# Theory Factorial

theory Factorial
imports Groups_List
```(*  Title:      HOL/Factorial.thy
Author:     Jacques D. Fleuriot
Author:     Lawrence C Paulson
Author:     Chaitanya Mangla
Author:     Manuel Eberl
*)

section ‹Factorial Function, Rising Factorials›

theory Factorial
imports Groups_List
begin

subsection ‹Factorial Function›

context semiring_char_0
begin

definition fact :: "nat ⇒ 'a"
where fact_prod: "fact n = of_nat (∏{1..n})"

lemma fact_prod_Suc: "fact n = of_nat (prod Suc {0..<n})"
by (cases n)
atLeastLessThanSuc_atLeastAtMost)

lemma fact_prod_rev: "fact n = of_nat (∏i = 0..<n. n - i)"
using prod.atLeastAtMost_rev [of "λi. i" 1 n]
by (cases n)
atLeastLessThanSuc_atLeastAtMost)

lemma fact_0 [simp]: "fact 0 = 1"

lemma fact_1 [simp]: "fact 1 = 1"

lemma fact_Suc_0 [simp]: "fact (Suc 0) = 1"

lemma fact_Suc [simp]: "fact (Suc n) = of_nat (Suc n) * fact n"
by (simp add: fact_prod atLeastAtMostSuc_conv algebra_simps)

lemma fact_2 [simp]: "fact 2 = 2"

lemma fact_split: "k ≤ n ⟹ fact n = of_nat (prod Suc {n - k..<n}) * fact (n - k)"
by (simp add: fact_prod_Suc prod.union_disjoint [symmetric]
ivl_disj_un ac_simps of_nat_mult [symmetric])

end

lemma of_nat_fact [simp]: "of_nat (fact n) = fact n"

lemma of_int_fact [simp]: "of_int (fact n) = fact n"
by (simp only: fact_prod of_int_of_nat_eq)

lemma fact_reduce: "n > 0 ⟹ fact n = of_nat n * fact (n - 1)"
by (cases n) auto

lemma fact_nonzero [simp]: "fact n ≠ (0::'a::{semiring_char_0,semiring_no_zero_divisors})"
apply (induct n)
apply auto
using of_nat_eq_0_iff
apply fastforce
done

lemma fact_mono_nat: "m ≤ n ⟹ fact m ≤ (fact n :: nat)"
by (induct n) (auto simp: le_Suc_eq)

lemma fact_in_Nats: "fact n ∈ ℕ"
by (induct n) auto

lemma fact_in_Ints: "fact n ∈ ℤ"
by (induct n) auto

context
assumes "SORT_CONSTRAINT('a::linordered_semidom)"
begin

lemma fact_mono: "m ≤ n ⟹ fact m ≤ (fact n :: 'a)"
by (metis of_nat_fact of_nat_le_iff fact_mono_nat)

lemma fact_ge_1 [simp]: "fact n ≥ (1 :: 'a)"
by (metis le0 fact_0 fact_mono)

lemma fact_gt_zero [simp]: "fact n > (0 :: 'a)"
using fact_ge_1 less_le_trans zero_less_one by blast

lemma fact_ge_zero [simp]: "fact n ≥ (0 :: 'a)"

lemma fact_not_neg [simp]: "¬ fact n < (0 :: 'a)"

lemma fact_le_power: "fact n ≤ (of_nat (n^n) :: 'a)"
proof (induct n)
case 0
then show ?case by simp
next
case (Suc n)
then have *: "fact n ≤ (of_nat (Suc n ^ n) ::'a)"
by (rule order_trans) (simp add: power_mono del: of_nat_power)
have "fact (Suc n) = (of_nat (Suc n) * fact n ::'a)"
also have "… ≤ of_nat (Suc n) * of_nat (Suc n ^ n)"
by (simp add: * ordered_comm_semiring_class.comm_mult_left_mono del: of_nat_power)
also have "… ≤ of_nat (Suc n ^ Suc n)"
by (metis of_nat_mult order_refl power_Suc)
finally show ?case .
