Invertibility of factorials

This file contains lemmas providing sufficient conditions for the cast of n! to a (semi)ring A to be a unit.

theorem IsUnit.natCast_factorial_of_le {A : Type u_1} [Semiring A] {n : ℕ} (hn_fac : IsUnit ↑n.factorial) {m : ℕ} (hmn : m ≤ n) : IsUnit ↑m.factorial

theorem IsUnit.natCast_factorial_of_lt {A : Type u_1} [Semiring A] {n : ℕ} (hn_fac : IsUnit ↑(n - 1).factorial) {m : ℕ} (hmn : m < n) : IsUnit ↑m.factorial

theorem IsUnit.natCast_factorial_of_algebra {A : Type u_1} [Semiring A] (K : Type u_2) [Semifield K] [CharZero K] [Algebra K A] (n : ℕ) : IsUnit ↑n.factorial

If A is an algebra over a characteristic-zero (semi)field, then n! is a unit.

theorem IsUnit.natCast_factorial_iff_of_charP {A : Type u_1} [Ring A] (p : ℕ) [Fact (Nat.Prime p)] [CharP A p] {n : ℕ} : IsUnit ↑n.factorial ↔ n < p