implies 1 y

k

k

1

2

2

(1 z

2

);and Re(b

k

) k

1z

2

4

;and

Re(a

k

)

k

4

[

(yz x)

2

1 z

2

+2e

4

(w y)

2

]

(ii) jkj 1=

p

1 z

2

implies jj

p

1 z

2

1 y

k

1

2

jj

p

1 z

2

;and Re(b

k

)

1

4jj

p

1 z

2

and

Re(a

k

)

1

jj

1

4

p

1 z

2

[

(yz x)

2

1 z

2

+

1

4

(w y)

2

]

(iii) jb

k

j

1

2

and Re(b

k

) 0

(iv) jv

k

j 2=(1 z

2

) +jwj +1

(v) jc

k

j 3=(1 z

2

) +jwj +1

(vi) Re(a

k

)Re(b

k

) Re(c

k

)

2

(vii) Re(a

k

) is a non-decreasing sequence.

Proof.All these points are easy to check from what was done above.The proof of

(i) follows by induction;it is true when k = 0 and,assuming the inequality for k;

we get

1 y

k+1

= (1 y

k

)y

k

+

2

(1 z

2

)(37)

k

1

2

2

(1 z

2

)(1 jj

p

1 z

2

) +

2

(1 z

2

)

k

1

2

2

(1 z

2

) +

1

2

2

(1 z

2

)(2 jkj

p

1 z

2

)

(k +1)

1

2

2

(1 z

2

)

Besides,for all p k we have y

p

1 jj

p

1 z

2

e

2jj

p

1z

2

since 1 t e

2t

if

t 2 [0;

1

2

].Hence

(y

0

::: y

p1

)

2

e

4jpj

p

1z

2

e

4

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 25

The inequality for Re(a

k

) in (i) now follows instantly from (36) and the fact that

0.

Point (ii) follows from the fact (granted by (37)) that 1 y

k

cjj implies 1

y

k+1

cjj for any real number c with

p

1 z

2

c 0.

We have 2b

k

= iz + (1 y

k

)= and 0 1 y

k

jj

p

1 z

2

so j2b

k

j

p

z

2

+(1 z

2

) 1 yields (iii).Similarly v

k

v = (v

0

v)y

0

::: y

k1

,hence

2v

k

= i(y xz)=(1 z

2

) +i(w y)y

0

::: y

k1

Since jy

i

j 1 for all i,we get j2v

k

j jy xzj=j1 z

2

j + jw yj so j2v

k

j

2=j1z

2

j+jwj+1,and we have (iv) and also (v) because of (iii) and v

k

:= c

k

+

yzx

1z

2

b

k

:

For (vi);we have Re(a

k

)

1y

k

2

2

yzx

1z

2

2

= Re(b

k

)

yzx

1z

2

2

,so (recall that the v

k

belong to iR)

Re(a

k

)Re(b

k

)

Re(b

k

)

yz x

1 z

2

2

= Re(c

k

)

2

Finally (vii) is easily checked from (36).

7.Proof of the domination condition for small values of the

parameters s;t;

In this section,we give a domination estimate for a particular type of trigonometric

sum that will arise in the proof of local limit theorem.We then apply these estimates

to treat the part of the integral that yields a contribution to the limit,that is when

s;t; are small.

7.1.Estimating a trigonometric sum.Let us consider throughout this section a

sequence of independent and identically distributed randomvariables (A

k

;B

k

;C

k

;D

k

)

k1

in R

4

.We assume that the distribution has compact support in R

4

;and that

E(A

k

) = E(B

k

) = E(C

k

) = 0.Let us use the shorthand

X to denote the expectation

E(X) of a random variable X.We x the following notations for the correlations

x =

A

1

B

1

q

A

21

B

2

1

;y =

A

1

C

1

q

A

21

C

2

1

;z =

B

1

C

1

q

B

2

1

C

2

1

We also assume that A

1

is not identically 0 and that the distribution of the marginal

(B

k

;C

k

) is not degenerate,i.e.is not supported on a line.This is equivalent to the

condition jzj < 1.

Fix w 2 Rand then consider the trigonometric product for r; in R,

(38)

n

=

Y

1kn

e

ir(A

k

=

p

A

21

wC

k

=

p

C

2

1

)

e

iD

k

!

Y

1p<qn

e

iB

q

C

p

=

p

B

2

1

C

2

1

!

e

i

2

z(C

1

+:::+C

n

)

2

=

C

2

1

The following proposition yields the desired estimate for the trigonometric sumE(

n

).

26 E.BREUILLARD

Proposition 7.1.Let us x D > 0 a positive number and m 1 an integer.

For any integer n 1 and any distribution (A

1

;B

1

;C

1

;D

1

),of compact support

and as described above,the following estimate holds uniformly when r varies in

[D

log

2m

n

p

n

;D

log

2m

n

p

n

] and varies in [D

log n

n

;D

log n

n

]n[

2

n

p

1z

2

;

2

n

p

1z

2

],

jE(

n

)j exp(

njj

p

1 z

2

8

+

r

2

jj

C

16

p

1 z

2

) +

1 +jwj

1 z

2

4

O(

log

6m

n

p

n

)

where C =

(w y)

2

+

(yzx)

2

1z

2

.

Similarly,if r varies in [D

log

2m

n

p

n

;D

log

2m

n

p

n

] and varies in [

2

n

p

1z

2

;

2

n

p

1z

2

],we

obtain

jE(

n

)j exp[nr

2

Ce

4

4

] +

1 +jwj

1 z

2

4

O(

log

6m

n

p

n

)

The constant in O() depends only on D and on the size of the distribution

M = maxfjA

1

j=

q

A

21

;jB

1

j=

q

B

2

1

;jC

1

j=

q

C

2

1

;jD

1

jg

Proof.Let n 2 N and r; 2 R be as in the statement of the proposition.Let U

0

= 0

and for k 1,

U

k

= (C

1

+:::+C

k

)=

p

C

2

We rst set a few notations.Let q

k

be the quadratic form

q

k

(u;v) = a

k

u

2

+b

k

v

2

+2c

k

uv

where the coecients are dened right below.Then let

0

= 1 and for k 1

k

= e

ir(A

1

+:::+A

k

)=

p

A

21

Y

1p<qk

e

iB

q

C

p

=

p

B

2

1

C

2

1

!

e

i(D

1

+:::+D

k

)

And nally set

P

k

(r;) = E(

nk

e

q

k

(r;U

nk

)

)

We shall dene the coecients a

k

;b

k

;c

k

recursively as follows.We set:

a

k+1

= a

k

+

1

2

2

2

c

2k

+2ic

k

y(39)

b

k+1

= b

k

+

1

2

2

2

b

2k

+2ib

k

z

c

k+1

= c

k

+

x

2

2

2

b

k

c

k

+ib

k

y +ic

k

z

and the initial values are taken to be:a

0

= 0;c

0

=

i

2

w (recall that w 2 R),and

b

0

=

i

2

z.

These relations are precisely the recurrence relations (30) dened in the last section.

