# Theory HOL-Computational_Algebra.Polynomial

```(*  Title:      HOL/Computational_Algebra/Polynomial.thy
Author:     Brian Huffman
Author:     Clemens Ballarin
Author:     Amine Chaieb
Author:     Florian Haftmann
*)

section ‹Polynomials as type over a ring structure›

theory Polynomial
imports
Complex_Main
"HOL-Library.More_List"
"HOL-Library.Infinite_Set"
Factorial_Ring
begin

context semidom_modulo
begin

lemma not_dvd_imp_mod_neq_0:
‹a mod b ≠ 0› if ‹¬ b dvd a›
using that mod_0_imp_dvd [of a b] by blast

end

subsection ‹Auxiliary: operations for lists (later) representing coefficients›

definition cCons :: "'a::zero ⇒ 'a list ⇒ 'a list"  (infixr "##" 65)
where "x ## xs = (if xs = [] ∧ x = 0 then [] else x # xs)"

lemma cCons_0_Nil_eq [simp]: "0 ## [] = []"

lemma cCons_Cons_eq [simp]: "x ## y # ys = x # y # ys"

lemma cCons_append_Cons_eq [simp]: "x ## xs @ y # ys = x # xs @ y # ys"

lemma cCons_not_0_eq [simp]: "x ≠ 0 ⟹ x ## xs = x # xs"

lemma strip_while_not_0_Cons_eq [simp]:
"strip_while (λx. x = 0) (x # xs) = x ## strip_while (λx. x = 0) xs"
proof (cases "x = 0")
case False
then show ?thesis by simp
next
case True
show ?thesis
proof (induct xs rule: rev_induct)
case Nil
with True show ?case by simp
next
case (snoc y ys)
then show ?case
by (cases "y = 0") (simp_all add: append_Cons [symmetric] del: append_Cons)
qed
qed

lemma tl_cCons [simp]: "tl (x ## xs) = xs"

subsection ‹Definition of type ‹poly››

typedef (overloaded) 'a poly = "{f :: nat ⇒ 'a::zero. ∀⇩∞ n. f n = 0}"
morphisms coeff Abs_poly
by (auto intro!: ALL_MOST)

setup_lifting type_definition_poly

lemma poly_eq_iff: "p = q ⟷ (∀n. coeff p n = coeff q n)"
by (simp add: coeff_inject [symmetric] fun_eq_iff)

lemma poly_eqI: "(⋀n. coeff p n = coeff q n) ⟹ p = q"

lemma MOST_coeff_eq_0: "∀⇩∞ n. coeff p n = 0"
using coeff [of p] by simp

subsection ‹Degree of a polynomial›

definition degree :: "'a::zero poly ⇒ nat"
where "degree p = (LEAST n. ∀i>n. coeff p i = 0)"

lemma coeff_eq_0:
assumes "degree p < n"
shows "coeff p n = 0"
proof -
have "∃n. ∀i>n. coeff p i = 0"
using MOST_coeff_eq_0 by (simp add: MOST_nat)
then have "∀i>degree p. coeff p i = 0"
unfolding degree_def by (rule LeastI_ex)
with assms show ?thesis by simp
qed

lemma le_degree: "coeff p n ≠ 0 ⟹ n ≤ degree p"
by (erule contrapos_np, rule coeff_eq_0, simp)

lemma degree_le: "∀i>n. coeff p i = 0 ⟹ degree p ≤ n"
unfolding degree_def by (erule Least_le)

lemma less_degree_imp: "n < degree p ⟹ ∃i>n. coeff p i ≠ 0"
unfolding degree_def by (drule not_less_Least, simp)

subsection ‹The zero polynomial›

instantiation poly :: (zero) zero
begin

lift_definition zero_poly :: "'a poly"
is "λ_. 0"
by (rule MOST_I) simp

instance ..

end

lemma coeff_0 [simp]: "coeff 0 n = 0"
by transfer rule

lemma degree_0 [simp]: "degree 0 = 0"
by (rule order_antisym [OF degree_le le0]) simp

assumes "p ≠ 0"
shows "coeff p (degree p) ≠ 0"
proof (cases "degree p")
case 0
from ‹p ≠ 0› obtain n where "coeff p n ≠ 0"
then have "n ≤ degree p"
by (rule le_degree)
with ‹coeff p n ≠ 0› and ‹degree p = 0› show "coeff p (degree p) ≠ 0"
by simp
next
case (Suc n)
from ‹degree p = Suc n› have "n < degree p"
by simp
then have "∃i>n. coeff p i ≠ 0"
by (rule less_degree_imp)
then obtain i where "n < i" and "coeff p i ≠ 0"
by blast
from ‹degree p = Suc n› and ‹n < i› have "degree p ≤ i"
by simp
also from ‹coeff p i ≠ 0› have "i ≤ degree p"
by (rule le_degree)
finally have "degree p = i" .
with ‹coeff p i ≠ 0› show "coeff p (degree p) ≠ 0" by simp
qed

lemma leading_coeff_0_iff [simp]: "coeff p (degree p) = 0 ⟷ p = 0"

lemma eq_zero_or_degree_less:
assumes "degree p ≤ n" and "coeff p n = 0"
shows "p = 0 ∨ degree p < n"
proof (cases n)
case 0
with ‹degree p ≤ n› and ‹coeff p n = 0› have "coeff p (degree p) = 0"
by simp
then have "p = 0" by simp
then show ?thesis ..
next
case (Suc m)
from ‹degree p ≤ n› have "∀i>n. coeff p i = 0"
with ‹coeff p n = 0› have "∀i≥n. coeff p i = 0"
with ‹n = Suc m› have "∀i>m. coeff p i = 0"
then have "degree p ≤ m"
by (rule degree_le)
with ‹n = Suc m› have "degree p < n"
then show ?thesis ..
