# Theory HOL-Computational_Algebra.Polynomial_Factorial

(*  Title:      HOL/Computational_Algebra/Polynomial_Factorial.thy
Author:     Manuel Eberl
*)

section ‹Polynomials, fractions and rings›

theory Polynomial_Factorial
imports
Complex_Main
Polynomial
Normalized_Fraction
begin

subsection ‹Lifting elements into the field of fractions›

definition to_fract :: "'a :: idom  'a fract"
where "to_fract x = Fract x 1"
― ‹FIXME: more idiomatic name, abbreviation›

lemma to_fract_0 [simp]:
by (simp add: to_fract_def eq_fract Zero_fract_def)

lemma to_fract_1 [simp]:
by (simp add: to_fract_def eq_fract One_fract_def)

lemma to_fract_add [simp]: "to_fract (x + y) = to_fract x + to_fract y"

lemma to_fract_diff [simp]: "to_fract (x - y) = to_fract x - to_fract y"

lemma to_fract_uminus [simp]: "to_fract (-x) = -to_fract x"

lemma to_fract_mult [simp]: "to_fract (x * y) = to_fract x * to_fract y"

lemma to_fract_eq_iff [simp]: "to_fract x = to_fract y  x = y"

lemma to_fract_eq_0_iff [simp]: "to_fract x = 0  x = 0"
by (simp add: to_fract_def Zero_fract_def eq_fract)

lemma to_fract_quot_of_fract:
assumes "snd (quot_of_fract x) = 1"
shows   "to_fract (fst (quot_of_fract x)) = x"
proof -
have "x = Fract (fst (quot_of_fract x)) (snd (quot_of_fract x))" by simp
also note assms
finally show ?thesis by (simp add: to_fract_def)
qed

lemma Fract_conv_to_fract: "Fract a b = to_fract a / to_fract b"

lemma quot_of_fract_to_fract [simp]: "quot_of_fract (to_fract x) = (x, 1)"
unfolding to_fract_def by transfer (simp add: normalize_quot_def)

lemma snd_quot_of_fract_to_fract [simp]: "snd (quot_of_fract (to_fract x)) = 1"
unfolding to_fract_def by (rule snd_quot_of_fract_Fract_whole) simp_all

subsection ‹Lifting polynomial coefficients to the field of fractions›

abbreviation (input) fract_poly :: 'a::idom poly  'a fract poly
where "fract_poly  map_poly to_fract"

abbreviation (input) unfract_poly ::
where "unfract_poly  map_poly (fst  quot_of_fract)"

lemma fract_poly_smult [simp]: "fract_poly (smult c p) = smult (to_fract c) (fract_poly p)"
by (simp add: smult_conv_map_poly map_poly_map_poly o_def)

lemma fract_poly_0 [simp]:

lemma fract_poly_1 [simp]:

by (intro poly_eqI) (simp_all add: coeff_map_poly)

lemma fract_poly_diff [simp]:

by (intro poly_eqI) (simp_all add: coeff_map_poly)

lemma to_fract_sum [simp]: "to_fract (sum f A) = sum (λx. to_fract (f x)) A"
by (cases "finite A", induction A rule: finite_induct) simp_all

lemma fract_poly_mult [simp]:

by (intro poly_eqI) (simp_all add: coeff_map_poly coeff_mult)

lemma fract_poly_eq_iff [simp]: "fract_poly p = fract_poly q  p = q"
by (auto simp: poly_eq_iff coeff_map_poly)

lemma fract_poly_eq_0_iff [simp]: "fract_poly p = 0  p = 0"
using fract_poly_eq_iff[of p 0] by (simp del: fract_poly_eq_iff)

lemma fract_poly_dvd: "p dvd q  fract_poly p dvd fract_poly q"
by auto

lemma prod_mset_fract_poly:
"(x∈#A. map_poly to_fract (f x)) = fract_poly (prod_mset (image_mset f A))"
by (induct A) (simp_all add: ac_simps)

lemma is_unit_fract_poly_iff:

proof safe
assume A: "p dvd 1"
with fract_poly_dvd [of p 1] show "is_unit (fract_poly p)"
by simp
from A show "content p = 1"
by (auto simp: is_unit_poly_iff normalize_1_iff)
next
assume A:  and B: "content p = 1"
from A obtain c where c: "fract_poly p = [:c:]" by (auto simp: is_unit_poly_iff)
{
fix n :: nat assume "n > 0"
have "to_fract (coeff p n) = coeff (fract_poly p) n" by (simp add: coeff_map_poly)
also note c
also from n > 0 have "coeff [:c:] n = 0" by (simp add: coeff_pCons split: nat.splits)
finally have "coeff p n = 0" by simp
}
hence "degree p  0" by (intro degree_le) simp_all
with B show "p dvd 1" by (auto simp: is_unit_poly_iff normalize_1_iff elim!: degree_eq_zeroE)
qed