As was discussed there,Lemma 6.1 applies and,as soon as jj D

log n

n

and jrj

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 27

D

log

2m

n

p

n

,we see that b

k

and c

k

are uniformly bounded (by const=(1 z

2

) +jwj +

1) and that Re(a

k

)Re(b

k

) (Re(c

k

))

2

.Also note that P

0

= E(

n

e

q

0

(r;U

n

)

) =

E(

n

e

i

2

zU

2

n

riwU

n

),so that

P

0

(r;) = E(

n

):

And

P

n

= E(e

q

n

(r;0)

) = e

a

n

r

2

:

In order to obtain the desired estimate on E(

n

),that is on P

0

(r;),we are going

to start from P

0

(r;) and proceed by induction on k.To achieve this goal,we need

the following key induction step:

Claim:For all k and n with k n 1,and if jj D

log n

n

and jrj D

log

2m

n

p

n

,we

have

(40) P

k

(t;s) = e

b

k

2

+i

D

P

k+1

(t;s) +

1 +jwj

1 z

2

4

O(

log

6m

n

n

p

n

)

where the constant in O depends only on D and on the size M of the support of the

distribution (A

1

;:::;D

1

).

This crucial estimate follows from the computation made below.First,note that

q

k

(r;U

nk

) = q

k

(r;U

nk1

+C

nk

=

p

C

2

)

= q

k

(r;U

nk1

) +b

k

2

C

2

nk

=

C

2

+

2b

k

2

U

nk1

C

nk

=

p

C

2

+2c

k

rC

nk

=

p

C

2

We also recall that A

n

is the event fmax

0kn

jU

k

j

p

nlog ng.

We will use the expansion e

t

= 1 +t +t

2

=2 +O(t

3

) near t = 0.Applying Lemma

5.1 and bearing in mind that is centered we can write

P

k

= E

nk1

e

irA

nk

=

p

A

2

e

iB

nk

U

nk1

=

p

B

2

e

iD

nk

e

q

k

(r;U

nk

)

= E(1

A

n

nk1

e

q

k

(r;U

nk1

)

e

irA

nk

=

p

A

2

e

iB

nk

U

nk1

=

p

B

2

e

iD

nk

e

b

k

2

C

2

nk

=

C

2

2b

k

2

U

nk1

C

nk

=

p

C

2

2c

k

rC

nk

=

p

C

2

) +e

c

0

log

2

n

= E(1

A

n

nk1

e

q

k

(r;U

nk1

)

(1 +irA

nk

=

p

A

2

+iB

nk

U

nk1

=

p

B

2

+

iD

nk

b

k

2

C

2

nk

=

C

2

2b

k

2

U

nk1

C

nk

=

p

C

2

2c

k

rC

nk

=

p

C

2

+

1

2

"

irA

nk

=

p

A

2

+iB

nk

U

nk1

=

p

B

2

+iD

nk

b

k

2

C

2

nk

=

C

2

2b

k

2

U

nk1

C

nk

=

p

C

2

2c

k

rC

nk

=

p

C

2

#

2

+O(

log

6m

n

n

p

n

))) +e

c

0

log

2

n

28 E.BREUILLARD

Expanding further,we get

P

k

= E(

nk1

e

q

k

(r;U

nk1

)

[1 b

k

2

+i

D r

2

(

1

2

2

2

c

2k

+2ic

k

y)

2

U

2

nk1

(

1

2

2

2

b

2k

+2ib

k

z) 2rU

nk1

(

x

2

2

2

b

k

c

k

+ib

k

y +ic

k

z)]

+O(

log

6m

n

n

p

n

))

= E(

nk1

e

q

k

(r;U

nk1

)

[exp

b

k

2

+i

D a

k+1

r

2

b

k+1

2

U

2

nk1

2c

k+1

rU

nk1

+O(

log

6m

n

n

p

n

)] +O(

log

6m

n

n

p

n

)

where the coecients a

k+1

,b

k+1

and c

k+1

are the ones dened above.Hence we

indeed obtain

P

k

= e

b

k

2

+i

D

P

k+1

+O(

log

6m

n

n

p

n

)

This computation makes sense as long as b

k

and c

k

remain uniformly bounded

when n grows,and we checked that it is indeed the case.We also need to insure that

Re(q

k

) 0 everywhere,i.e.Re(a

k

)Re(b

k

) (Re(c

k

))

2

and Re(a

k

) 0,but we also

checked that above.

With the help of Lemma 5.1 and the remark following it,we verify that the constant

involved in the O in the above calculations can be taken of the form c D

4

K

4

;where

c > 0 depends only on the size of the support of the distribution of (A

1

;B

1

;C

1

;D

1

)

and where K is that number that bounds c

k

and b

k

.Hence the claim (40) is

proved.

We can now iterate (40),going from P

0

to P

n

.We deduce

E(

n

) = e

2

P

n1

k=0

b

k

e

in

D

e

a

n

r

2

+O(

log

6m

n

p

n

)

The constant involved here in the O is bounded by some cD

4

(1 +jwj)

4

=(1 z

2

)

4

where c is a constant depending only on M = maxfjA

1

j=

q

A

21

;jB

1

j=

q

B

2

1

;jC

1

j=

q

C

2

1

;jD

1

jg.

jE(

n

)j e

2

P

n1

k=0

Reb

k

e

r

2

Rea

n

+O(

log

6m

n

p

n

)

From Lemma 6.1,if jj

2

p

1z

2

1

n

then Reb

k

1

4jj

p

1 z

2

for all k n=2 and

Re(b

k

) 0 for all k.So the rst factor in the above equation leads to the bound

e

njj

p

1z

2

=8

.While Re(a

n

)

1

jj

1

4(1z

2

)

1=2

1

4

(w y)

2

+

(yzx)

2

1z

2

.

If jj

2

p

1z

2

1

n

,then Re(a

n

) Re(a

n=2

)

n

4

e

4

(w y)

2

+

(yzx)

2

1z

2

.

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 29

So we obtain the desired inequalities.

7.2.Domination condition.The purpose of this subsection is to give the precise

estimate we wanted in the course of the proof of the local limit theorem (control of

the part of the integral where all parameters s;t; are small).This is explained is

the following proposition:

Proposition 7.2.Let be a probability measure on the Heisenberg group with the

properties described in the introduction.Let

n

be the random variable dened in

(19) at the beginning of section 5.There exist positive numbers c

1

> 0 and c

2

> 0

depending only on ,such that,if D > 0 denotes some positive number,then the

following estimates hold uniformly

(i) when s and t vary in [D

log

8

n

p

n

;D

log

8

n

p

n

] and varies in [D

log n

n

;D

log n

n

]n[

c

2

n

;

c

2

n

],

jE(

n

)j exp(c

1

njj +

t

2

+s

2

jj

) +O(

log

24

n

p

n

)

(ii) when s and t vary in [D

log

8

n

p

n

;D

log

8

n

p

n

] and varies in [

c

2

n

;

c

2

n

],

jE(

n

)j exp(c

1

n(t

2

+s

2

)) +O(

log

24

n

p

n

)

where the constant in O depends only on and on D.

