# Ming Li Talk about Bioinformatics

Βιοτεχνολογία

2 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

83 εμφανίσεις

Lecture 19. Reduction: More
Undecidable problems

It turns out that many problems are undecidable.

In fact, many problems are even “harder” than being
“undecidable”. You can actually have an infinite
hierarchy of undecidable problems
---

each level
contains problems that are “more undecidable” than
those in the previous level.

Here is another undecidable problem

The Post
correspondence problem.

Input: list of pairs of strings (x
1
, y
1
), … ,(x
n
,y
n
)

Question: Does there exist a finite nonempty list of
indices i
1
, i
2
, ... , i
r

such that

x
i1

x
i2

... x
ir

= y
i1

y
i2

... y
ir

?

Example of the Post
correspondence problem

Input might be

(x
1
, y
1
) = (1, 111)

(x
2
, y
2
) = (10111, 10)

(x
3
, y
3
) = (10, 0)

and a corresponding solution is

(i
1
, i
2
, i
3
, i
4
) = (2,1,1,3)

which gives x
i1

x
i2

x
i3

x
i4

= y
i1

y
i2

y
i3

y
i4

= 101111110

Exercise: is the following instance solvable? If so, provide it:

(x
1
, y
1
) = (0, 1)

(x
2
, y
2
) = (1, 011)

(x
3
, y
3
) = (011, 0)

Emil Post proved in 1944 that the Post correspondence problem
is unsolvable.

Reductions: proving problems
unsolvable

If we know a problem A is unsolvable (e.g.
the Turing’s Halting problem), in order to
show another problem B to be unsolvable, we
“reduce” A to B via computable process. If
this can be done, then B is also unsolvable,
since otherwise we can always reduce A to B
and solve B, and then get solution for A, a
contradiction to the fact that A is unsolvable.

Examples of reductions.

Claim. “Does program P halt on all inputs?“ is unsolvable.

Proof. Suppose it is solvable. I.e. a program "alwayshalts" that
takes a program P as an input and returns "true" iff P halts on all
inputs. We show how to use "alwayshalts" to solve the original
halting problem. Given P and I, we create a new program as
follows:

Reduce();

/* simply run P on I

{ run P on I; }

Now call alwayshalts(Reduce()). Since no matter what the input
is to “Reduce”, it runs P on I, if alwayshalts(Reduce) returns
"true", then P halts on input I. If alwayshalts(Reduce) returns
"false", then P doesn't halt on input I. Thus calling
"alwayshalts(Reduce)“ solves the halting problem for program P
and input I. But we know that is unsolvable, a contradiction.
QED

One more example.

Claim. "does P index an array out of bounds on input I?“ is
unsolvable.

Proof. Suppose the problem is solvable. I.e. a program
"arrayindex" takes a program P and input I as input and returns
"true" iff P attempts to index an array out of bounds on input I.
Reduction:

void indextest(J:input);

x : array[1..1] of integer;

simulate P on input I, making sure P never indexes an

array out of bounds (by run
-
time checking)

if (P halts on I) x[2] := 1; /* array out of bound

Function "indextest" has been defined in such a way that it
indexes an array out of bounds if and only if P halts on I. We
have now reduced halting problem to “array
-
out
-
of
-
bound”.

Reductions

understand it better

We say
a computational problem A reduces to a problem B
if you can solve A given access to an algorithm to solve B
as a subroutine
.

Example: "primality testing" reduces to "compute prime
factorization". To see this, we need to see how to test n for
primality, given an algorithm to compute the prime factorization
of n. That's easy: we just compute the prime factorization of n
and see if it consists of a single prime.

Example: "compute the median of a list" reduces to "sort a list".
Again, this is easy: first, sort the list of n numbers; then look for
the element in position ceil(n/2).

Example: The problem of determining the median reduces to the
problem of computing the k
-
th smallest. Less obviously, the
problem of determining the k
-
th smallest reduces to the problem
of computing the median. (Why?)

Reduction continues

If problem A reduces to problem B, we say that "problem B is at
least as hard as problem A“ (note: not time complexity). This is
sensible, because knowing an algorithm for B allows us to solve
A, but not necessarily the other way around. We sometimes
write A ≤ B. This notation is very helpful, because the inequality
goes from the "easier" problem to the "harder" problem.

There are two facts about reductions:

if A reduces to B, and B is solvable, then A is solvable.

if A reduces to B, and A is unsolvable, then B is unsolvable.

It suffices to prove the first, because the second is just the
contrapositive. The proof is trivial: since B is solvable, there is
an algorithm to solve it. Now A is solvable using an algorithm for
B. So A is solvable
.

One more example

Claim. Program
-
equivalence problem ("given two
Java programs P1 and P2, do they have the same
input
-
output behavior on all inputs"?) is unsolvable.

Proof. We reduce the halting problem to it. To show
that
, we must figure out a way to solve the halting
problem given an algorithm for the program
-
equivalence problem. We let P1 be the program that
ignores its input and runs P on input i, returning "1" if
P halts on i (and not halting otherwise). We let P2 be
the program that ignores its input and returns "1".
Then P1 and P2 have the same input
-
output behavior
on all inputs if and only if P halts on i. So we have
reduced halting to the program
-
equivalence problem.
It follows that program
-
equivalence is unsolvable.

Foolproof virus testing is impossible

A computer virus is a piece of code that, when you
run it, it somehow causes the operating system of the
computer to be altered. Obviously, computer viruses
are undesirable, and millions of dollars are spent
every year on testing software for viruses and
removing them. We'd like to have a
perfect

virus
tester.

Unfortunately, such a thing cannot exist. Suppose
there were a perfect virus tester. Such a thing might
be a function

virusfree(P:program, I:input) that returns "false" iff
program P run on input I alters the operating system.
Of course the program "virusfree" must itself not be a
virus
---

not alter the operating system when it is run.

Two proofs

Proof 1 (by reducing the halting problem to it).
Given a Halting problem instance (M,x),
construct new program P such that P is a
virus iff M(x) halts:

M

x

P

Alter the OS

halt

Direct proof

Proof 2. Create a program S that on input W checks to
see if running W on W is safe according to
"virusfree". If it is safe, then S alters the operating
system. Otherwise, S does nothing.

What happens when we run S on input S? If running
S on S is safe, then virusfree(S, S) returns "true". But
then S alters the operating system, so running S on S
is not safe.

If running S on S is not safe, then virusfree(S, S)
returns "false". But then running S on S does nothing,
so virusfree(S, S) should have returned "true".

Neither answer is correct, so a perfect virus tester
cannot exist. QED