lemma fract_poly_is_unit: "p dvd 1  fract_poly p dvd 1"
using fract_poly_dvd[of p 1] by simp

lemma fract_poly_smult_eqE:
fixes c ::
assumes "fract_poly p = smult c (fract_poly q)"
obtains a b
where "c = to_fract b / to_fract a" "smult a p = smult b q" "coprime a b" "normalize a = a"
proof -
define a b where "a = fst (quot_of_fract c)" and "b = snd (quot_of_fract c)"
have "smult (to_fract a) (fract_poly q) = smult (to_fract b) (fract_poly p)"
by (subst smult_eq_iff) (simp_all add: a_def b_def Fract_conv_to_fract [symmetric] assms)
hence "fract_poly (smult a q) = fract_poly (smult b p)" by (simp del: fract_poly_eq_iff)
hence "smult b p = smult a q" by (simp only: fract_poly_eq_iff)
moreover have "c = to_fract a / to_fract b" "coprime b a" "normalize b = b"
by (simp_all add: a_def b_def coprime_quot_of_fract [of c] ac_simps
normalize_snd_quot_of_fract Fract_conv_to_fract [symmetric])
ultimately show ?thesis by (intro that[of a b])
qed

subsection ‹Fractional content›

abbreviation (input) Lcm_coeff_denoms
::
where "Lcm_coeff_denoms p  Lcm (snd ` quot_of_fract ` set (coeffs p))"

definition fract_content ::
where
"fract_content p =
(let d = Lcm_coeff_denoms p in Fract (content (unfract_poly (smult (to_fract d) p))) d)"

definition primitive_part_fract ::
where
"primitive_part_fract p =
primitive_part (unfract_poly (smult (to_fract (Lcm_coeff_denoms p)) p))"

lemma primitive_part_fract_0 [simp]:

lemma fract_content_eq_0_iff [simp]:

unfolding fract_content_def Let_def Zero_fract_def
by (subst eq_fract) (auto simp: Lcm_0_iff map_poly_eq_0_iff)

lemma content_primitive_part_fract [simp]:
fixes p ::
shows
unfolding primitive_part_fract_def
by (rule content_primitive_part)
(auto simp: primitive_part_fract_def map_poly_eq_0_iff Lcm_0_iff)

lemma content_times_primitive_part_fract:

proof -
define p' where "p' = unfract_poly (smult (to_fract (Lcm_coeff_denoms p)) p)"
have
unfolding primitive_part_fract_def p'_def
by (subst map_poly_map_poly) (simp_all add: o_assoc)
also have " = smult (to_fract (Lcm_coeff_denoms p)) p"
proof (intro map_poly_idI, unfold o_apply)
fix c assume "c  set (coeffs (smult (to_fract (Lcm_coeff_denoms p)) p))"
then obtain c' where c: "c'  set (coeffs p)" "c = to_fract (Lcm_coeff_denoms p) * c'"
by (auto simp add: Lcm_0_iff coeffs_smult split: if_splits)
note c(2)
also have "c' = Fract (fst (quot_of_fract c')) (snd (quot_of_fract c'))"
by simp
also have
unfolding to_fract_def by (subst mult_fract) simp_all
also have "snd (quot_of_fract ) = 1"
by (intro snd_quot_of_fract_Fract_whole dvd_mult2 dvd_Lcm) (insert c(1), auto)
finally show "to_fract (fst (quot_of_fract c)) = c"
by (rule to_fract_quot_of_fract)
qed
also have "p' = smult (content p') (primitive_part p')"
by (rule content_times_primitive_part [symmetric])
also have
also have  by simp
finally have  .
thus ?thesis
by (subst (asm) smult_eq_iff)
(auto simp add: Let_def p'_def Fract_conv_to_fract field_simps Lcm_0_iff fract_content_def)
qed

lemma fract_content_fract_poly [simp]:
proof -
have
by (auto simp: set_coeffs_map_poly)
hence
by (simp add: fract_content_def to_fract_def fract_collapse map_poly_map_poly del: Lcm_1_iff)
also have
by (intro map_poly_idI) simp_all
finally show ?thesis .
qed

lemma content_decompose_fract:
fixes p ::
obtains c p' where "p = smult c (map_poly to_fract p')" "content p' = 1"
proof (cases "p = 0")
case True
hence   by simp_all
thus ?thesis ..
next
case False
thus ?thesis
by (rule that[OF content_times_primitive_part_fract [symmetric] content_primitive_part_fract])
qed

