f

#### help with problems on computability needed!!

```Hi all, I was wondering if anyone would be able to help me with the
following problems.  I'm really bad at these... that's why I have to
retake this course on computational theory and complexity... I could
use any help I can get!

First question:
Consider a table whose rows are labelled with TMs, whose columns are
labelled with TM encodings, and where the entry at (Mi, <Mj>) contains
a 1 if Mi accepts <Mj> in at most |<Mj>|+t steps, and a 0 otherwise.
Use a diagonalization argument to show that for every t, there exists a
language L which is decidable in general, but not decidable with lag t.
We say that a language L is decidable with lag t if there exists a TM M
deciding L, which also satifies the requirement that on every input w,
M halts in at most |w| + t steps.

This second question is more tricky, I think:
Let M1,M2, . . . ,Mi, . . . be the list of all TMs in lexicographical
order.
For every positive integer k, define f(k) to be the index in the above
list of the k-th TM M such that L(M) = NULL. Formally,

f(k) = max{i : there exists J, which is a subset of {1 . . . i - 1}
such that (|J| = k - 1 and for all j belonging to {1 . . . i - 1},
L(Mj) = NULL iff j belongs to J)}

Notice that f is well defined for every k, since there are infinitely
many TMs M with L(M) = NULL. For example, if L(M2) = L(M5) = NULL and
L(M1),L(M3),L(M4) are not empty, then f(1) = 2 and f(2) = 5.
Prove that f is not computable.

I hope these aren't too difficult, although they are making me scratch
my head!  Thanks for all your help, guys!!!

```
 0
ahtwong (2)
6/16/2005 3:12:34 AM
comp.theory 5139 articles. 1 followers. marty.musatov (1143) is leader.

1 Replies
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```Hey, that's quite a mouthful. But you're registered to a course, right?
So you are *entitled* for help from the most qualified person - your
prof.
There are too many students who are shy of approaching their
professors,
who can actually turn out to be quite willing to help. Try it!
There is hardly another way to overcome the difficulties so common with
the theory courses.

```
 0
sixholes (1)
6/17/2005 10:13:21 PM

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