qed

lemma coeff_0_degree_minus_1: "coeff rrr dr = 0 ⟹ degree rrr ≤ dr ⟹ degree rrr ≤ dr - 1"
using eq_zero_or_degree_less by fastforce

subsection ‹List-style constructor for polynomials›

lift_definition pCons :: "'a::zero ⇒ 'a poly ⇒ 'a poly"
is "λa p. case_nat a (coeff p)"
by (rule MOST_SucD) (simp add: MOST_coeff_eq_0)

lemmas coeff_pCons = pCons.rep_eq

lemma coeff_pCons_0 [simp]: "coeff (pCons a p) 0 = a"
by transfer simp

lemma coeff_pCons_Suc [simp]: "coeff (pCons a p) (Suc n) = coeff p n"

lemma degree_pCons_le: "degree (pCons a p) ≤ Suc (degree p)"
by (rule degree_le) (simp add: coeff_eq_0 coeff_pCons split: nat.split)

lemma degree_pCons_eq: "p ≠ 0 ⟹ degree (pCons a p) = Suc (degree p)"
by (simp add: degree_pCons_le le_antisym le_degree)

lemma degree_pCons_0: "degree (pCons a 0) = 0"
proof -
have "degree (pCons a 0) ≤ Suc 0"
by (metis (no_types) degree_0 degree_pCons_le)
then show ?thesis
by (metis coeff_0 coeff_pCons_Suc degree_0 eq_zero_or_degree_less less_Suc0)
qed

lemma degree_pCons_eq_if [simp]: "degree (pCons a p) = (if p = 0 then 0 else Suc (degree p))"

lemma pCons_0_0 [simp]: "pCons 0 0 = 0"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)

lemma pCons_eq_iff [simp]: "pCons a p = pCons b q ⟷ a = b ∧ p = q"
proof safe
assume "pCons a p = pCons b q"
then have "coeff (pCons a p) 0 = coeff (pCons b q) 0"
by simp
then show "a = b"
by simp
next
assume "pCons a p = pCons b q"
then have "coeff (pCons a p) (Suc n) = coeff (pCons b q) (Suc n)" for n
by simp
then show "p = q"
qed

lemma pCons_eq_0_iff [simp]: "pCons a p = 0 ⟷ a = 0 ∧ p = 0"
using pCons_eq_iff [of a p 0 0] by simp

lemma pCons_cases [cases type: poly]:
obtains (pCons) a q where "p = pCons a q"
proof
show "p = pCons (coeff p 0) (Abs_poly (λn. coeff p (Suc n)))"
by transfer
(simp_all add: MOST_inj[where f=Suc and P="λn. p n = 0" for p] fun_eq_iff Abs_poly_inverse
split: nat.split)
qed

lemma pCons_induct [case_names 0 pCons, induct type: poly]:
assumes zero: "P 0"
assumes pCons: "⋀a p. a ≠ 0 ∨ p ≠ 0 ⟹ P p ⟹ P (pCons a p)"
shows "P p"
proof (induct p rule: measure_induct_rule [where f=degree])
case (less p)
obtain a q where "p = pCons a q" by (rule pCons_cases)
have "P q"
proof (cases "q = 0")
case True
then show "P q" by (simp add: zero)
next
case False
then have "degree (pCons a q) = Suc (degree q)"
by (rule degree_pCons_eq)
with ‹p = pCons a q› have "degree q < degree p"
by simp
then show "P q"
by (rule less.hyps)
qed
have "P (pCons a q)"
proof (cases "a ≠ 0 ∨ q ≠ 0")
case True
with ‹P q› show ?thesis by (auto intro: pCons)
next
case False
with zero show ?thesis by simp
qed
with ‹p = pCons a q› show ?case
by simp
qed

lemma degree_eq_zeroE:
fixes p :: "'a::zero poly"
assumes "degree p = 0"
obtains a where "p = pCons a 0"
proof -
obtain a q where p: "p = pCons a q"
by (cases p)
with assms have "q = 0"
by (cases "q = 0") simp_all
with p have "p = pCons a 0"
by simp
then show thesis ..
qed

subsection ‹Quickcheck generator for polynomials›

quickcheck_generator poly constructors: "0 :: _ poly", pCons

subsection ‹List-style syntax for polynomials›

syntax "_poly" :: "args ⇒ 'a poly"  ("[:(_):]")
translations
"[:x, xs:]" ⇌ "CONST pCons x [:xs:]"
"[:x:]" ⇌ "CONST pCons x 0"
"[:x:]" ↽ "CONST pCons x (_constrain 0 t)"

subsection ‹Representation of polynomials by lists of coefficients›

primrec Poly :: "'a::zero list ⇒ 'a poly"
where
[code_post]: "Poly [] = 0"
| [code_post]: "Poly (a # as) = pCons a (Poly as)"

lemma Poly_replicate_0 [simp]: "Poly (replicate n 0) = 0"
by (induct n) simp_all

lemma Poly_eq_0: "Poly as = 0 ⟷ (∃n. as = replicate n 0)"
by (induct as) (auto simp add: Cons_replicate_eq)

lemma Poly_append_replicate_zero [simp]: "Poly (as @ replicate n 0) = Poly as"
by (induct as) simp_all

lemma Poly_snoc_zero [simp]: "Poly (as @ [0]) = Poly as"
using Poly_append_replicate_zero [of as 1] by simp

lemma Poly_cCons_eq_pCons_Poly [simp]: "Poly (a ## p) = pCons a (Poly p)"

lemma Poly_on_rev_starting_with_0 [simp]: "hd as = 0 ⟹ Poly (rev (tl as)) = Poly (rev as)"
by (cases as) simp_all

lemma degree_Poly: "degree (Poly xs) ≤ length xs"
by (induct xs) simp_all

lemma coeff_Poly_eq [simp]: "coeff (Poly xs) = nth_default 0 xs"
by (induct xs) (simp_all add: fun_eq_iff coeff_pCons split: nat.splits)

definition coeffs :: "'a poly ⇒ 'a::zero list"
where "coeffs p = (if p = 0 then [] else map (λi. coeff p i) [0 ..< Suc (degree p)])"

lemma coeffs_eq_Nil [simp]: "coeffs p = [] ⟷ p = 0"

lemma not_0_coeffs_not_Nil: "p ≠ 0 ⟹ coeffs p ≠ []"
by simp

lemma coeffs_0_eq_Nil [simp]: "coeffs 0 = []"
by simp

lemma coeffs_pCons_eq_cCons [simp]: "coeffs (pCons a p) = a ## coeffs p"
proof -
have *: "∀m∈set ms. m > 0 ⟹ map (case_nat x f) ms = map f (map (λn. n - 1) ms)"
for ms :: "nat list" and f :: "nat ⇒ 'a" and x :: "'a"
by (induct ms) (auto split: nat.split)
show ?thesis
