Computing n! modulo pᵏ for small p

Revision en9, by adamant, 2023-05-23 03:14:42

Hi everyone!

There is an article on cp-algorithms about how to compute $$$n!$$$ modulo prime number $$$p$$$. Today I was wondering, how to do it if we need to compute modulo $$$p^k$$$ rather than just $$$p$$$. Turns out one can do it efficiently if $$$p$$$ is small.

Motivational example

Xiaoxu Guo Contest 3 — Binomial Coefficient. Given $$$1 \leq n,k \leq 10^{18}$$$, find $$$\binom{n}{k}$$$ modulo $$$2^{32}$$$.

There are also some projecteuler problems that require it, including with several queries of distinct $$$n$$$ and $$$k$$$.

Task formulation and outline of result

To clarify the task a bit, our ultimate goal here is to be able to compute e.g. binomial coefficients modulo $$$p^k$$$. Thus, what we really need is to represent $$$n! = p^t a$$$, where $$$\gcd(a, p)=1$$$, and then report $$$t$$$ and $$$a$$$ modulo $$$p^k$$$. This is sufficient to also compute $$$\binom{n}{r}$$$ modulo $$$p^k$$$.

We will show that, assuming that polynomial multiplication of size $$$n$$$ requires $$$O(n \log n)$$$ operations, and assuming that arithmetic operations modulo $$$p^k$$$ take $$$O(1)$$$, we can find $$$t$$$ and $$$a$$$ in $$$O(d^2 + dk\log k)$$$, where $$$d=\log_p n$$$. It requires $$$O(pk\log^2 k)$$$ pre-computation that takes $$$O(pk \log k)$$$ memory.

Algorithm with polynomials

Great thanks to Endagorion for sharing this algorithm with me!

Let $$$k=\lfloor \frac{n}{p} \rfloor$$$, then we can represent $$$n!$$$ as

$$$ n! = p^k k! \prod\limits_{\substack{1 \leq j \leq n \\ \gcd(j, n)=1}}j. $$$

So, there is a part contributed by numbers divisible by $$$p$$$, which can be reduced to computing $$$k!$$$, and a part contributed by numbers not divisible by $$$p$$$. Let $$$n = a_0 + a_1 p + \dots + a_d p^d$$$, then the later can be represented as follows:

$$$ \prod\limits_{\substack{1 \leq j \leq n \\ \gcd(j, n)=1}}j = \prod\limits_{i=0}^d \prod\limits_{\substack{1 \leq j \leq a_i p^i \\\gcd(j, p)=1}} \left(\left\lfloor\frac{n}{p^{i+1}} \right\rfloor p^{i+1}+j\right). $$$

To compute it quickly, we can define a family of polynomial $$$P_{i,a}(x)$$$ for $$$0 \leq a \leq p$$$ such that

$$$ P_{i, a}(x) = \prod\limits_{\substack{1 \leq j\leq a p^{i-1} \\ \gcd(j, p)=1}}(xp^{i}+j), $$$

so the value of the factorial would be represented as

$$$ n! = p^k k! \prod\limits_{i=0}^d P_{i+1,a_i}\left(\left\lfloor\frac{n}{p^{i+1}} \right\rfloor\right), $$$

and expanding $$$k!$$$ it then rewrites into

$$$ n! = \prod\limits_{i=0}^{d} p^{\left\lfloor \frac{n}{p^{i+1}} \right\rfloor} \prod\limits_{j=0}^{d-i} P_{j+1,a_{i+j}}\left(\left\lfloor\frac{n}{p^{i+j+1}} \right\rfloor\right), $$$

which simplifies as

$$$ \boxed{n! = \prod\limits_{i=0}^{d} p^{\left\lfloor \frac{n}{p^{i+1}} \right\rfloor} \prod\limits_{j=0}^{i} P_{j+1,a_{i}}\left(\left\lfloor\frac{n}{p^{i+1}} \right\rfloor\right)} $$$

Now, what would it take us to use this setup? First of all, notice that $$$P_{i,a}(x)$$$ can be computed from one another:

$$$ P_{i,a}(x) = \prod\limits_{\substack{1 \leq j\leq a p^{i-1} \\ \gcd(j, p)=1}}(xp^{i}+j) = \prod\limits_{t=0}^{a-1}\prod\limits_{\substack{1 \leq j\leq p^{i-1} \\ \gcd(j, p)=1}}(xp^{i}+tp^{i-1}+j)=\prod\limits_{t=0}^{a-1} P_{i-1,p}(px+t). $$$

Note that for shorter and more consistent implementation, this recurrent formula also mostly works for $$$i=1$$$ if we put $$$P_{0, p}(x) = x+1$$$, but for $$$P_{1,p}$$$ we should go up to $$$p-2$$$ instead of $$$p-1$$$. We should also note that for $$$P_{i,a}(x)$$$, we only care for coefficients up to $$$x^{\lfloor x/i \rfloor}$$$, as the larger ones are divisible by $$$p^k$$$.

This allows to compute $$$P_{i,a}(x)$$$ in $$$O(\frac{pk \log k}{i})$$$ for all $$$a$$$ for a given $$$i$$$. Over all $$$i$$$ from $$$1$$$ to $$$k$$$ it sums up to $$$O(pk \log^2 k)$$$ time and $$$O(p k \log k)$$$ memory. Then, evaluating all the polynomials for any specific $$$a$$$ requires $$$O(d^2 + dk \log k)$$$ operations, where $$$d = \log_p n$$$.