qed

end

lemma fact_less_mono_nat: "0 < m ⟹ m < n ⟹ fact m < (fact n :: nat)"
by (induct n) (auto simp: less_Suc_eq)

lemma fact_less_mono: "0 < m ⟹ m < n ⟹ fact m < (fact n :: 'a::linordered_semidom)"
by (metis of_nat_fact of_nat_less_iff fact_less_mono_nat)

lemma fact_ge_Suc_0_nat [simp]: "fact n ≥ Suc 0"
by (metis One_nat_def fact_ge_1)

lemma dvd_fact: "1 ≤ m ⟹ m ≤ n ⟹ m dvd fact n"
by (induct n) (auto simp: dvdI le_Suc_eq)

lemma fact_ge_self: "fact n ≥ n"
by (cases "n = 0") (simp_all add: dvd_imp_le dvd_fact)

lemma fact_dvd: "n ≤ m ⟹ fact n dvd (fact m :: 'a::linordered_semidom)"
by (induct m) (auto simp: le_Suc_eq)

lemma fact_mod: "m ≤ n ⟹ fact n mod (fact m :: 'a::{semidom_modulo, linordered_semidom}) = 0"

lemma fact_div_fact:
assumes "m ≥ n"
shows "fact m div fact n = ∏{n + 1..m}"
proof -
obtain d where "d = m - n"
by auto
with assms have "m = n + d"
by auto
have "fact (n + d) div (fact n) = ∏{n + 1..n + d}"
proof (induct d)
case 0
show ?case by simp
next
case (Suc d')
have "fact (n + Suc d') div fact n = Suc (n + d') * fact (n + d') div fact n"
by simp
also from Suc.hyps have "… = Suc (n + d') * ∏{n + 1..n + d'}"
unfolding div_mult1_eq[of _ "fact (n + d')"] by (simp add: fact_mod)
also have "… = ∏{n + 1..n + Suc d'}"
finally show ?case .
qed
with ‹m = n + d› show ?thesis by simp
qed

lemma fact_num_eq_if: "fact m = (if m = 0 then 1 else of_nat m * fact (m - 1))"
by (cases m) auto

lemma fact_div_fact_le_pow:
assumes "r ≤ n"
shows "fact n div fact (n - r) ≤ n ^ r"
proof -
have "r ≤ n ⟹ ∏{n - r..n} = (n - r) * ∏{Suc (n - r)..n}" for r
by (subst prod.insert[symmetric]) (auto simp: atLeastAtMost_insertL)
with assms show ?thesis
by (induct r rule: nat.induct) (auto simp add: fact_div_fact Suc_diff_Suc mult_le_mono)
qed

lemma fact_numeral: "fact (numeral k) = numeral k * fact (pred_numeral k)"
― ‹Evaluation for specific numerals›
by (metis fact_Suc numeral_eq_Suc of_nat_numeral)

subsection ‹Pochhammer's symbol: generalized rising factorial›

text ‹See 🌐‹https://en.wikipedia.org/wiki/Pochhammer_symbol›.›

context comm_semiring_1
begin

definition pochhammer :: "'a ⇒ nat ⇒ 'a"
where pochhammer_prod: "pochhammer a n = prod (λi. a + of_nat i) {0..<n}"

lemma pochhammer_prod_rev: "pochhammer a n = prod (λi. a + of_nat (n - i)) {1..n}"
using prod.atLeastLessThan_rev_at_least_Suc_atMost [of "λi. a + of_nat i" 0 n]

lemma pochhammer_Suc_prod: "pochhammer a (Suc n) = prod (λi. a + of_nat i) {0..n}"

lemma pochhammer_Suc_prod_rev: "pochhammer a (Suc n) = prod (λi. a + of_nat (n - i)) {0..n}"

lemma pochhammer_0 [simp]: "pochhammer a 0 = 1"

lemma pochhammer_1 [simp]: "pochhammer a 1 = a"

lemma pochhammer_Suc0 [simp]: "pochhammer a (Suc 0) = a"