by (simp add: * coeffs_def upt_conv_Cons coeff_pCons map_decr_upt del: upt_Suc)
qed

lemma length_coeffs: "p ≠ 0 ⟹ length (coeffs p) = degree p + 1"

lemma coeffs_nth: "p ≠ 0 ⟹ n ≤ degree p ⟹ coeffs p ! n = coeff p n"
by (auto simp: coeffs_def simp del: upt_Suc)

lemma coeff_in_coeffs: "p ≠ 0 ⟹ n ≤ degree p ⟹ coeff p n ∈ set (coeffs p)"
using coeffs_nth [of p n, symmetric] by (simp add: length_coeffs)

lemma not_0_cCons_eq [simp]: "p ≠ 0 ⟹ a ## coeffs p = a # coeffs p"

lemma Poly_coeffs [simp, code abstype]: "Poly (coeffs p) = p"
by (induct p) auto

lemma coeffs_Poly [simp]: "coeffs (Poly as) = strip_while (HOL.eq 0) as"
proof (induct as)
case Nil
then show ?case by simp
next
case (Cons a as)
from replicate_length_same [of as 0] have "(∀n. as ≠ replicate n 0) ⟷ (∃a∈set as. a ≠ 0)"
by (auto dest: sym [of _ as])
with Cons show ?case by auto
qed

lemma no_trailing_coeffs [simp]:
"no_trailing (HOL.eq 0) (coeffs p)"
by (induct p)  auto

lemma strip_while_coeffs [simp]:
"strip_while (HOL.eq 0) (coeffs p) = coeffs p"
by simp

lemma coeffs_eq_iff: "p = q ⟷ coeffs p = coeffs q"
(is "?P ⟷ ?Q")
proof
assume ?P
then show ?Q by simp
next
assume ?Q
then have "Poly (coeffs p) = Poly (coeffs q)" by simp
then show ?P by simp
qed

lemma nth_default_coeffs_eq: "nth_default 0 (coeffs p) = coeff p"
by (simp add: fun_eq_iff coeff_Poly_eq [symmetric])

lemma [code]: "coeff p = nth_default 0 (coeffs p)"

lemma coeffs_eqI:
assumes coeff: "⋀n. coeff p n = nth_default 0 xs n"
assumes zero: "no_trailing (HOL.eq 0) xs"
shows "coeffs p = xs"
proof -
from coeff have "p = Poly xs"
with zero show ?thesis by simp
qed

lemma degree_eq_length_coeffs [code]: "degree p = length (coeffs p) - 1"

lemma length_coeffs_degree: "p ≠ 0 ⟹ length (coeffs p) = Suc (degree p)"
by (induct p) (auto simp: cCons_def)

lemma [code abstract]: "coeffs 0 = []"
by (fact coeffs_0_eq_Nil)

lemma [code abstract]: "coeffs (pCons a p) = a ## coeffs p"
by (fact coeffs_pCons_eq_cCons)

lemma set_coeffs_subset_singleton_0_iff [simp]:
"set (coeffs p) ⊆ {0} ⟷ p = 0"
by (auto simp add: coeffs_def intro: classical)

lemma set_coeffs_not_only_0 [simp]:
"set (coeffs p) ≠ {0}"

lemma forall_coeffs_conv:
"(∀n. P (coeff p n)) ⟷ (∀c ∈ set (coeffs p). P c)" if "P 0"
using that by (auto simp add: coeffs_def)
(metis atLeastLessThan_iff coeff_eq_0 not_less_iff_gr_or_eq zero_le)

instantiation poly :: ("{zero, equal}") equal
begin

definition [code]: "HOL.equal (p::'a poly) q ⟷ HOL.equal (coeffs p) (coeffs q)"

instance
by standard (simp add: equal equal_poly_def coeffs_eq_iff)

end

lemma [code nbe]: "HOL.equal (p :: _ poly) p ⟷ True"
by (fact equal_refl)

definition is_zero :: "'a::zero poly ⇒ bool"
where [code]: "is_zero p ⟷ List.null (coeffs p)"

lemma is_zero_null [code_abbrev]: "is_zero p ⟷ p = 0"

subsubsection ‹Reconstructing the polynomial from the list›
― ‹contributed by Sebastiaan J.C. Joosten and René Thiemann›

definition poly_of_list :: "'a::comm_monoid_add list ⇒ 'a poly"
where [simp]: "poly_of_list = Poly"

lemma poly_of_list_impl [code abstract]: "coeffs (poly_of_list as) = strip_while (HOL.eq 0) as"
by simp

subsection ‹Fold combinator for polynomials›

definition fold_coeffs :: "('a::zero ⇒ 'b ⇒ 'b) ⇒ 'a poly ⇒ 'b ⇒ 'b"
where "fold_coeffs f p = foldr f (coeffs p)"

lemma fold_coeffs_0_eq [simp]: "fold_coeffs f 0 = id"

lemma fold_coeffs_pCons_eq [simp]: "f 0 = id ⟹ fold_coeffs f (pCons a p) = f a ∘ fold_coeffs f p"
by (simp add: fold_coeffs_def cCons_def fun_eq_iff)

lemma fold_coeffs_pCons_0_0_eq [simp]: "fold_coeffs f (pCons 0 0) = id"

lemma fold_coeffs_pCons_coeff_not_0_eq [simp]:
"a ≠ 0 ⟹ fold_coeffs f (pCons a p) = f a ∘ fold_coeffs f p"

lemma fold_coeffs_pCons_not_0_0_eq [simp]:
"p ≠ 0 ⟹ fold_coeffs f (pCons a p) = f a ∘ fold_coeffs f p"

subsection ‹Canonical morphism on polynomials -- evaluation›

definition poly :: ‹'a::comm_semiring_0 poly ⇒ 'a ⇒ 'a›
where ‹poly p a = horner_sum id a (coeffs p)›

lemma poly_eq_fold_coeffs:
‹poly p = fold_coeffs (λa f x. a + x * f x) p (λx. 0)›
by (induction p) (auto simp add: fun_eq_iff poly_def)

lemma poly_0 [simp]: "poly 0 x = 0"

lemma poly_pCons [simp]: "poly (pCons a p) x = a + x * poly p x"
by (cases "p = 0 ∧ a = 0") (auto simp add: poly_def)

lemma poly_altdef: "poly p x = (∑i≤degree p. coeff p i * x ^ i)"
for x :: "'a::{comm_semiring_0,semiring_1}"
proof (induction p rule: pCons_induct)
case 0
then show ?case
by simp
next
case (pCons a p)
show ?case
proof (cases "p = 0")
case True
then show ?thesis by simp
next
case False
let ?p' = "pCons a p"
note poly_pCons[of a p x]
also note pCons.IH
also have "a + x * (∑i≤degree p. coeff p i * x ^ i) =
coeff ?p' 0 * x^0 + (∑i≤degree p. coeff ?p' (Suc i) * x^Suc i)"
by (simp add: field_simps sum_distrib_left coeff_pCons)
also note sum.atMost_Suc_shift[symmetric]
also note degree_pCons_eq[OF ‹p ≠ 0›, of a, symmetric]
finally show ?thesis .