Proof.We are going to apply the results of the last section.In order to do so,we

need to choose carefully the variables A

1

;B

1

;C

1

;D

1

as well as the coecient w.Take

D

1

= Z=

p

X

2

Y

2

,C

1

= Y=

p

Y

2

,B

1

= X=

p

X

2

and A

1

= cos()

X

p

X

2

+2 sin()

Y

p

Y

2

and w =

sin()

()

,where ()

2

=

A

21

= 1 +3 sin

2

() +4z sin() cos() and () 0.An

easy computation shows that 8 ()

2

(1 z

2

)=5.Clearly the variables A

1

;B

1

and C

1

are linearly dependent,hence = 0.But B

1

and C

1

are linearly independent

since is aperiodic,hence jzj < 1.

Then the two expressions

n

in (19) and (38) agree if we change into =

p

X

2

Y

2

and let r and be determined by

t =

r cos()

()

p

X

2

s =

r sin()

()

p

Y

2

30 E.BREUILLARD

From the above inequality on (),we conclude that if M

X;Y

= maxf

1

X

2

;

1

Y

2

g and

m

X;Y

= minf

1

X

2

;

1

Y

2

g;

m

X;Y

1 z

2

5

(t

2

+s

2

)

r

2

X

2

Y

2

8M

X;Y

(t

2

+s

2

)

To compute the constant C appearing in Proposition (7.1),let us rst compute x;y

and z.

z =

XY =

p

X

2

Y

2

y =

z cos() +2 sin()

()

x =

cos() +2z sin()

()

hence

C = (w y)

2

+

(yz x)

2

1 z

2

=

sin() +z cos()

()

2

+

(1 z

2

) cos

2

()

()

2

=

1

()

2

[1 2z cos() sin()]

1

()

2

(1 jzj)

1 z

2

2 8

Also note that jwj 1=()

p

5=(1 z

2

).The rst estimate in (7:1) now yields

jE(

n

)j exp(

njj

p

X

2

Y

2

p

1 z

2

8

+

r

2

jj

C

4

p

1 z

2

) +O(

log

6

n

p

n

)

exp(

p

X

2

Y

2

p

1 z

2

njj

8

+

r

2

jj

X

2

Y

2

1

64

) +O(

log

6

n

p

n

)

exp(

q

X

2

Y

2

XY

2

njj

8

+

M

X;Y

(t

2

+s

2

)

64 jj

) +O(

log

6

n

p

n

)

exp(c

1

njj +

(t

2

+s

2

)

jj

) +O(

log

6

n

p

n

)

where we can take c

1

p

X

2

Y

2

XY

2

8

minf1;M

X;Y

=8g,and the constant in O in

the last line depends only on and D.

We thus have obtained (i),and (ii) follows similarly with c

1

e

4

80

(

X

2

Y

2

XY

2

).

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 31

8.Proofs of the main theorems

We can now nish the proof of Theorem 2.1.Let f and g be as in the statement

of Theorem 2.1.Recall that the number was dened in (5).As was remarked at

the end of section 3,we can write

n

2

h(

n

)f;gi = A

n

+B

n

where A

n

and B

n

are dened as follows:

A

n

= n

2

Z

jjD

log n

n

h

(

n

)f

;g

i

H

d

and there is a constant C depending on D, and the size of the set f;9(t;s);F

g

(g)(s;t;) 6=

0g such that

(41) jA

n

j

C

n

k

0

2

kfk

L

2

(G)

kgk

L

2

(G)

as soon as the integer k

0

satises D > k

0

=c (where c > 0 is a constant depending on

only dened in (13)).And

(42) B

n

=

Z

G

dx

(2)

3=2

f(x)I

n

(x)

with

I

n

(x) = n

2

Z

jjD

log n

n

E(

n

(t;s;));e

i

F

(g)(t y;s +2y;)

L

2

(R

2

)

d

where,keeping the notations of Theorem 2.1 F

(g) is the Fourier transform dened

in (4),

n

(t;s;) is dened in (19) and x = (x;y;z) 2 G.

Splitting the integral on R

2

in the expression of I

n

(x) above into the parts when

j(t;s)j D

log

8

n

p

n

on the one hand and j(t;s)j D

log

8

n

p

n

on the other hand,we can

write:(43) I

n

(x) = I

S

n

(x) +I

L

n

(x)

and assert that if kxk = maxfjxj;jyj;jzjg and 2jj are less than say K > 1 then

F

(g)(t y;s + 2y;) 6= 0 implies that maxfjtj;jsj;jjg A(1 + K

2

) where

A > 0 is a number such that the support of F

(g) lies inside [A;A]

3

.Then we may

write:

jI

L

n

(x)j n

2

kF

(g)k

1

Z

jjD

log n

n

Z

D

log

8

n

p

n

j(t;s)jA(1+K

2

)

jE(

n

(t;s;))jdtdsd

n

2

kF

(g)k

1

Z

jjD

log n

n

O(

1

nlog

2

(n)

)d(44)

kF

(g)k

1

O(

1

log(n)

)

32 E.BREUILLARD

where line 44 is granted by the two Propositions 5.2 and 5.4.The constant involved

here in O depends only on ,D and the size A of the support of F

(g) and on the

maximum of kxk.In particular it is uniform when x varies in compact subsets of G.

We can now concentrate on the part I

S

n

(x) of the integral which actually gives a

contribution to the limit.From Proposition 7.2,we deduce that if j(t;s)j Dlog

8

n

and c

2

jj Dlog n

jE(

n

(

t

p

n

;

s

p

n

;

n

))j e

c

1

(jj+t

2

=jj+s

2

=jj)

+O(

log

24

n

p

n

)

and if j(t;s)j Dlog

8

n and jj c

2

jE(

n

(

t

p

n

;

s

p

n

;

n

))j e

c

1

(t

2

+s

2

)

+O(

log

24

n

p

n

)

where the constant in O depends only on and D.But one can check that the

function

(t;s;) 7!e

c(jj+t

2

=jj+s

2

=jj)

is integrable over R

3

.And

(t;s;) 7!e

c(t

2

+s

2

)

is integrable in (t;s;) 2 R

2

[c

2

;c

2

].Moreover

Z

jjDlog n

Z

j(t;s)jDlog

8

n

O(

log

24

n

p

n

) = O(

log

41

n

p

n

)!0

And nally,from the central limit theorem (see [Tut] or [Rau]) the following limit

holds point-wise in t;s;:

E(

n

(

t

p

n

;

s

p

n

;

n

))!E(e

i(tX+sY +(ZY

2

))

)

where (X;Y;Z) is the limit random variable with gaussian distribution

1

as men-

tioned in the introduction.Hence if we use the shorthand t

n

:= t=

p

n,s

n

= s=

p

n and

n

:= =n we have (see the denition of

n

= (

t

p

n

;

s

p

n

;

n

) in (18) above),uniformly

when x varies in compact subsets of G

lim

n!+1

E(

n

(t

n

;s

n

;

n

))e

i

n

F

(g)(t

n

n

y;s

n

+2

n

y;

n

) = E(e

i(tX+sY +(ZY

2

))

)

F

(g)(0)

