Natural number multiplicity

This file contains lemmas about the multiplicity function (the maximum prime power divding a number).

Main results

There are natural number versions of some basic lemmas about multiplicity.

There are also lemmas about the multiplicity of primes in factorials and in binomial coefficients.

theorem nat.multiplicity_eq_card_pow_dvd {m n b : ℕ} :

m ≠ 1 → 0 < n → n ≤ b → multiplicity m n = ↑((finset.filter (λ (i : ℕ), m ^ i ∣ n) (finset.Ico 1 b)).card)

The multiplicity of a divisor m of n, is the cardinality of the set of positive natural numbers i such that p ^ i divides n. The set is expressed by filtering Ico 1 b where b is any bound at least n

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theorem nat.prime.multiplicity_one {p : ℕ} :

nat.prime p → multiplicity p 1 = 0

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theorem nat.prime.multiplicity_mul {p m n : ℕ} :

nat.prime p → multiplicity p (m * n) = multiplicity p m + multiplicity p n

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theorem nat.prime.multiplicity_pow {p m n : ℕ} :

nat.prime p → multiplicity p (m ^ n) = n •ℕ multiplicity p m

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theorem nat.prime.multiplicity_self {p : ℕ} :

nat.prime p → multiplicity p p = 1

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theorem nat.prime.multiplicity_pow_self {p n : ℕ} :

nat.prime p → multiplicity p (p ^ n) = ↑n

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theorem nat.prime.multiplicity_fact {p : ℕ} (hp : nat.prime p) {n b : ℕ} :

n ≤ b → multiplicity p n.fact = ↑((finset.Ico 1 b).sum (λ (i : ℕ), n / p ^ i))

The multiplicity of a prime in fact n is the sum of the quotients n / p ^ i. This sum is expressed over the set Ico 1 b where b is any bound at least n

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theorem nat.prime.pow_dvd_fact_iff {p n r b : ℕ} :

nat.prime p → n ≤ b → (p ^ r ∣ n.fact ↔ r ≤ (finset.Ico 1 b).sum (λ (i : ℕ), n / p ^ i))

A prime power divides fact n iff it is at most the sum of the quotients n / p ^ i. This sum is expressed over the set Ico 1 b where b is any bound at least n

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theorem nat.prime.multiplicity_choose_aux {p n b k : ℕ} :

nat.prime p → k ≤ n → (finset.Ico 1 b).sum (λ (i : ℕ), n / p ^ i) = (finset.Ico 1 b).sum (λ (i : ℕ), k / p ^ i) + (finset.Ico 1 b).sum (λ (i : ℕ), (n - k) / p ^ i) + (finset.filter (λ (i : ℕ), p ^ i ≤ k % p ^ i + (n - k) % p ^ i) (finset.Ico 1 b)).card

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theorem nat.prime.multiplicity_choose {p n k b : ℕ} :

nat.prime p → k ≤ n → n ≤ b → multiplicity p (n.choose k) = ↑((finset.filter (λ (i : ℕ), p ^ i ≤ k % p ^ i + (n - k) % p ^ i) (finset.Ico 1 b)).card)

The multiplity of p in choose n k is the number of carries when k and n - k are added in base p. The set is expressed by filtering Ico 1 b where b is any bound at least n.

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