qed
qed

lemma poly_0_coeff_0: "poly p 0 = coeff p 0"
by (cases p) (auto simp: poly_altdef)

subsection ‹Monomials›

lift_definition monom :: "'a ⇒ nat ⇒ 'a::zero poly"
is "λa m n. if m = n then a else 0"

lemma coeff_monom [simp]: "coeff (monom a m) n = (if m = n then a else 0)"
by transfer rule

lemma monom_0: "monom a 0 = [:a:]"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)

lemma monom_Suc: "monom a (Suc n) = pCons 0 (monom a n)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)

lemma monom_eq_0 [simp]: "monom 0 n = 0"
by (rule poly_eqI) simp

lemma monom_eq_0_iff [simp]: "monom a n = 0 ⟷ a = 0"

lemma monom_eq_iff [simp]: "monom a n = monom b n ⟷ a = b"

lemma degree_monom_le: "degree (monom a n) ≤ n"
by (rule degree_le, simp)

lemma degree_monom_eq: "a ≠ 0 ⟹ degree (monom a n) = n"

lemma coeffs_monom [code abstract]:
"coeffs (monom a n) = (if a = 0 then [] else replicate n 0 @ [a])"
by (induct n) (simp_all add: monom_0 monom_Suc)

lemma fold_coeffs_monom [simp]: "a ≠ 0 ⟹ fold_coeffs f (monom a n) = f 0 ^^ n ∘ f a"
by (simp add: fold_coeffs_def coeffs_monom fun_eq_iff)

lemma poly_monom: "poly (monom a n) x = a * x ^ n"
for a x :: "'a::comm_semiring_1"
by (cases "a = 0", simp_all) (induct n, simp_all add: mult.left_commute poly_eq_fold_coeffs)

lemma monom_eq_iff': "monom c n = monom d m ⟷  c = d ∧ (c = 0 ∨ n = m)"
by (auto simp: poly_eq_iff)

lemma monom_eq_const_iff: "monom c n = [:d:] ⟷ c = d ∧ (c = 0 ∨ n = 0)"
using monom_eq_iff'[of c n d 0] by (simp add: monom_0)

abbreviation lead_coeff:: "'a::zero poly ⇒ 'a"
where "lead_coeff p ≡ coeff p (degree p)"

"p = 0 ⟹ lead_coeff (pCons a p) = a"
by auto

by (cases "c = 0") (simp_all add: degree_monom_eq)

lemma last_coeffs_eq_coeff_degree:
"last (coeffs p) = lead_coeff p" if "p ≠ 0"
using that by (simp add: coeffs_def)

begin

lift_definition plus_poly :: "'a poly ⇒ 'a poly ⇒ 'a poly"
is "λp q n. coeff p n + coeff q n"
proof -
fix q p :: "'a poly"
show "∀⇩∞n. coeff p n + coeff q n = 0"
using MOST_coeff_eq_0[of p] MOST_coeff_eq_0[of q] by eventually_elim simp
qed

lemma coeff_add [simp]: "coeff (p + q) n = coeff p n + coeff q n"

instance
proof
fix p q r :: "'a poly"
show "(p + q) + r = p + (q + r)"
show "p + q = q + p"
show "0 + p = p"
qed

end

begin

lift_definition minus_poly :: "'a poly ⇒ 'a poly ⇒ 'a poly"
is "λp q n. coeff p n - coeff q n"
proof -
fix q p :: "'a poly"
show "∀⇩∞n. coeff p n - coeff q n = 0"
using MOST_coeff_eq_0[of p] MOST_coeff_eq_0[of q] by eventually_elim simp
qed

lemma coeff_diff [simp]: "coeff (p - q) n = coeff p n - coeff q n"

instance
proof
fix p q r :: "'a poly"
show "p + q - p = q"
show "p - q - r = p - (q + r)"
qed

end

begin

lift_definition uminus_poly :: "'a poly ⇒ 'a poly"
is "λp n. - coeff p n"
proof -
fix p :: "'a poly"
show "∀⇩∞n. - coeff p n = 0"
using MOST_coeff_eq_0 by simp
qed

lemma coeff_minus [simp]: "coeff (- p) n = - coeff p n"

instance
proof
fix p q :: "'a poly"
show "- p + p = 0"
show "p - q = p + - q"
qed

end

lemma add_pCons [simp]: "pCons a p + pCons b q = pCons (a + b) (p + q)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)

lemma minus_pCons [simp]: "- pCons a p = pCons (- a) (- p)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)

lemma diff_pCons [simp]: "pCons a p - pCons b q = pCons (a - b) (p - q)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)

lemma degree_add_le_max: "degree (p + q) ≤ max (degree p) (degree q)"
by (rule degree_le) (auto simp add: coeff_eq_0)

lemma degree_add_le: "degree p ≤ n ⟹ degree q ≤ n ⟹ degree (p + q) ≤ n"

lemma degree_add_less: "degree p < n ⟹ degree q < n ⟹ degree (p + q) < n"

lemma degree_add_eq_right: assumes "degree p < degree q" shows "degree (p + q) = degree q"
proof (cases "q = 0")
case False
show ?thesis
proof (rule order_antisym)
show "degree (p + q) ≤ degree q"
show "degree q ≤ degree (p + q)"
by (simp add: False assms coeff_eq_0 le_degree)
qed
qed (use assms in auto)

lemma degree_add_eq_left: "degree q < degree p ⟹ degree (p + q) = degree p"

lemma degree_minus [simp]: "degree (- p) = degree p"

by (metis coeff_minus degree_minus)

lemma degree_diff_le_max: "degree (p - q) ≤ max (degree p) (degree q)"
for p q :: "'a::ab_group_add poly"
using degree_add_le [where p=p and q="-q"] by simp

lemma degree_diff_le: "degree p ≤ n ⟹ degree q ≤ n ⟹ degree (p - q) ≤ n"
for p q :: "'a::ab_group_add poly"
using degree_add_le [of p n "- q"] by simp

lemma degree_diff_less: "degree p < n ⟹ degree q < n ⟹ degree (p - q) < n"
for p q :: "'a::ab_group_add poly"
using degree_add_less [of p n "- q"] by simp

lemma add_monom: "monom a n + monom b n = monom (a + b) n"
by (rule poly_eqI) simp

lemma diff_monom: "monom a n - monom b n = monom (a - b) n"
by (rule poly_eqI) simp

lemma minus_monom: "- monom a n = monom (- a) n"
by (rule poly_eqI) simp

lemma coeff_sum: "coeff (∑x∈A. p x) i = (∑x∈A. coeff (p x) i)"
by (induct A rule: infinite_finite_induct) simp_all

lemma monom_sum: "monom (∑x∈A. a x) n = (∑x∈A. monom (a x) n)"
by (rule poly_eqI) (simp add: coeff_sum)

fun plus_coeffs :: "'a::comm_monoid_add list ⇒ 'a list ⇒ 'a list"
where
"plus_coeffs xs [] = xs"
| "plus_coeffs [] ys = ys"
| "plus_coeffs (x # xs) (y # ys) = (x + y) ## plus_coeffs xs ys"

lemma coeffs_plus_eq_plus_coeffs [code abstract]:
"coeffs (p + q) = plus_coeffs (coeffs p) (coeffs q)"
proof -
have *: "nth_default 0 (plus_coeffs xs ys) n = nth_default 0 xs n + nth_default 0 ys n"
for xs ys :: "'a list" and n
proof (induct xs ys arbitrary: n rule: plus_coeffs.induct)
case (3 x xs y ys n)
then show ?case
by (cases n) (auto simp add: cCons_def)
qed simp_all
have **: "no_trailing (HOL.eq 0) (plus_coeffs xs ys)"
if "no_trailing (HOL.eq 0) xs" and "no_trailing (HOL.eq 0) ys"
for xs ys :: "'a list"
using that by (induct xs ys rule: plus_coeffs.induct) (simp_all add: cCons_def)
show ?thesis
by (rule coeffs_eqI) (auto simp add: * nth_default_coeffs_eq intro: **)
qed

lemma coeffs_uminus [code abstract]:
"coeffs (- p) = map uminus (coeffs p)"
proof -
have eq_0: "HOL.eq 0 ∘ uminus = HOL.eq (0::'a)"
show ?thesis
by (rule coeffs_eqI) (simp_all add: nth_default_map_eq nth_default_coeffs_eq no_trailing_map eq_0)
qed

