## Thursday, November 30, 2017

### Kolmogorov Complexity and the Primes

Bill's post on how to derive the non-finiteness of the primes from Van der Waerden's theorem reminds me of a nice proof using Kolmogorov complexity.

A quick primer: Fixed some universal programming language. Let C(x), the Kolmogorov complexity of x, be the length of the smallest program that outputs x. One can show by a simple counting argument for every n there is an x such that C(x) ≥ n. We call such x "random".

Suppose we had a finite list of primes p1…pk. Then any number m can be expressed as p1e1···pkek. Pick n large, a random x of length n and let m be the number x expresses in binary. We can compute m from e1,…,ek and a constant amount of other information, remembering that k is a constant. Each ei is at most log m and so we can describe all of them in O(log log m) bits and thus C(m) = O(log log m). But roughly C(m) = C(x)  ≥  n = log m, a contradiction.

But we can do better. Again pick n large, a random x of length n and let m be the number x expresses in binary. Let pi be the largest prime that divides m where pi is the ith prime. We can describe m by pi and m/pi, or by i and m/pi. So we have C(m) ≤ C(i,m/pi) ≤ C(i) + C(m/pi) + 2 log C(pi) ≤ log i + log m/pi + 2 log log i + c. The 2 log C(pi) term is needed to specify the separation between the program for i and the program for m/pi.

Since C(m) ≥ log m, we have
log m ≤ log i + log (m/pi)+ 2 log log i + c
log m ≤ log i + log m - log pi + 2 log log i + c
log pi ≤ log i + 2 log log i + c
pi ≤ O(i (log i)2)

The prime number theorem has pi approximately i log i, so we get just a log factor off from optimal with simple Kolmogorov complexity.

I wrote a short introduction to Kolmogorov complexity with this proof. I originally got the proof from the great text on Kolmogorov complexity from Li and Vitányi and they give credit to Piotr Berman and John Tromp.

1. It should be mentioned that the bound can be improved by a better encoding, like first writing down the length of C(pi), so we get log i + log m/pi + log log i + 2 log log log i + c, and I think pi<=O(i log i (log log i)^2) follows. Of course this can be further improved.

2. I like the first simpler proof better because Chebyshev used something almost as simple as the second proof that gets within a constant factor of the right density. The idea is to look at the prime factorization of N=(2n choose n) = (2n)!/(n!)^2.

Consider any prime p and the exponent of p in m!. We get [m/p] terms divisible by p and [m/p^2] divisible by p^2, etc so the exponent of p in m! is just
[m/p]+[m/p^2]+[m/p^3]+... where [ ] is the floor function and we can go on forever since the terms become 0 eventually.

Therefore the exponent of p dividing (2n choose n) is precisely
[2n/p]+[2n/p^2}+[2n/[p^3] +...
- 2[n/p]-2[n/p^2}- 2[n/p^3] -...
= [2n/p]-2[n/p] + [2n/p^2]-2[n/p^2] +...+ [2n/p^k]-2[n/p^k]
where k is the largest power of p smaller than 2n.
Now for any number m, 2[n/m] > 2(n/m -1) = 2n/m - 2 so
[2n/m]-2[n/m] = 1 since it is <= 2n/m - 2[n/m] < 2.
Therefore the exponent of p in (2n choose n) is at most k and hence it contributes a factor at most p^k <= 2n to
N=(2n choose n).

Trivially N=(2n)!/(n!)^2 is at least 2^n. (I want to keep this elementary so I am willing to lose the near factor of 2 in the exponent.) Each prime dividing N must be at most 2n and its contribution to the product N is at most 2n by the above argument. Therefore the # of primes smaller than 2n must be at least log_{2n} N > n/ log_2 (2n) which gives the right asymptotics.

3. As said, this shows only the inequality for p_i that are factors in random numbers. The previous argument shows that this is true for infinitely many i, but to get this for all large i something else is needed (as Alexey Milovanov, if I remember correctly, pointed out)