lemma pochhammer_Suc: "pochhammer a (Suc n) = pochhammer a n * (a + of_nat n)"
by (simp add: pochhammer_prod atLeast0_lessThan_Suc ac_simps)

end

lemma pochhammer_nonneg:
fixes x :: "'a :: linordered_semidom"
shows "x > 0 ⟹ pochhammer x n ≥ 0"
by (induction n) (auto simp: pochhammer_Suc intro!: mult_nonneg_nonneg add_nonneg_nonneg)

lemma pochhammer_pos:
fixes x :: "'a :: linordered_semidom"
shows "x > 0 ⟹ pochhammer x n > 0"
by (induction n) (auto simp: pochhammer_Suc intro!: mult_pos_pos add_pos_nonneg)

lemma pochhammer_of_nat: "pochhammer (of_nat x) n = of_nat (pochhammer x n)"

lemma pochhammer_of_int: "pochhammer (of_int x) n = of_int (pochhammer x n)"

lemma pochhammer_rec: "pochhammer a (Suc n) = a * pochhammer (a + 1) n"
by (simp add: pochhammer_prod prod.atLeast0_lessThan_Suc_shift ac_simps)

lemma pochhammer_rec': "pochhammer z (Suc n) = (z + of_nat n) * pochhammer z n"
by (simp add: pochhammer_prod prod.atLeast0_lessThan_Suc ac_simps)

lemma pochhammer_fact: "fact n = pochhammer 1 n"

lemma pochhammer_of_nat_eq_0_lemma: "k > n ⟹ pochhammer (- (of_nat n :: 'a:: idom)) k = 0"

lemma pochhammer_of_nat_eq_0_lemma':
assumes kn: "k ≤ n"
shows "pochhammer (- (of_nat n :: 'a::{idom,ring_char_0})) k ≠ 0"
proof (cases k)
case 0
then show ?thesis by simp
next
case (Suc h)
then show ?thesis
using Suc kn
done
qed

lemma pochhammer_of_nat_eq_0_iff:
"pochhammer (- (of_nat n :: 'a::{idom,ring_char_0})) k = 0 ⟷ k > n"
(is "?l = ?r")
using pochhammer_of_nat_eq_0_lemma[of n k, where ?'a='a]
pochhammer_of_nat_eq_0_lemma'[of k n, where ?'a = 'a]

lemma pochhammer_0_left:
"pochhammer 0 n = (if n = 0 then 1 else 0)"
by (induction n) (simp_all add: pochhammer_rec)

lemma pochhammer_eq_0_iff: "pochhammer a n = (0::'a::field_char_0) ⟷ (∃k < n. a = - of_nat k)"

lemma pochhammer_eq_0_mono:
"pochhammer a n = (0::'a::field_char_0) ⟹ m ≥ n ⟹ pochhammer a m = 0"
unfolding pochhammer_eq_0_iff by auto

lemma pochhammer_neq_0_mono:
"pochhammer a m ≠ (0::'a::field_char_0) ⟹ m ≥ n ⟹ pochhammer a n ≠ 0"
unfolding pochhammer_eq_0_iff by auto

lemma pochhammer_minus:
"pochhammer (- b) k = ((- 1) ^ k :: 'a::comm_ring_1) * pochhammer (b - of_nat k + 1) k"
proof (cases k)
case 0
then show ?thesis by simp
next
case (Suc h)
have eq: "((- 1) ^ Suc h :: 'a) = (∏i = 0..h. - 1)"
using prod_constant [where A="{0.. h}" and y="- 1 :: 'a"]
by auto
with Suc show ?thesis
using pochhammer_Suc_prod_rev [of "b - of_nat k + 1"]
by (auto simp add: pochhammer_Suc_prod prod.distrib [symmetric] eq of_nat_diff simp del: prod_constant)
qed

lemma pochhammer_minus':
"pochhammer (b - of_nat k + 1) k = ((- 1) ^ k :: 'a::comm_ring_1) * pochhammer (- b) k"

lemma pochhammer_same: "pochhammer (- of_nat n) n =
((- 1) ^ n :: 'a::{semiring_char_0,comm_ring_1,semiring_no_zero_divisors}) * fact n"