lemma [code]: "p - q = p + - q"
for p q :: "'a::ab_group_add poly"

lemma poly_add [simp]: "poly (p + q) x = poly p x + poly q x"
proof (induction p arbitrary: q)
case (pCons a p)
then show ?case
by (cases q) (simp add: algebra_simps)
qed auto

lemma poly_minus [simp]: "poly (- p) x = - poly p x"
for x :: "'a::comm_ring"
by (induct p) simp_all

lemma poly_diff [simp]: "poly (p - q) x = poly p x - poly q x"
for x :: "'a::comm_ring"
using poly_add [of p "- q" x] by simp

lemma poly_sum: "poly (∑k∈A. p k) x = (∑k∈A. poly (p k) x)"
by (induct A rule: infinite_finite_induct) simp_all

lemma degree_sum_le: "finite S ⟹ (⋀p. p ∈ S ⟹ degree (f p) ≤ n) ⟹ degree (sum f S) ≤ n"
proof (induct S rule: finite_induct)
case empty
then show ?case by simp
next
case (insert p S)
then have "degree (sum f S) ≤ n" "degree (f p) ≤ n"
by auto
then show ?case
unfolding sum.insert[OF insert(1-2)] by (metis degree_add_le)
qed

lemma poly_as_sum_of_monoms':
assumes "degree p ≤ n"
shows "(∑i≤n. monom (coeff p i) i) = p"
proof -
have eq: "⋀i. {..n} ∩ {i} = (if i ≤ n then {i} else {})"
by auto
from assms show ?thesis
by (simp add: poly_eq_iff coeff_sum coeff_eq_0 sum.If_cases eq
if_distrib[where f="λx. x * a" for a])
qed

lemma poly_as_sum_of_monoms: "(∑i≤degree p. monom (coeff p i) i) = p"
by (intro poly_as_sum_of_monoms' order_refl)

lemma Poly_snoc: "Poly (xs @ [x]) = Poly xs + monom x (length xs)"
by (induct xs) (simp_all add: monom_0 monom_Suc)

subsection ‹Multiplication by a constant, polynomial multiplication and the unit polynomial›

lift_definition smult :: "'a::comm_semiring_0 ⇒ 'a poly ⇒ 'a poly"
is "λa p n. a * coeff p n"
proof -
fix a :: 'a and p :: "'a poly"
show "∀⇩∞ i. a * coeff p i = 0"
using MOST_coeff_eq_0[of p] by eventually_elim simp
qed

lemma coeff_smult [simp]: "coeff (smult a p) n = a * coeff p n"

lemma degree_smult_le: "degree (smult a p) ≤ degree p"
by (rule degree_le) (simp add: coeff_eq_0)

lemma smult_smult [simp]: "smult a (smult b p) = smult (a * b) p"
by (rule poly_eqI) (simp add: mult.assoc)

lemma smult_0_right [simp]: "smult a 0 = 0"
by (rule poly_eqI) simp

lemma smult_0_left [simp]: "smult 0 p = 0"
by (rule poly_eqI) simp

lemma smult_1_left [simp]: "smult (1::'a::comm_semiring_1) p = p"
by (rule poly_eqI) simp

lemma smult_add_right: "smult a (p + q) = smult a p + smult a q"
by (rule poly_eqI) (simp add: algebra_simps)

lemma smult_add_left: "smult (a + b) p = smult a p + smult b p"
by (rule poly_eqI) (simp add: algebra_simps)

lemma smult_minus_right [simp]: "smult a (- p) = - smult a p"
for a :: "'a::comm_ring"
by (rule poly_eqI) simp

lemma smult_minus_left [simp]: "smult (- a) p = - smult a p"
for a :: "'a::comm_ring"
by (rule poly_eqI) simp

lemma smult_diff_right: "smult a (p - q) = smult a p - smult a q"
for a :: "'a::comm_ring"
by (rule poly_eqI) (simp add: algebra_simps)

lemma smult_diff_left: "smult (a - b) p = smult a p - smult b p"
for a b :: "'a::comm_ring"
by (rule poly_eqI) (simp add: algebra_simps)

lemmas smult_distribs =
smult_diff_left smult_diff_right

lemma smult_pCons [simp]: "smult a (pCons b p) = pCons (a * b) (smult a p)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)

lemma smult_monom: "smult a (monom b n) = monom (a * b) n"
by (induct n) (simp_all add: monom_0 monom_Suc)

lemma smult_Poly: "smult c (Poly xs) = Poly (map ((*) c) xs)"
by (auto simp: poly_eq_iff nth_default_def)

lemma degree_smult_eq [simp]: "degree (smult a p) = (if a = 0 then 0 else degree p)"
for a :: "'a::{comm_semiring_0,semiring_no_zero_divisors}"
by (cases "a = 0") (simp_all add: degree_def)

lemma smult_eq_0_iff [simp]: "smult a p = 0 ⟷ a = 0 ∨ p = 0"
for a :: "'a::{comm_semiring_0,semiring_no_zero_divisors}"

lemma coeffs_smult [code abstract]:
"coeffs (smult a p) = (if a = 0 then [] else map (Groups.times a) (coeffs p))"
for p :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
proof -
have eq_0: "HOL.eq 0 ∘ times a = HOL.eq (0::'a)" if "a ≠ 0"
using that by (simp add: fun_eq_iff)
show ?thesis
by (rule coeffs_eqI) (auto simp add: no_trailing_map nth_default_map_eq nth_default_coeffs_eq eq_0)
qed

lemma smult_eq_iff:
fixes b :: "'a :: field"
assumes "b ≠ 0"
shows "smult a p = smult b q ⟷ smult (a / b) p = q"
(is "?lhs ⟷ ?rhs")
proof
assume ?lhs
also from assms have "smult (inverse b) … = q"
by simp
finally show ?rhs
next
assume ?rhs
with assms show ?lhs by auto
qed

instantiation poly :: (comm_semiring_0) comm_semiring_0
begin

definition "p * q = fold_coeffs (λa p. smult a q + pCons 0 p) p 0"

lemma mult_poly_0_left: "(0::'a poly) * q = 0"

lemma mult_pCons_left [simp]: "pCons a p * q = smult a q + pCons 0 (p * q)"
by (cases "p = 0 ∧ a = 0") (auto simp add: times_poly_def)

lemma mult_poly_0_right: "p * (0::'a poly) = 0"
by (induct p) (simp_all add: mult_poly_0_left)

lemma mult_pCons_right [simp]: "p * pCons a q = smult a p + pCons 0 (p * q)"
by (induct p) (simp_all add: mult_poly_0_left algebra_simps)

lemmas mult_poly_0 = mult_poly_0_left mult_poly_0_right

lemma mult_smult_left [simp]: "smult a p * q = smult a (p * q)"

lemma mult_smult_right [simp]: "p * smult a q = smult a (p * q)"

lemma mult_poly_add_left: "(p + q) * r = p * r + q * r"
for p q r :: "'a poly"
by (induct r) (simp_all add: mult_poly_0 smult_distribs algebra_simps)

instance
proof
fix p q r :: "'a poly"
show 0: "0 * p = 0"
by (rule mult_poly_0_left)
show "p * 0 = 0"
by (rule mult_poly_0_right)
show "(p + q) * r = p * r + q * r"
show "(p * q) * r = p * (q * r)"
show "p * q = q * p"
by (induct p) (simp_all add: mult_poly_0)
qed

end

lemma coeff_mult_degree_sum:
"coeff (p * q) (degree p + degree q) = coeff p (degree p) * coeff q (degree q)"
by (induct p) (simp_all add: coeff_eq_0)

instance poly :: ("{comm_semiring_0,semiring_no_zero_divisors}") semiring_no_zero_divisors
proof
fix p q :: "'a poly"
assume "p ≠ 0" and "q ≠ 0"
have "coeff (p * q) (degree p + degree q) = coeff p (degree p) * coeff q (degree q)"
by (rule coeff_mult_degree_sum)
also from ‹p ≠ 0› ‹q ≠ 0› have "coeff p (degree p) * coeff q (degree q) ≠ 0"
by simp
finally have "∃n. coeff (p * q) n ≠ 0" ..
then show "p * q ≠ 0"
qed

instance poly :: (comm_semiring_0_cancel) comm_semiring_0_cancel ..

