Mid0's blog

nCr % prime power

By Mid0, history, 9 years ago, In English

i'm trying to calculate nCr mod prime power , i found this comment which is really helpful but the formula as i understand doesn't apply to some numbers and i don't know how it works , could anyone explain it in more details ?

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9 years ago, # |
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Lucas's theorem. WIKI

int calc(int n, int k, int mod) {
  int res = 1;
  while (n || k) {
    res = (res * c[n % mod][k % mod]) % mod;
    n /= mod;
    k /= mod;
  }
  return res;
}
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    9 years ago, # ^ |
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    this works well for primes , but it doesn't work for prime power . for example try 28 choose 3 mod 27 it gives zero while the correct answer is nine .

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    9 years ago, # ^ |
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    Lucas' theorem stated that p must be a prime.
    The OP said he needed nCr modulo a prime power (pSomeNumber).

    I'm not sure whether Lucas' theorem still holds.

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9 years ago, # |
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Once I've coded it in Python, maybe it'll help. (See nCk_mod_prime_power function)

Based on this paper by Andrew Granville.

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