unfolding pochhammer_minus

lemma pochhammer_product': "pochhammer z (n + m) = pochhammer z n * pochhammer (z + of_nat n) m"
proof (induct n arbitrary: z)
case 0
then show ?case by simp
next
case (Suc n z)
have "pochhammer z (Suc n) * pochhammer (z + of_nat (Suc n)) m =
z * (pochhammer (z + 1) n * pochhammer (z + 1 + of_nat n) m)"
also note Suc[symmetric]
also have "z * pochhammer (z + 1) (n + m) = pochhammer z (Suc (n + m))"
by (subst pochhammer_rec) simp
finally show ?case
by simp
qed

lemma pochhammer_product:
"m ≤ n ⟹ pochhammer z n = pochhammer z m * pochhammer (z + of_nat m) (n - m)"
using pochhammer_product'[of z m "n - m"] by simp

lemma pochhammer_times_pochhammer_half:
fixes z :: "'a::field_char_0"
shows "pochhammer z (Suc n) * pochhammer (z + 1/2) (Suc n) = (∏k=0..2*n+1. z + of_nat k / 2)"
proof (induct n)
case 0
then show ?case
next
case (Suc n)
define n' where "n' = Suc n"
have "pochhammer z (Suc n') * pochhammer (z + 1 / 2) (Suc n') =
(pochhammer z n' * pochhammer (z + 1 / 2) n') * ((z + of_nat n') * (z + 1/2 + of_nat n'))"
(is "_ = _ * ?A")
also have "?A = (z + of_nat (Suc (2 * n + 1)) / 2) * (z + of_nat (Suc (Suc (2 * n + 1))) / 2)"
(is "_ = ?B")
also note Suc[folded n'_def]
also have "(∏k=0..2 * n + 1. z + of_nat k / 2) * ?B = (∏k=0..2 * Suc n + 1. z + of_nat k / 2)"
finally show ?case
qed

lemma pochhammer_double:
fixes z :: "'a::field_char_0"
shows "pochhammer (2 * z) (2 * n) = of_nat (2^(2*n)) * pochhammer z n * pochhammer (z+1/2) n"
proof (induct n)
case 0
then show ?case by simp
next
case (Suc n)
have "pochhammer (2 * z) (2 * (Suc n)) = pochhammer (2 * z) (2 * n) *
(2 * (z + of_nat n)) * (2 * (z + of_nat n) + 1)"
also note Suc
also have "of_nat (2 ^ (2 * n)) * pochhammer z n * pochhammer (z + 1/2) n *
(2 * (z + of_nat n)) * (2 * (z + of_nat n) + 1) =
of_nat (2 ^ (2 * (Suc n))) * pochhammer z (Suc n) * pochhammer (z + 1/2) (Suc n)"
finally show ?case .
qed

lemma fact_double:
"fact (2 * n) = (2 ^ (2 * n) * pochhammer (1 / 2) n * fact n :: 'a::field_char_0)"
using pochhammer_double[of "1/2::'a" n] by (simp add: pochhammer_fact)

lemma pochhammer_absorb_comp: "(r - of_nat k) * pochhammer (- r) k = r * pochhammer (-r + 1) k"
(is "?lhs = ?rhs")
for r :: "'a::comm_ring_1"
proof -
have "?lhs = - pochhammer (- r) (Suc k)"
by (subst pochhammer_rec') (simp add: algebra_simps)
also have "… = ?rhs"
by (subst pochhammer_rec) simp
finally show ?thesis .
qed

subsection ‹Misc›

lemma fact_code [code]:
"fact n = (of_nat (fold_atLeastAtMost_nat (( * )) 2 n 1) :: 'a::semiring_char_0)"
proof -
have "fact n = (of_nat (∏{1..n}) :: 'a)"
also have "∏{1..n} = ∏{2..n}"
by (intro prod.mono_neutral_right) auto
also have "… = fold_atLeastAtMost_nat (( * )) 2 n 1"