lemma coeff_mult: "coeff (p * q) n = (∑i≤n. coeff p i * coeff q (n-i))"
proof (induct p arbitrary: n)
case 0
show ?case by simp
next
case (pCons a p n)
then show ?case
by (cases n) (simp_all add: sum.atMost_Suc_shift del: sum.atMost_Suc)
qed

lemma degree_mult_le: "degree (p * q) ≤ degree p + degree q"
proof (rule degree_le)
show "∀i>degree p + degree q. coeff (p * q) i = 0"
by (induct p) (simp_all add: coeff_eq_0 coeff_pCons split: nat.split)
qed

lemma mult_monom: "monom a m * monom b n = monom (a * b) (m + n)"

instantiation poly :: (comm_semiring_1) comm_semiring_1
begin

lift_definition one_poly :: "'a poly"
is "λn. of_bool (n = 0)"
by (rule MOST_SucD) simp

lemma coeff_1 [simp]:
"coeff 1 n = of_bool (n = 0)"

lemma one_pCons:
"1 = [:1:]"
by (simp add: poly_eq_iff coeff_pCons split: nat.splits)

lemma pCons_one:
"[:1:] = 1"

instance

end

lemma poly_1 [simp]:
"poly 1 x = 1"

lemma one_poly_eq_simps [simp]:
"1 = [:1:] ⟷ True"
"[:1:] = 1 ⟷ True"

lemma degree_1 [simp]:
"degree 1 = 0"

lemma coeffs_1_eq [simp, code abstract]:
"coeffs 1 = [1]"

lemma smult_one [simp]:
"smult c 1 = [:c:]"

lemma monom_eq_1 [simp]:
"monom 1 0 = 1"

lemma monom_eq_1_iff:
"monom c n = 1 ⟷ c = 1 ∧ n = 0"
using monom_eq_const_iff [of c n 1] by auto

lemma monom_altdef:
"monom c n = smult c ([:0, 1:] ^ n)"
by (induct n) (simp_all add: monom_0 monom_Suc)

instance poly :: ("{comm_semiring_1,semiring_1_no_zero_divisors}") semiring_1_no_zero_divisors ..
instance poly :: (comm_ring) comm_ring ..
instance poly :: (comm_ring_1) comm_ring_1 ..
instance poly :: (comm_ring_1) comm_semiring_1_cancel ..

lemma degree_power_le: "degree (p ^ n) ≤ degree p * n"
by (induct n) (auto intro: order_trans degree_mult_le)

lemma coeff_0_power: "coeff (p ^ n) 0 = coeff p 0 ^ n"
by (induct n) (simp_all add: coeff_mult)

lemma poly_smult [simp]: "poly (smult a p) x = a * poly p x"
by (induct p) (simp_all add: algebra_simps)

lemma poly_mult [simp]: "poly (p * q) x = poly p x * poly q x"
by (induct p) (simp_all add: algebra_simps)

lemma poly_power [simp]: "poly (p ^ n) x = poly p x ^ n"
for p :: "'a::comm_semiring_1 poly"
by (induct n) simp_all

lemma poly_prod: "poly (∏k∈A. p k) x = (∏k∈A. poly (p k) x)"
by (induct A rule: infinite_finite_induct) simp_all

lemma degree_prod_sum_le: "finite S ⟹ degree (prod f S) ≤ sum (degree ∘ f) S"
proof (induct S rule: finite_induct)
case empty
then show ?case by simp
next
case (insert a S)
show ?case
unfolding prod.insert[OF insert(1-2)] sum.insert[OF insert(1-2)]
by (rule le_trans[OF degree_mult_le]) (use insert in auto)
qed

lemma coeff_0_prod_list: "coeff (prod_list xs) 0 = prod_list (map (λp. coeff p 0) xs)"
by (induct xs) (simp_all add: coeff_mult)

lemma coeff_monom_mult: "coeff (monom c n * p) k = (if k < n then 0 else c * coeff p (k - n))"
proof -
have "coeff (monom c n * p) k = (∑i≤k. (if n = i then c else 0) * coeff p (k - i))"
also have "… = (∑i≤k. (if n = i then c * coeff p (k - i) else 0))"
by (intro sum.cong) simp_all
also have "… = (if k < n then 0 else c * coeff p (k - n))"
by simp
finally show ?thesis .
qed

lemma monom_1_dvd_iff': "monom 1 n dvd p ⟷ (∀k<n. coeff p k = 0)"
proof
assume "monom 1 n dvd p"
then obtain r where "p = monom 1 n * r"
by (rule dvdE)
then show "∀k<n. coeff p k = 0"
next
assume zero: "(∀k<n. coeff p k = 0)"
define r where "r = Abs_poly (λk. coeff p (k + n))"
have "∀⇩∞k. coeff p (k + n) = 0"
by (subst cofinite_eq_sequentially, subst eventually_sequentially_seg,
subst cofinite_eq_sequentially [symmetric]) transfer
then have coeff_r [simp]: "coeff r k = coeff p (k + n)" for k
unfolding r_def by (subst poly.Abs_poly_inverse) simp_all
have "p = monom 1 n * r"
by (rule poly_eqI, subst coeff_monom_mult) (simp_all add: zero)
then show "monom 1 n dvd p" by simp
qed

subsection ‹Mapping polynomials›

definition map_poly :: "('a :: zero ⇒ 'b :: zero) ⇒ 'a poly ⇒ 'b poly"
where "map_poly f p = Poly (map f (coeffs p))"

lemma map_poly_0 [simp]: "map_poly f 0 = 0"

lemma map_poly_1: "map_poly f 1 = [:f 1:]"

lemma map_poly_1' [simp]: "f 1 = 1 ⟹ map_poly f 1 = 1"

lemma coeff_map_poly:
assumes "f 0 = 0"
shows "coeff (map_poly f p) n = f (coeff p n)"
by (auto simp: assms map_poly_def nth_default_def coeffs_def not_less Suc_le_eq coeff_eq_0
simp del: upt_Suc)

lemma coeffs_map_poly [code abstract]:
"coeffs (map_poly f p) = strip_while ((=) 0) (map f (coeffs p))"

lemma coeffs_map_poly':
assumes "⋀x. x ≠ 0 ⟹ f x ≠ 0"
shows "coeffs (map_poly f p) = map f (coeffs p)"
using assms
by (auto simp add: coeffs_map_poly strip_while_idem_iff
last_coeffs_eq_coeff_degree no_trailing_unfold last_map)

lemma set_coeffs_map_poly:
"(⋀x. f x = 0 ⟷ x = 0) ⟹ set (coeffs (map_poly f p)) = f ` set (coeffs p)"

lemma degree_map_poly:
assumes "⋀x. x ≠ 0 ⟹ f x ≠ 0"
shows "degree (map_poly f p) = degree p"
by (simp add: degree_eq_length_coeffs coeffs_map_poly' assms)

lemma map_poly_eq_0_iff:
assumes "f 0 = 0" "⋀x. x ∈ set (coeffs p) ⟹ x ≠ 0 ⟹ f x ≠ 0"
shows "map_poly f p = 0 ⟷ p = 0"
proof -
have "(coeff (map_poly f p) n = 0) = (coeff p n = 0)" for n
proof -
have "coeff (map_poly f p) n = f (coeff p n)"
also have "… = 0 ⟷ coeff p n = 0"
proof (cases "n < length (coeffs p)")
case True
then have "coeff p n ∈ set (coeffs p)"
by (auto simp: coeffs_def simp del: upt_Suc)
with assms show "f (coeff p n) = 0 ⟷ coeff p n = 0"
by auto
next
case False
then show ?thesis
by (auto simp: assms length_coeffs nth_default_coeffs_eq [symmetric] nth_default_def)
qed
finally show ?thesis .