Since the heat kernel corresponding to is a fastly decreasing smooth function

p(x;y;z) on R

3

(see [VSC]) it follows that E(e

i(tX+sY +(ZY

2

))

) is integrable in

(t;s;) 2 R

3

and we compute by the Fourier inversion formula

Z

R

3

E(e

i(tX+sY +(ZY

2

))

)dtdsd = (2)

3

p(0;0;0) = (2)

3

c()

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 33

Therefore,by Lebesgue's dominated convergence theorem,we get uniformly when

x varies in compact subsets

lim

n!+1

I

n

(x) = lim

n!+1

I

S

n

(x)

= n

2

Z

jjD

log n

n

Z

j(t;s)jD

log

8

n

p

n

E(

n

(t;s;))e

i

F

(g)(t y;s +2y;)dtdsd

= (2)

3

c()

F

(g)(0)

Integrating in x we nally obtain

lim

n!1

B

n

= lim

n!+1

Z

G

dx

(2)

3=2

f(x)I

n

(x)

= (2)

3=2

c()

F

(g)(0)

Z

G

f

= c()

Z

G

f

Z

G

g(45)

Combining (45) and (41) we get

(46) lim

n!1

n

2

h(

n

)f;gi = c()

Z

G

f

Z

G

g

as desired.

We also remark that any translation of g (to the left

z

g(x) = g(z

1

x) or to the

right g

z

(x) = g(xz)) has again a compactly supported Fourier transform F

(

z

g) and

F

(g

z

):If z remains in a compact subset,then the supports of F

(

z

g) and F

(g

z

)

also remain within a prescribed bounded set.Moreover kF

(g

z

)k

1

= kF

(

z

g)k

1

and

kg

z

k

2

= k

z

gk

2

are independent of z.This follows from the computation of F

(

z

g)

which yields

(47) F

(

z

g)(t;s;) = e

i

F

(g)(t;s (2z

y

z

x

);)

where z = (z

x

;z

y

;z

z

) and = z

x

t+z

y

s+(z

z

z

2

y

).Consequently all the calculations

above,and (46) in particular,hold uniformly for translates of g on compact subsets.

Now take f

n

a Dirac sequence of positive functions supported on a neighborhood

of 0 of diameter of order 1=n

3

.Then kf

n

k

22

= O(n

9

).Choosing D large enough (so

that A

n

in (41) remains negligeable) we see that we can replace f by f

n

in the above

calculations,hence

(48) lim

n!1

n

2

h(

n

)f

n

;gi = c()

Z

G

g

And the same holds uniformly when z varies in compact subsets and g is replaced

by g

z

or

z

g.

34 E.BREUILLARD

Assume now that F

(g) is also absolutely continuous (that is belongs to W

1;1

(R

3

),

i.e.its derivative distribution is a function in L

1

(R

3

)),then it follows

1

that g

is

real analytic and that kxk d

x

(g

) is a bounded function on R

3

(where kxk is the

max of the coordinates of x 2 R

3

).For any compact K we can then nd a constant

C = C(g;K) such that whenever y is small enough

supz2K

k

y

1

z

g

z

gk

1

C kyk

and

sup

z2G

k

y

1g

z

g

z

k

1

Ckyk

Hence uniformly for z in compact subsets,k

z

g f

n

z

gk

1

= O(1=n

3

) and kg

z

f

n

g

z

k

1

=

O(1=n

3

).From (48) it now follows that uniformly for z in compact subsets:

lim

n!1

n

2

Z

g(z

1

x)d

n

(x) = c()

Z

G

g

and(49) lim

n!1

n

2

Z

g(xz)d

n

(x) = c()

Z

G

g

We now deduce Theorem 1.1 from Theorem 2.1.

Proof.For all"> 0 one can nd

2

a strictly positive function h in L

1

(R

3

) such

that

b

h is C

1

and compactly supported,and such that there exists C > 0,with

h(x) kxk

6+"

C for x 2 R

3

large enough.Let g = h

1

.Now for all 2 R

3

and

z 2 G,e

i

1

(x)

g

z

(x) also has C

1

Fourier transform F

of compact support,hence

(49) holds for it uniformly when z varies in compact subsets.By Levy's criterion for

weak convergence of nite measures,this shows that the sequence of nite measures

d

z

n

= n

2

g

z

d

n

converges weakly (in the space of nite measures on G) to c()g

z

(x)dx

uniformly when z varies in compact subsets.Now let f be a function on G as in the

statement of the theorem.Then f=g is a bounded continuous function,hence

n

2

Z

f

z

d

n

=

Z

f

z

=g

z

d

z

n

!c()

Z

G

f

uniformly when z varies in compact subsets.

1

Note that if a function h(x) 2 L

1

(R) is such that

b

h(t) 2 C

c

(R) and

d

b

h

dt

2 L

1

(R) (i.e.

b

h absolutely

continuous) then x

dh

dx

is the Fourier transform of

d

dt

(t

b

h),hence is bounded.

2

It is enough to nd a function f 2 L

1

(R) such that

b

f is C

1

of compact support and f(x)

C

1+jxj

2+"

for some C > 0 (e.g.see [Breu] section 3.2).Then take h(x) = f(x)f(y)f(z) if x = (x;y;z):

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 35

9.Uniform local limit theorem for translates of a bounded set

We intend here to prove Theorem 1.2.The probability measure is assumed to

be aperiodic,centered and compactly supported.The letter denotes the associated

gaussian probability distribution.Its density function,the heat kernel,is denoted by

p

1

(x;y;z).It is a fastly decreasing smooth function on G.

We start by xing a non-negative function K on G such that F

(K) is smooth

and has compact support and F

(K)(0) = 1.Then we form a Dirac family (K

a

)

a>0

by letting K

a

(x) = a

4

K(d

a

(x)).Then F

(K

a

)() = F

(K)(d

1

a

()).Let us also write

K

_

a

(z) = K

a

(z

1

).They also form a Dirac family when a!+1.

Lemma 9.1.There are two sequences of positive numbers ("

n

)

n

and (a

n

)

n

,depending

only on ;with"

n

!0 and a

n

!+1,such that for all bounded Borel sets B G

for which maxfjy

1

j;jy

2

jg n= log n whenever y = (y

1

;y

2

;y

3

) 2 B,and all x 2 G,if

we set P

B

n

(x) =

n

(xB) and Q

Bn

(x) =

n

(xB),the following inequality holds for all

positive integers n;

(50) n

2

P

B

n

K

_

a

n

(x) Q

Bn

K

_

a

n

(x)

"

n

maxf1;jBjg

where jBj denotes the Haar measure of B.

Proof.We may write

P

B

n

K

_

a

(x) =

Z

G

n

(xz

1

B)K

_

a

(z)dz

=

Z

G

E(1

S

1

n

z2B

1

)K

a

(x

1

z)dz

= h(

n

)f;

x

gi

where

x

g is the translate

x

g(z):= g(x

1

z) and f:= 1

B

1 and g(z):= K

a

(z) (note

that K

a

is real).Then we can make use of the calculations performed in the previous

sections to estimate this scalar product.We use the notations introduced in section

8.

