20
$\begingroup$

Can one compute $(p-1)!$ modulo $p^2$ in time polynomial in $\log p$? I can do it modulo $p$! (The last one is an exclamation point, not a factorial.)

More generally, I would like to be able to compute $d!$ modulo $p^N$ in time polynomial in $N \log p$.

$\endgroup$
3
  • $\begingroup$ How do you compute $(p-1)!$ modulo $p$ in time $\log p$? $\endgroup$ Mar 25, 2011 at 19:53
  • 3
    $\begingroup$ For $(p-1)!$ modulo $p$: Wilson's theorem and Primes is in P? $\endgroup$ Mar 25, 2011 at 20:11
  • $\begingroup$ May be relevant. $\endgroup$
    – metamorphy
    Dec 24, 2018 at 22:02

2 Answers 2

12
$\begingroup$

I believe that the question whether $(p-1)! \bmod p^2$ is computable in polynomial time is still open. The best I am aware of is $O(p^{1/2+\epsilon})$, but I am not a specialist on this issue.

The computation of $(p-1)! \bmod p^2$ plays a role in certain p-adic algorithms to compute zeta functions of hypersurfaces in $\mathbb{P}^n$. There is an algorithm of David Harvey for zeta functions of hypersurfaces, where your question plays a role. I.e., the fact that this is not known that $(p-1)! \bmod p^2$ can be computed in less than $O(p^{1/2+\epsilon})$ is an obstruction to improve the complexity of this algorithm. (See the slides "Computing zeta functions of projective surfaces in large characteristic" on http://www.cims.nyu.edu/~harvey/.)

$\endgroup$
5
$\begingroup$

As you may know, a Wilson prime is a prime $p$ such that $(p-1)!\equiv-1\pmod{p^2}$. Only three are known (5, 13, and 563), despite searches going up to $5\times10^8$. Presumably, the people doing these searches have looked into your question, so maybe it's a good idea to go through the literature on Wilson primes. The Wikipedia article will get you started.

$\endgroup$
1

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.