qed
then show ?thesis by (auto simp: poly_eq_iff)
qed

lemma map_poly_smult:
assumes "f 0 = 0""⋀c x. f (c * x) = f c * f x"
shows "map_poly f (smult c p) = smult (f c) (map_poly f p)"
by (intro poly_eqI) (simp_all add: assms coeff_map_poly)

lemma map_poly_pCons:
assumes "f 0 = 0"
shows "map_poly f (pCons c p) = pCons (f c) (map_poly f p)"
by (intro poly_eqI) (simp_all add: assms coeff_map_poly coeff_pCons split: nat.splits)

lemma map_poly_map_poly:
assumes "f 0 = 0" "g 0 = 0"
shows "map_poly f (map_poly g p) = map_poly (f ∘ g) p"
by (intro poly_eqI) (simp add: coeff_map_poly assms)

lemma map_poly_id [simp]: "map_poly id p = p"

lemma map_poly_id' [simp]: "map_poly (λx. x) p = p"

lemma map_poly_cong:
assumes "(⋀x. x ∈ set (coeffs p) ⟹ f x = g x)"
shows "map_poly f p = map_poly g p"
proof -
from assms have "map f (coeffs p) = map g (coeffs p)"
by (intro map_cong) simp_all
then show ?thesis
by (simp only: coeffs_eq_iff coeffs_map_poly)
qed

lemma map_poly_monom: "f 0 = 0 ⟹ map_poly f (monom c n) = monom (f c) n"
by (intro poly_eqI) (simp_all add: coeff_map_poly)

lemma map_poly_idI:
assumes "⋀x. x ∈ set (coeffs p) ⟹ f x = x"
shows "map_poly f p = p"
using map_poly_cong[OF assms, of _ id] by simp

lemma map_poly_idI':
assumes "⋀x. x ∈ set (coeffs p) ⟹ f x = x"
shows "p = map_poly f p"
using map_poly_cong[OF assms, of _ id] by simp

lemma smult_conv_map_poly: "smult c p = map_poly (λx. c * x) p"
by (intro poly_eqI) (simp_all add: coeff_map_poly)

subsection ‹Conversions›

lemma of_nat_poly:
"of_nat n = [:of_nat n:]"
by (induct n) (simp_all add: one_pCons)

lemma of_nat_monom:
"of_nat n = monom (of_nat n) 0"

lemma degree_of_nat [simp]:
"degree (of_nat n) = 0"

"lead_coeff (of_nat n) = of_nat n"

lemma of_int_poly:
"of_int k = [:of_int k:]"
by (simp only: of_int_of_nat of_nat_poly) simp

lemma of_int_monom:
"of_int k = monom (of_int k) 0"

lemma degree_of_int [simp]:
"degree (of_int k) = 0"

"lead_coeff (of_int k) = of_int k"

lemma numeral_poly: "numeral n = [:numeral n:]"
proof -
have "numeral n = of_nat (numeral n)"
by simp
also have "… = [:of_nat (numeral n):]"
finally show ?thesis
by simp
qed

lemma numeral_monom:
"numeral n = monom (numeral n) 0"

lemma degree_numeral [simp]:
"degree (numeral n) = 0"

"lead_coeff (numeral n) = numeral n"

lemma dvd_smult:
assumes "p dvd q"
shows "p dvd smult a q"
proof -
from assms obtain k where "q = p * k" ..
then have "smult a q = p * smult a k" by simp
then show "p dvd smult a q" ..
qed

lemma dvd_smult_cancel: "p dvd smult a q ⟹ a ≠ 0 ⟹ p dvd q"
for a :: "'a::field"
by (drule dvd_smult [where a="inverse a"]) simp

lemma dvd_smult_iff: "a ≠ 0 ⟹ p dvd smult a q ⟷ p dvd q"
for a :: "'a::field"
by (safe elim!: dvd_smult dvd_smult_cancel)

lemma smult_dvd_cancel:
assumes "smult a p dvd q"
shows "p dvd q"
proof -
from assms obtain k where "q = smult a p * k" ..
then have "q = p * smult a k" by simp
then show "p dvd q" ..
qed

lemma smult_dvd: "p dvd q ⟹ a ≠ 0 ⟹ smult a p dvd q"
for a :: "'a::field"
by (rule smult_dvd_cancel [where a="inverse a"]) simp

lemma smult_dvd_iff: "smult a p dvd q ⟷ (if a = 0 then q = 0 else p dvd q)"
for a :: "'a::field"
by (auto elim: smult_dvd smult_dvd_cancel)

lemma is_unit_smult_iff: "smult c p dvd 1 ⟷ c dvd 1 ∧ p dvd 1"
proof -
have "smult c p = [:c:] * p" by simp
also have "… dvd 1 ⟷ c dvd 1 ∧ p dvd 1"
proof safe
assume *: "[:c:] * p dvd 1"
then show "p dvd 1"
by (rule dvd_mult_right)
from * obtain q where q: "1 = [:c:] * p * q"
by (rule dvdE)
have "c dvd c * (coeff p 0 * coeff q 0)"
by simp
also have "… = coeff ([:c:] * p * q) 0"
also note q [symmetric]
finally have "c dvd coeff 1 0" .
then show "c dvd 1" by simp
next
assume "c dvd 1" "p dvd 1"
from this(1) obtain d where "1 = c * d"
by (rule dvdE)
then have "1 = [:c:] * [:d:]"
then have "[:c:] dvd 1"
by (rule dvdI)
from mult_dvd_mono[OF this ‹p dvd 1›] show "[:c:] * p dvd 1"
by simp
qed
finally show ?thesis .
qed

subsection ‹Polynomials form an integral domain›

instance poly :: (idom) idom ..