As noted at the beginning of section 8,we can write

n

2

h(S

n

)f;

x

gi = A

n

() +B

n

()

where A

n

is controlled by the estimation (41) and B

n

given by (42).Since kfk

L

2

(G)

=

p

jBj and k

x

gk

L

2

(G)

= a

2

kKk

L

2

(G)

we obtain (take k

0

= 3 and D = 3=c)

(51) jA

n

()j C(a)

a

2

n

p

jBj kKk

L

2

(G)

where C(a) is a positive constant depending only on a.Integrating the decomposition

I

n

(y) = I

S

n

(y)+I

L

n

(y) with respect to f(y)dy (see equations (42) and (43));we obtain

that B

n

() can be written as a sum B

S

n

() + B

L

n

().Note that kF

(

x

K

a

)k

1

=

36 E.BREUILLARD

kF

(K)k

1

as follows from (47).Spliting B

L

n

() into part when" j(t;s)j D

log

8

n

n

and the part when j(t;s)j "we have from (42) and (44)

B

L

n

() = B

L

0

n

() +B

L

1

n

()

(52) jB

L

0

n

()j jBj kF

(K)k

1

O

(

1

log n

)

as follows from Proposition 5.4,and

(53) jB

L

1

n

()j

jBj kF

(K)k

1

(2)

3=2

L n

2

sup

Tj(t;s)j";jjD

log n

n

jE(

n

(t;s;))j

where T is the size of the support of the functions F

(

x

g) and L is the Lebesgue

measure of that support:

(t;s) 7!F

(K)(

t y

2

a

;

s +2(y

2

x

2

) +x

1

a

;

a

2

)

where x = (x

1

;x

2

;x

3

) and y = (y

1

;y

2

;y

3

).Note that L is a xed multiple of a

2

.

Since jj D

log n

n

in (44) and by assumption maxfjy

1

j;jy

2

jg n= log n,if we sup-

pose additionally that maxfjx

1

j;jx

2

jg 2n= log n then the constant T in the above

equation (53) is bounded by some xed function of a.In Proposition 5.2 we showed

that there is a constant C

T

such that

sup

Tj(t;s)j";jjD

log n

n

jE(

n

(t;s;))j

C

T

n

3

Hence for some number (a) < +1

(54) jB

L

n

()j jBj

(a)

log n

The estimations above can be carried out in a similar way for instead of :In

particular

Q

Bn

K

_

a

n

(x) = h(

n

)f;

x

gi

= A

n

() +B

n

()

The term A

n

() is dealt with in exaclty the same way since estimate (41) is also valid

for and we have

(55) jA

n

()j C(a)

a

2

n

p

jBj kKk

L

2

(G)

In order to control B

n

() we made use of the compact support assumption on .For

we can use the following direct argument because F

(p

1

) is integrable since p

1

(x;y;z)

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 37

decays rapidly when (x;y;z) is large.Recall that (

t

)

t

is a stable semi-group,i.e.

n

=

n

1

= d

p

n

(

1

).From (44) we have

jB

L

n

()j

1

(2)

3=2

jBj kF

(K)k

1

n

2

Z

j(t;s)jD

log

8

n

p

n

jE(

n

(t;s;))jdtdsd

1

(2)

3=2

jBj kF

(K)k

1

Z

j(t;s)jDlog

8

n

jF

(p

1

)(t;s;)jdtdsd

1

(2)

3=2

jBj kF

(K)k

1

o(1)(56)

Therefore it remains to treat B

S

n

() B

S

n

():From (42) and (43),we have

(57)

B

S

n

() B

S

n

()

A n

2

Z

jjD

log n

n

Z

j(t;s)jD

log

8

n

p

n

jE(

n

(t;s;)) E(

n

(t;s;))jdtdsd

with

A =

1

(2)

3=2

jBj kF

(K)k

1

Now the integral on the right hand side tends to 0 as it follows from Lebesgue's

dominated converge theorem like we did in section 8.In section 7 we showed that

there exists an integrable function (t;s;) such that for n suciently large and for

all (t;s;) such that jj Dlog n and j(t;s)j Dlog

8

n,

E(

n

(

t

p

n

;

s

p

n

;

n

))

(t;s;) +O(

log

6

n

p

n

)

On the other hand F

(p

1

) is integrable and

E(

n

(

t

p

n

;

s

p

n

;

n

)) = F

(p

1

)(t;s;)

And by the central limit theorem,we had point-wise

lim

n!1

E(

n

(

t

p

n

;

s

p

n

;

n

)) E(

n

(

t

p

n

;

s

p

n

;

n

)) = 0

Hence by Lebesgue's dominated convergence theorem,

lim

n!+1

n

2

Z

jjD

log n

n

Z

j(t;s)jD

log

8

n

p

n

jE(

n

(t;s;)) E(

n

(t;s;))jdtdsd = 0

Therefore (57) reads:

(58)

B

S

n

() B

S

n

()

jBj o(1)

38 E.BREUILLARD

And nally combining (51),(55),(56),(54) and (58),we have that for some sequence

"

n

!0

n

2

P

B

n

K

_

a

(x) Q

Bn

K

_

a

(x)

"

n

jBj +

(a)

log n

jBj +C(a)

a

2

n

2 kKk

2

p

jBj

whenever x satises maxfjx

1

j;jx

2

jg 2n= log n.But we can choose a sequence

a

n

!+1 such that

(a

n

)

log n

and C(a

n

)

a

2n

n

tend to 0:Changing"

n

if necessary,we

obtain for these values of x

n

2

P

B

n

K

_

a

n

(x) Q

Bn

K

_

a

n

(x)

"

n

maxfjBj;

p

jBjg

Now let us examine the case when x takes large values,i.e.when maxfjx

1

j;jx

2

jg

2n= log n.Then

P

B

n

K

_

a

n

(x) =

Z

G

n

(xzB)K

a

n

(z)dz P(jT

n

j or jU

n

j n=2 log n)+

Z

jzj

n

2 log n

K

a

n

(z)dz

where U

n

is as before the sum Y

1

+:::+Y

n

and T

n

= X

1

+:::+X

n

.Now since F

(K)

is smooth,K decays rapidly and in particular

Z

jzj

n

2 log n

K

a

n

(z)dz =

Z

jzja

n

n

2 log n

K(z)dz = O(

1

n

k

)

for any k 0.Hence we get from Lemma 5.1

P

B

n

K

_

a

n

(x) = o(1=n

2

)

Similarly the same holds for Q

Bn

K

_

a

n

(x) when maxfjx

1

j;jx

2

jg 2n= log n.Changing

("

n

)

n

if necessary,we obtain the desired conclusion,i.e.inequality (50) for all x.