As it requires some manipulations with polynomials, I implemented it in Python with sympy just as a proof of concept:

Code

Algorithm with $$$p$$$-adic logarithms and exponents

The algorithm above requires some heavy machinery on polynomials. I also found out an alternative approach that allows to compute $$$t$$$ and $$$a$$$ for any given $$$n$$$ divisible by $$$p$$$ in $$$O(pk)$$$. It doesn't rely on polynomial operations, except for Lagrange interpolation.

Let $$$n=pt+b$$$, where $$$0 \leq b < p$$$, then we can represent its factorial as

$$$ n! = p^t t! \prod\limits_{i=1}^{b} \prod\limits_{j=0}^{t} \left(pj+i\right)\prod\limits_{i=b+1}^{p-1} \prod\limits_{j=0}^{t-1} \left(pj+i\right). $$$

We can further rewrite it as

$$$ n! = p^t t! (p-1)!^t b! \prod\limits_{i=1}^{b} \prod\limits_{j=0}^{t} \left(1+\frac{j}{i}p\right)\prod\limits_{i=b+1}^{p-1} \prod\limits_{j=0}^{t-1} \left(1+\frac{j}{i}p\right) $$$

Let's learn how to compute the product

$$$ A(b, t) = \prod\limits_{i=1}^b \prod\limits_{j=0}^t \left(1+\frac{j}{i}p\right), $$$

as with it we can represent the factorial as

$$$ n! = p^t t! (p-1)!^t b! A(b, t)\frac{A(p-1,t-1)}{A(b,t-1)}. $$$

We can take a $$$p$$$-adic logarithm (please refer to this article for definitions and properties) from the product to get

$$$ \boxed{\log A(b, t) = \sum\limits_{i=1}^{b} \sum\limits_{j=0}^{t} \log\left(1+\frac{j}{i}p\right)=\sum\limits_{z=1}^\infty \frac{(-1)^{z+1}p^z}{z} \sum\limits_{i=1}^{b} i^{-z} \sum\limits_{j=0}^{t} j^z} $$$

We can precompute sums of $$$i^{-z}$$$ for each $$$z$$$ up to $$$k$$$ and $$$b$$$ up to $$$p-1$$$ in $$$O(pk)$$$, and we can find the sum of $$$j^z$$$ for $$$j$$$ from $$$0$$$ to $$$t$$$ in $$$O(z)$$$ via Lagrange interpolation. Therefore, with $$$O(pk)$$$ precomputation, we can compute $$$\log A(b, t)$$$ in $$$O(k^2)$$$, which then allows to find $$$n!$$$ in $$$O(dk^2)$$$, where $$$d = \log_p n$$$. I implemented it in Python, as a proof of concept, but I didn't bother with faster sums over $$$i$$$ and $$$j$$$:

Code

Note: Due to inherent properties of $$$p$$$-adic logarithms and exponents, the algorithm only works with $$$p>2$$$, and it might return $$$-n!$$$ instead with $$$p=2$$$. This is because $$$\exp \log z = -z$$$ with $$$p=2$$$ when $$$z$$$ has remainder $$$3$$$ modulo $$$4$$$. It is accounted for in the code by tracking the number of integers below $$$n$$$ that have remainder $$$3$$$ modulo $$$4$$$.

Some other approaches

There is also a blog by prabowo, based apparently on another blog by Min_25, and a scholarly article by Andrew Granville. I'm not sure whether they talk about the same algorithms as described here.

Tags tutorial, p-adic numbers, factorial

History

 
 
 
 
Revisions
 
 
  Rev. Lang. By When Δ Comment
en27 English adamant 2023-05-31 14:22:05 2 Tiny change: '\ \gcd(j, n)=1}}j = \' -> '\ \gcd(j, p)=1}}j = \'
en26 English adamant 2023-05-31 14:21:17 2 Tiny change: '\ \gcd(j, n)=1}}j.\n$' -> '\ \gcd(j, p)=1}}j.\n$'
en25 English adamant 2023-05-31 14:10:41 60
en24 English adamant 2023-05-31 14:10:03 531
en23 English adamant 2023-05-31 14:04:45 256
en22 English adamant 2023-05-31 13:58:15 2269
en21 English adamant 2023-05-31 02:45:28 41
en20 English adamant 2023-05-31 02:29:49 308
en19 English adamant 2023-05-31 02:26:05 308
en18 English adamant 2023-05-31 02:20:22 1037 Tiny change: 'lgorithm w' -> 'lgorithm when $p^k$ is also small\n\n\n\n#### Algorithm w'
en17 English adamant 2023-05-23 18:48:21 57
en16 English adamant 2023-05-23 17:46:45 41
en15 English adamant 2023-05-23 17:45:16 42
en14 English adamant 2023-05-23 17:44:45 1268
en13 English adamant 2023-05-23 03:22:28 22 Tiny change: 'omputation. It doesn' -> 'omputation, where $d = \log_p n$. It doesn'
en12 English adamant 2023-05-23 03:21:59 30 Tiny change: '$p$ in $O(pk)$. It doesn' -> '$p$ in $O(dk^2)$ with $O(pk)$ precomputation. It doesn'
en11 English adamant 2023-05-23 03:20:54 216
en10 English adamant 2023-05-23 03:19:19 175
en9 English adamant 2023-05-23 03:14:42 10
en8 English adamant 2023-05-23 03:07:15 184
en7 English adamant 2023-05-23 02:57:31 5 Tiny change: 'k, a = fct_test(t)\n ' -> 'k, a = fct(t)\n '
en6 English adamant 2023-05-23 00:40:56 153
en5 English adamant 2023-05-23 00:04:31 106
en4 English adamant 2023-05-22 23:23:37 625
en3 English adamant 2023-05-22 23:23:03 62
en2 English adamant 2023-05-22 23:22:24 339
en1 English adamant 2023-05-22 23:16:26 7960 Initial revision (published)