instance poly :: ("{ring_char_0, comm_ring_1}") ring_char_0
by standard (auto simp add: of_nat_poly intro: injI)

lemma degree_mult_eq: "p ≠ 0 ⟹ q ≠ 0 ⟹ degree (p * q) = degree p + degree q"
for p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
by (rule order_antisym [OF degree_mult_le le_degree]) (simp add: coeff_mult_degree_sum)

lemma dvd_imp_degree:
‹degree x ≤ degree y› if ‹x dvd y› ‹x ≠ 0› ‹y ≠ 0›
for x y :: ‹'a::{comm_semiring_1,semiring_no_zero_divisors} poly›
proof -
from ‹x dvd y› obtain z where ‹y = x * z› ..
with ‹x ≠ 0› ‹y ≠ 0› show ?thesis
qed

lemma degree_prod_eq_sum_degree:
fixes A :: "'a set"
and f :: "'a ⇒ 'b::idom poly"
assumes f0: "∀i∈A. f i ≠ 0"
shows "degree (∏i∈A. (f i)) = (∑i∈A. degree (f i))"
using assms
by (induction A rule: infinite_finite_induct) (auto simp: degree_mult_eq)

lemma degree_mult_eq_0:
"degree (p * q) = 0 ⟷ p = 0 ∨ q = 0 ∨ (p ≠ 0 ∧ q ≠ 0 ∧ degree p = 0 ∧ degree q = 0)"
for p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
by (auto simp: degree_mult_eq)

lemma degree_power_eq: "p ≠ 0 ⟹ degree ((p :: 'a :: idom poly) ^ n) = n * degree p"
by (induction n) (simp_all add: degree_mult_eq)

lemma degree_mult_right_le:
fixes p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
assumes "q ≠ 0"
shows "degree p ≤ degree (p * q)"
using assms by (cases "p = 0") (simp_all add: degree_mult_eq)

lemma coeff_degree_mult: "coeff (p * q) (degree (p * q)) = coeff q (degree q) * coeff p (degree p)"
for p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
by (cases "p = 0 ∨ q = 0") (auto simp: degree_mult_eq coeff_mult_degree_sum mult_ac)

lemma dvd_imp_degree_le: "p dvd q ⟹ q ≠ 0 ⟹ degree p ≤ degree q"
for p q :: "'a::{comm_semiring_1,semiring_no_zero_divisors} poly"
by (erule dvdE, hypsubst, subst degree_mult_eq) auto

lemma divides_degree:
fixes p q :: "'a ::{comm_semiring_1,semiring_no_zero_divisors} poly"
assumes "p dvd q"
shows "degree p ≤ degree q ∨ q = 0"
by (metis dvd_imp_degree_le assms)

lemma const_poly_dvd_iff:
fixes c :: "'a::{comm_semiring_1,semiring_no_zero_divisors}"
shows "[:c:] dvd p ⟷ (∀n. c dvd coeff p n)"
proof (cases "c = 0 ∨ p = 0")
case True
then show ?thesis
by (auto intro!: poly_eqI)
next
case False
show ?thesis
proof
assume "[:c:] dvd p"
then show "∀n. c dvd coeff p n"
by (auto simp: coeffs_def)
next
assume *: "∀n. c dvd coeff p n"
define mydiv where "mydiv x y = (SOME z. x = y * z)" for x y :: 'a
have mydiv: "x = y * mydiv x y" if "y dvd x" for x y
using that unfolding mydiv_def dvd_def by (rule someI_ex)
define q where "q = Poly (map (λa. mydiv a c) (coeffs p))"
from False * have "p = q * [:c:]"
by (intro poly_eqI)
(auto simp: q_def nth_default_def not_less length_coeffs_degree coeffs_nth
intro!: coeff_eq_0 mydiv)
then show "[:c:] dvd p"
by (simp only: dvd_triv_right)
qed
qed

lemma const_poly_dvd_const_poly_iff [simp]: "[:a:] dvd [:b:] ⟷ a dvd b"
for a b :: "'a::{comm_semiring_1,semiring_no_zero_divisors}"
by (subst const_poly_dvd_iff) (auto simp: coeff_pCons split: nat.splits)

for p q :: "'a::{comm_semiring_0, semiring_no_zero_divisors} poly"
by (cases "p = 0 ∨ q = 0") (auto simp: coeff_mult_degree_sum degree_mult_eq)

for f :: "'a ⇒ 'b::{comm_semiring_1, semiring_no_zero_divisors} poly"
by (induction A rule: infinite_finite_induct) (auto simp: lead_coeff_mult)

for p :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
proof -
have "smult c p = [:c:] * p" by simp
finally show ?thesis .
qed

by simp

for p :: "'a::{comm_semiring_1,semiring_no_zero_divisors} poly"

subsection ‹Polynomials form an ordered integral domain›

definition pos_poly :: "'a::linordered_semidom poly ⇒ bool"
where "pos_poly p ⟷ 0 < coeff p (degree p)"

lemma pos_poly_pCons: "pos_poly (pCons a p) ⟷ pos_poly p ∨ (p = 0 ∧ 0 < a)"

lemma not_pos_poly_0 [simp]: "¬ pos_poly 0"

lemma pos_poly_add: "pos_poly p ⟹ pos_poly q ⟹ pos_poly (p + q)"
proof (induction p arbitrary: q)
case (pCons a p)
then show ?case
qed auto

lemma pos_poly_mult: "pos_poly p ⟹ pos_poly q ⟹ pos_poly (p * q)"

lemma pos_poly_total: "p = 0 ∨ pos_poly p ∨ pos_poly (- p)"
for p :: "'a::linordered_idom poly"
by (induct p) (auto simp: pos_poly_pCons)

lemma pos_poly_coeffs [code]: "pos_poly p ⟷ (let as = coeffs p in as ≠ [] ∧ last as > 0)"
(is "?lhs ⟷ ?rhs")
proof
assume ?rhs
then show ?lhs
by (auto simp add: pos_poly_def last_coeffs_eq_coeff_degree)
next
assume ?lhs
then have *: "0 < coeff p (degree p)"
then have "p ≠ 0"
by auto
with * show ?rhs
qed

instantiation poly :: (linordered_idom) linordered_idom
begin

definition "x < y ⟷ pos_poly (y - x)"

definition "x ≤ y ⟷ x = y ∨ pos_poly (y - x)"

definition "¦x::'a poly¦ = (if x < 0 then - x else x)"

definition "sgn (x::'a poly) = (if x = 0 then 0 else if 0 < x then 1 else - 1)"

instance
proof
fix x y z :: "'a poly"
show "x < y ⟷ x ≤ y ∧ ¬ y ≤ x"
unfolding less_eq_poly_def less_poly_def
then show "x ≤ y ⟹ y ≤ x ⟹ x = y"
using less_eq_poly_def less_poly_def by force
show "x ≤ x"
show "x ≤ y ⟹ y ≤ z ⟹ x ≤ z"
show "x ≤ y ⟹ z + x ≤ z + y"
show "x ≤ y ∨ y ≤ x"
unfolding less_eq_poly_def
using pos_poly_total [of "x - y"]
by auto
show "x < y ⟹ 0 < z ⟹ z * x < z * y"
by (simp add: less_poly_def right_diff_distrib [symmetric] pos_poly_mult)
show "¦x¦ = (if x < 0 then - x else x)"
by (rule abs_poly_def)
show "sgn x = (if x = 0 then 0 else if 0 < x then 1 else - 1)"
by (rule sgn_poly_def)
qed

end

text ‹TODO: Simplification rules for comparisons›

subsection ‹Synthetic division and polynomial roots›

subsubsection ‹Synthetic division›

text ‹Synthetic division is simply division by the linear polynomial \<^term>‹x - c›.›

definition synthetic_divmod :: "'a::comm_semiring_0 poly ⇒ 'a ⇒ 'a poly × 'a"
where "synthetic_divmod p c = fold_coeffs (λa (q, r). (pCons r q, a + c * r)) p (0, 0)"

definition synthetic_div :: "'a::comm_semiring_0 poly ⇒ 'a ⇒ 'a poly"
where "synthetic_div p c = fst (synthetic_divmod p c)"

lemma synthetic_divmod_0 [simp]: "synthetic_divmod 0 c = (0, 0)"