The restrictions on the size of B in the above lemma disappear if we make the

additional assumption (Cramer's condition) that

sup

t

2

+s

2

1

jE(e

i(tX+sY )

)j < 1

Indeed in this case,we can control E(

n

(t;s;)) uniformly for arbitrary large values

of t and s (see remark (5:3)).Therefore we can take T = +1in the estimation (53)

above and the restriction on B is unecessary.We obtain

Lemma 9.2.If we suppose additionally that satises Cramer's condition (9),then

there are two sequences of positive numbers ("

n

)

n

and (a

n

)

n

,depending only on ;

with"

n

!0 and a

n

!+1,such that for all bounded Borel sets B G and all

x 2 G,if we set P

B

n

(x) =

n

(xB) and Q

Bn

(x) =

n

(xB),the following inequality

holds for all positive integers n;

n

2

P

B

n

K

_

an

(x) Q

Bn

K

_

an

(x)

"

n

maxf1;jBjg

where jBj denotes the Haar measure of B.

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 39

Lemma 9.3.Let (a

n

)

n

be a sequence of positive numbers such that a

n

!+1.Then

there exists another sequence ("

n

)

n

with"

n

!0 and a sequence of neighborhoods of

identity (U

n

) converging to identity,such that for all bounded Borel sets B G,the

following inequality holds for all positive integers n;

n

2

Q

Bn

(x) Q

Bn

K

_

a

n

(x)

p(jV

n

BnBj +"

n

jBj)

where jBj denotes the Haar measure of B and where we have set Q

Bn

(x) =

n

(xB)

and p = max

jp

1

(g)j;g 2 G

> 0:

Proof.The proof is straightforward.We rst note that for any bounded Borel set B,

n

(B) pjBj=n

2

.Then we simply write:

jQ

Bn

K

_

a

n

(x) Q

Bn

(x)j

Z

jQ

Bn

(xz) Q

Bn

(x)jK

a

n

(z)dz

Z

z2U

1

n

n

(x(zBB)K

a

n

(z)dz

+2p

jBj

n

2

Z

z =2U

1

n

K

a

n

(z)dz

p(jU

n

BnBj +"

n

jBj)=n

2

where U

n

is a sequence of neighborhoods of identity tending to identity such that

R

z =2U

1

n

K

a

n

(z)dz tends to 0 at innity.

Now,let us complete the proof of Theorem (1:2).Let B be an arbitrary bounded

Borel set satisfying the condition of Lemma (9:1),that is maxfjy

1

j;jy

2

jg n= log n

whenever y = (y

1

;y

2

;y

3

) 2 B (resp.satisfying no additional condition if we assume

Cramer's condition).In the sequel like above,the Landau notations o and O will

correspond to functions depending only on .We keep notations of Lemma (9:1),

P

U

n

B

n

K

_

a

n

(x) =

Z

n

(xzU

n

B)K

a

n

(z)dz

n

(xB)

Z

U

n

K

a

n

(z)dz

where U

n

is as in Lemma 9.3.Now making use of Lemma 9:1 we get uniformly in

x 2 G,

P

B

n

(x) (1 +o(1))P

U

n

B

n

K

_

a

n

(x)(59)

(1 +o(1))[Q

U

n

B

n

K

_

a

n

(x) +(1 +jU

n

Bj)o(1=n

2

)]

40 E.BREUILLARD

And by Lemma 9:3,

Q

U

n

B

n

K

_

a

n

(x) Q

U

n

B

n

(x) +

p

n

2

(jU

2

n

BnBj +"

n

jU

n

Bj)

Q

Bn

(x) +

p

n

2

jU

n

BnBj +

p

n

2

(jU

2

n

BnBj +"

n

jU

n

Bj)

Q

Bn

(x) +

p

n

2

(2jU

2

n

BnBj +"

n

jU

n

Bj)

In particular we have

Q

U

n

B

n

K

_

a

n

(x)

4p

n

2

jU

2

n

Bj

and,from (59);

(60) P

B

n

(x) jU

2

n

BjO(

1

n

2

) +o(

1

n

2

)

Additionally,

(61) P

B

n

(x) Q

Bn

(x) +

2p

n

2

jU

2

n

BnBj +(jU

2

n

Bj +1)o(

1

n

2

)

Now let us turn to the other direction of the inequality.We have,making use of

(60)

P

B

n

K

_

a

n

(x) =

Z

n

(xzB)K

a

n

(z)dz

Z

U

n

n

(xzB)K

a

n

(z)dz +

Z

U

c

n

n

(xzB)K

a

n

(z)dz

n

(xU

n

B)

Z

U

n

K

a

n

(z)dz +(jU

2

n

Bj +1)o(

1

n

2

)

P

U

n

B

n

(x) +(jU

2

n

Bj +1)o(

1

n

2

)

But from (60),

P

U

n

B

n

(x) P

B

n

(x) =

n

(x(U

n

BnB))

jU

2

n

(U

n

BnB)jO(

1

n

2

) +o(

1

n

2

)

Hence

P

B

n

K

_

a

n

(x) P

B

n

(x) +jU

2

n

(U

n

BnB)jO(

1

n

2

) +(jU

2

n

Bj +1)o(

1

n

2

)

Now it follows from Lemma (9:1) that

Q

Bn

K

_

a

n

(x) P

B

n

K

_

a

n

(x) +(1 +jU

n

Bj)o(1=n

2

)

and from Lemma (9:3)

Q

Bn

(x) Q

Bn

K

_

a

n

(x) +

p

n

2

(jU

n

BnBj +"

n

jBj)

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 41

Combining the last three inequalities,we get

(62) Q

Bn

(x) P

B

n

(x) +jU

2

n

(U

n

BnB)jO(

1

n

2

) +(jU

2

n

Bj +1)o(

1

n

2

)

Equations (61) and (62) yield the desired result

(63) jP

B

n

(x) Q

Bn

(x)j jU

2

n

(U

n

BnB)jO(

1

n

2

) +(jU

2

n

Bj +1)o(

1

n

2

)

But clearly

T

n0

U

2

n

(U

n

BnB) is contained in

Bn

B.Hence for every bounded mea-

surable set B such that j@Bj = 0 we have

(64) lim

n!0

sup

x2G

n

2

j

n

(xB)

n

(xB)j = 0

And if satises Cramer's condition the estimate (63) holds without the above re-

striction on B:Hence (63) holds uniformly on y for all By:We conclude

(65) lim

n!0

sup

x;y2G

n

2

j

n

(xBy)

n

(xBy)j = 0

Remark 9.4.It is easy to see from (63) that the limits in (64) and (65) are uniform

in B when B ranges over the set of balls for a given norm on G lying in a given

compact subset of G.

Finally note that Theorem 1.3 follows instantly from the inequality (60) above.

10.Applications

10.1.An equidistribution result for bounded uniformly continuous func-

tions.In this section,we intend to give a proof of Corollary (1:4).The proof splits

into two steps.

Lemma 10.1.Suppose f is a continuous and bounded function on G satisfying

condition (2) of Corollary (1:4) then

lim

n!1

Z

G

f(g)d

n

(g) =`

where =

1

is an arbitrary gaussian measure on G.

Proof.Let (

t

)

t

be the one-parameter semigroup of gaussian measures in which

is embedded.By scaling invariance,

t

coincide with the image of

1

under the

automorphism d

p

t

of G,where d

t

(x;y;z) = (tx;ty;t

2

z).Hence

Z

G

f(g)d

n

(g) =

Z

G

f d

p

n

(g)p(g)dg

where p(g) is the density of .It is known that p as well as its derivatives are smooth,

fastly decreasing functions on G.Let L > 0 be a Lipschitz constant for p in the sense

42 E.BREUILLARD

that jp(g

1

) p(g

2

)j Lkg

1

g

2

k for all g

1

;g

2

2 G where kgk = maxfjxj;jyj;jzjg

C for g = (x;y;z):Fix"> 0 and let C > 0 be such that

Z

kgkC

p(g)dg "

We now have

Z

kgkC

f(d

p

n

(g))p(g)dg

"kfk

1

Let us denote by R(g;h) the rectangle [x;x + h) [y;y + h) [z;z + h) where

g = (x;y;z) 2 G and h > 0.We can nd a subdivision of the hypercube fkgk Cg

by small cubes of the form R(g

i

;h).Hence

Z

kgkC

f(d

p

n

(g))p(g)dg =

X

Z

R(g

i

;h)

f(

p

nx;

p

ny;nz)p(g)dg

Also

X

Z

R(g

i

;h)

f(

p

nx;

p

ny;nz)p(g)dg

X

p(g

i

)

Z

R(g

i

;h)

f(

p

nx;

p

ny;nz)dg

Lhkfk

1

(2C)

3

Now,note that,by viewing R(g;h) as a dierence of several rectangles in R

3

,the

assumption (2) made on f easily implies that for all g 2 G and h > 0

(66) lim

T!+1

1

T

2

Z

f(x)

R(g;h)

(d

1=

p

T

(x))dx =` h

3

since

R(g;h)

d

1=

p

T

can be written as a nite sumof terms of the form

[0;T

1

]:::[0;T

3

]

for some positive or negative T

1;

:::;T

3

.Hence by (66)

lim

T!+1

X

p(g

i

)

Z

R(g

i

;h)

f(

p

nx;

p

ny;nz)dg =`

X

p(g

i

)h

3

But

X

p(g

i

)h

3

1

Z

kgkC

p(g)dg +

X

Z

R(g

i

;h)

jp(g) p(g

i

)j dg

"+Lh(2C)

3

Therefore,combining the above inequalities,for n large enough

Z

f(d

p

n

(g))p(g)dg `

`("+Lh(2C)

3

) +Lhkfk

1

(2C)

3

+"kfk +"

We nally obtain the desired result since h can be taken arbitrarily small.

The second step is about comparing the integrals with respect to the probability

measure and its associated gaussian distribution .Here,we make use of the

uniform version of the local limit theorem (Theorem 1.2).Namely,

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 43

Lemma 10.2.Let f be a bounded and uniformly continuous function (with respect

to either right or left uniform structure on G).Let be a compactly supported

aperiodic and centered probability measure on G.And let be its associated gaussian

distribution.Then we have

lim

n!+1

Z

f(g)d

n

(g)

Z

f(g)d

n

(g)

= 0

Proof.We may assume kfk

1

1.Fix"> 0 and let!> 0 be a modulus of

continuity for f relatively to",i.e.jf(ux) f(x)j "if kuk !and x 2 G,

where kgk = maxfjxj;jyj;jzjg for g = (x;y;z) 2 G.As follows from the central limit

theorem,we can nd a number C > 0 such that if we let A

n

= fg = (x;y;z) 2

G;jxj;jyj C

p

n and jzj Cng,we have for large n

n

(A

cn

) "

and

n

(A

cn

) "

We can then nd a cover B

n

of the cube A

n

by less than O(n

2

=!

4

) disjoint translates

R

!

h of a small cube of the form R

!

= d

!

(R) where h 2 d

!

(G(Z)),d

!

is the dilation

on G with coecient of contraction!,and R is a fundamental domain for the co-

compact lattice G(Z) in G.Now we can write

Z

fd

n

Z

fd

n

Z

B

n

fd

n

Z

B

n

fd

n

+2"

X

i

f(h

i

) j

n

(Rh

i

)

n

(Rh

i

)j +4"

O(

n

2

!

4

) sup

h2G

j

n

(Rh)

n

(Rh)j +4"

Here we can apply the uniform local limit theorem (Theorem 1.2) and get

lim

n!1

n

2

sup

h2G

j

n

(Rh)

n

(Rh)j = 0

Thus,we obtain the desired result.

The proof of Corollary 1.4 now follows immediately from the combination of the

last two lemmas.

10.2.Unipotent randomwalks and equidistribution on homogeneous spaces.

Here we shall conclude this paper and give a proof of Theorem 1.5.

Let G be a connected real Lie group and a lattice in G,that is,a discrete

subgroup of G such that the homogeneous space G= bears a nite Borel measure

invariant by the left action of G.An element u 2 G is called Ad-unipotent,when

the automorphism Ad(u) 2 GL(g) of the Lie algebra g of G is unipotent,i.e.every

eigenvalue of Ad(u) equals 1.A subgroup U G is called Ad-unipotent or simply

44 E.BREUILLARD

unipotent is every element u 2 U is Ad-unipotent.The action of U on G= is called

a unipotent ow.

In the early nineties,in a series of papers (see [Rat1-3]),M.Ratner proved the

validity in full generality of the Raghunathan-Dani conjectures for the action of

connected Ad-unipotent subgroups on G=.These results have had a number of

far reaching applications (some of which were obtained earlier by proving special

cases of the conjecture,like in Margulis'proof of the Oppenheim conjecture (1986)),

especially to number theory and lattice points counting problems (see the recent

survey [Bab]).The results,can be summarized as follows.First,if U is a connected

Ad-unipotent subgroup of G,for every x 2 G=,the orbit Ux has a\nice algebraic"

closure,that is,there exists a closed subgroup H G such that

Ux = Hx is closed

and bears a unique H-invariant probability measure m

x

.Secondly,every U-ergodic

probability measure on G= is of the form m

x

for some x 2 G=.We refer the reader

to the surveys [Rat3] and [Sta] for a detailed exposition of these results and further

references (see also [MaT] for an alternative proof).

One of the main steps in the proof of the latter conjecture is the following equidis-

tribution theorem for the action of one-parameter unipotent ows:

Theorem 10.3 (M.Ratner).Suppose G is a Lie group and a lattice in G.Let

U = fu(t);t 2 Rg be a one-parameter Ad-unipotent subgroup of G.Then for any

x 2 G=,there is a closed subgroup H of G,such that Hx is closed and bears an

H-invariant probability m

x

,and the orbit Ux is equidistributed in Hx with respect to

m

x

.In other words,for all continuous and bounded functions f on G=,we have

lim

T!+1

1

T

Z

T

0

f(u(t)x)dt =

Z

Hx

fdm

x

Let us emphasize the fact that this equidistribution holds for every point x 2 G=

and not only almost everywhere with respect to some U-ergodic measure.

10.2.1.A counter-example.In this paragraph,we illustrate the phenomenon de-

scribed in the introduction of a non-centered random walk that may diverge in G=.

More precisely,we have:

Proposition 10.4.Let G = SL

2

(R), = SL

2

(Z) and U be a one-parameter unipo-

tent subgroup of G.Let be a non-centered probability measure on U with standard

deviation < +1 and average d 6= 0.Then for any compact subset K in G= and

-almost all points x 2 G= ( is a G-invariant measure on G=),we have

(67) lim inf

n!+1

n

x

(K) = 0

Let S

n

= X

1

+:::+ X

n

be a sum of centered i.i.d.variables on the line with

standard deviation > 0:Then the random walk (S

n

+nd)

n

is non-centered with

drift d,and we can choose X

1

so that the probability law of S

n

+nd is precisely

n

.

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 45

We identify G= with the space of lattices in R

2

and we let kk be the standard

Euclidean norm on R

2

.Recall,that according to Mahler's criterion,a subset K

G= is relatively compact if and only if there exists > 0 such that kvk > for any

lattice x 2 K and any non-zero vector v 2 x.We set x

0

= Z

2

:

Let (u(t))

t

be a one-parameter unipotent subgroup in SL

2

(R).It stabilizes a line

D in R

2

and we let 2 R be the slope (that we assume nite) of this line.In the

canonical coordinates of R

2

;the action of u(t) reads:

u(t)

xy

=

x +t(y x)

y +t(y x)

for some 2 Rnf0g.

We say that a real is well approximable on both sides if for any"> 0 and

2 f1;1g one can nd integers x and y in Z

2

nf(0;0)g such that

jx(y x)j <"and x(y x) > 0

For the Lebesgue measure on R,almost all are well approximable on both sides.

Let us x an arbitrary compact subset K G= and let > 0 be small enough

so that kvk > for any lattice x 2 K and any non-zero vector v 2 x:Let

=

ft;u(t)x

0

=2 Kg.Then we claim:

Claim 10.5.If =2 Q is well approximable on both sides,then for any C > 0 and

n

0

> 0 there is an integer n > n

0

such that

S

n

+nd 2

whenever jS

n

j < C

p

n.

Proof of claim:For (x;y) 2 Z

2

nf(0;0)g we set A

x;y

= ft 2 R;ju(t)(x;y)j < g and

= j2j maxfjj;1g and t

x;y

=

x

(xy)

,I

x;y

= ft;jtj <

jxyj

g so that

[

x;y2Z

2

nf(0;0)g

A

x;y

If is small enough,since =2 Q is well approximable on both sides,one can nd

arbitrarily large integers x;y such that

(68) jx(y x)j <

4

and t

x;y

has the sign we want.Note that if is small enough,then t

x;y

+I

x;y

A

x;y

.

We can also assume jI

x;y

j > 2jdj.Hence,for some integer n,

(69) nd 2 t

x;y

+

1

2

I

x;y

Now if S

n

is not too large,i.e.if jS

n

j <

1

4

jI

x;y

j,then S

n

+nd 2 A

x;y

.On the

other hand,if n satises (69) then nd

2jxj

jjjxyj

:Thanks to (68),it follows that

jS

n

j <

1

4

jI

x;y

j whenever jS

n

j < C

p

n,with C

=

1

p

jdj=2

2

:Taking a smaller is

necessary,we obtain the desired conclusion.

46 E.BREUILLARD

Now the proof of the proposition follows easily from the central limit theorem:if

"> 0 one can nd C > 0 so that for all large enough n;P(jS

n

j < C

p

n) 1 ".

Hence;

(70) limsupP(u(S

n

+nd)x

0

=2 K) = 1

We thus have established (67) for x = x

0

and as soon as the slope of the line

D xed by U satises the diophantine condition dened above.If E is the set of

g 2 G such that (67) holds for x = g

1

x

0

;then E contains the set of g 2 G such that

the slope of gD is irrational and well approximable on both sides.The map from G

to R which sends g to the slope of gD has no critical points (it identies with left

translation on G=P).Hence E is a set of full Haar measure,and this ends the proof

of the proposition.

Remark 10.6.The same idea shows that a non-centered random walk can stay very

close to a closed orbit of U at arbitrary large time and with high probability.In

any case,it prevents the walk to equidistribute in the closure of the orbit.A similar

phenomenon arises as soon as the action of U on G= is not uniquely ergodic,i.e.

if U stabilizes a proper homogeneous subspace of G=.

10.2.2.Proof of Theorem 1.5.Making use of the equidistribution theorem above

(Theorem 10.3),N.Shah (cf.[Sha]) subsequently extended this result to the action

of an arbitrary simply connected Ad-unipotent subgroup U of G.Let us introduce

N.Shah's result.

Let U be any simply connected nilpotent Lie group and (v

1

;:::;v

k

) be a basis of

the Lie algebra u of U.This basis is called a triangular basis (or strong Malcev

basis) if the subspaces spanned by (v

i

;:::;v

k

) for any i are ideals of u,that is [v

i

;v

j

] 2

span(v

m

;:::;v

k

) where m = maxfi;jg + 1.Such a basis gives rise to polynomial

coordinates on U,i.e.the map

:R

k

!U

(t

1

;:::;t

k

) 7!exp(t

k

v

k

) ::: exp(t

1

v

1

)

is polynomial dieomorphism.It also sends the Lebesgue measure on R

k

to the Haar

measure on U.With this terminology,Shah proved (cf.[Sha] Cor.1.3.)

Theorem 10.7 (N.Shah).Suppose G is a Lie group and a lattice in G.Let U be

a simply connected Ad-unipotent subgroup of G.Let (v

1

;:::;v

k

) be a triangular basis

for U,and x 2 G=.Then for any continuous and bounded function f on G=,

lim

T

1

!1;:::;T

k

!1

1

T

1

:::T

k

Z

[0;T

1

]:::[0;T

k

]

f((t

1

;:::;t

k

)x)dt

1

:::dt

k

=

Z

Hx

fdm

x

where m

x

is the H-invariant probability measure on

Ux = Hx.

This theorem is precisely what we need to apply Corollary 1.4 to the situation

of Theorem 1.5.Keeping the notations of the statement of Theorem 1.5,let f be

LOCAL LIMIT THEOREMS AND EQUIDISTRIBUTION ON THE HEISENBERG GROUP 47

a compactly supported function on G=,and suppose that U is isomorphic to the

Heisenberg group with triangular basis given by (3) in section 2.Then the function

F(u) = f(ux) = f((u

x

;u

y

;u

z

)x) is a bounded uniformly continuous function for

the left uniform structure on U satisfying the condition of Corollary 1.4 with limit

`=

R

Hx

fdm

x

.Therefore this is the end of the proof and of this paper.

Acknowledgments 10.8.This work is part of the author's Yale University Ph.D.

thesis.I sincerely thank my supervisor Gregory Margulis for suggesting this problem

and oering his invaluable help and guidance throughout the last few years.I am

also very grateful to Yves Guivarc'h for our long discussions and to Martine Babillot

whose inspiring enthusiasm I now miss dearly.

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Emmanuel Breuillard,IHES,Le Bois-Marie,35 route de Chartres,F-91440 Bures-

sur-Yvette,FRANCE

E-mail address:emmanuel.breuillard@ihes.fr

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