lemma fract_poly_dvdD:
fixes p ::
assumes  "content p = 1"
shows   "p dvd q"
proof -
from assms(1) obtain r where r:  by (erule dvdE)
from content_decompose_fract[of r]
obtain c r' where r': "r = smult c (map_poly to_fract r')" "content r' = 1" .
from r r' have eq: "fract_poly q = smult c (fract_poly (p * r'))" by simp
from fract_poly_smult_eqE[OF this] obtain a b
where ab:
"c = to_fract b / to_fract a"
"smult a q = smult b (p * r')"
"coprime a b"
"normalize a = a" .
have "content (smult a q) = content (smult b (p * r'))" by (simp only: ab(2))
hence eq': "normalize b = a * content q" by (simp add: assms content_mult r' ab(4))
have "1 = gcd a (normalize b)" by (simp add: ab)
also note eq'
also have "gcd a (a * content q) = a" by (simp add: gcd_proj1_if_dvd ab(4))
finally have [simp]: "a = 1" by simp
from eq ab have "q = p * ([:b:] * r')" by simp
thus ?thesis by (rule dvdI)
qed

subsection ‹Polynomials over a field are a Euclidean ring›

context
begin

interpretation field_poly:
normalization_euclidean_semiring_multiplicative where zero = "0 :: 'a :: field poly"
and one = 1 and plus = plus and minus = minus
and times = times
and normalize = "λp. smult (inverse (lead_coeff p)) p"
and unit_factor = "λp. [:lead_coeff p:]"
and euclidean_size = "λp. if p = 0 then 0 else 2 ^ degree p"
and divide = divide and modulo = modulo
rewrites "dvd.dvd (times :: 'a poly  _) = Rings.dvd"
and
and
and
proof -
show "dvd.dvd (times :: 'a poly  _) = Rings.dvd"
show
show
show
show "class.normalization_euclidean_semiring_multiplicative divide plus minus (0 :: 'a poly) times 1
modulo (λp. if p = 0 then 0 else 2 ^ degree p)
proof (standard, fold dvd_dict)
fix p :: "'a poly"
by (cases "p = 0") simp_all
next
fix p :: "'a poly" assume "is_unit p"
then show "[:lead_coeff p:] = p"
by (elim is_unit_polyE) (auto simp: monom_0 one_poly_def field_simps)
next
fix p :: "'a poly" assume "p  0"
next
fix a b :: "'a poly" assume "is_unit a"
by (auto elim!: is_unit_polyE)
qed (auto simp: lead_coeff_mult Rings.div_mult_mod_eq intro!: degree_mod_less' degree_mult_right_le)
qed

lemma field_poly_irreducible_imp_prime:
"prime_elem p" if "irreducible p" for p :: "'a :: field poly"
using that by (fact field_poly.irreducible_imp_prime_elem)

lemma field_poly_prod_mset_prime_factorization:

if "p  0" for p :: "'a :: field poly"
using that by (fact field_poly.prod_mset_prime_factorization)

lemma field_poly_in_prime_factorization_imp_prime:
"prime_elem p" if
for p :: "'a :: field poly"
by (rule field_poly.prime_imp_prime_elem, rule field_poly.in_prime_factors_imp_prime)
(fact that)

subsection ‹Primality and irreducibility in polynomial rings›

lemma nonconst_poly_irreducible_iff:
fixes p ::
assumes "degree p  0"
shows
proof safe
assume p: "irreducible p"

from content_decompose[of p] obtain p' where p': "p = smult (content p) p'" "content p' = 1" .
hence "p = [:content p:] * p'" by simp
from p this have "[:content p:] dvd 1  p' dvd 1" by (rule irreducibleD)
moreover have "¬p' dvd 1"
proof
assume "p' dvd 1"
hence "degree p = 0" by (subst p') (auto simp: is_unit_poly_iff)
with assms show False by contradiction
qed
ultimately show [simp]: "content p = 1" by (simp add: is_unit_const_poly_iff)

show
proof (rule irreducibleI)
have "fract_poly p = 0  p = 0" by (intro map_poly_eq_0_iff) auto
with assms show  by auto
next
show "¬is_unit (fract_poly p)"
proof
assume
hence
by (auto simp: is_unit_poly_iff)
hence "degree p = 0" by (simp add: degree_map_poly)
with assms show False by contradiction
qed
next
fix q r assume qr: "fract_poly p = q * r"
from content_decompose_fract[of q]
obtain cg q' where q: "q = smult cg (map_poly to_fract q')" "content q' = 1" .
from content_decompose_fract[of r]
obtain cr r' where r: "r = smult cr (map_poly to_fract r')" "content r' = 1" .
from qr q r p have nz: "cg  0" "cr  0" by auto
from qr have eq: "fract_poly p = smult (cr * cg) (fract_poly (q' * r'))"
from fract_poly_smult_eqE[OF this] obtain a b
where ab: "cr * cg = to_fract b / to_fract a"
"smult a p = smult b (q' * r')" "coprime a b" "normalize a = a" .
hence "content (smult a p) = content (smult b (q' * r'))" by (simp only:)
with ab(4) have a: "a = normalize b" by (simp add: content_mult q r)
then have "normalize b = gcd a b"
by simp
with coprime a b have "normalize b = 1"
by simp
then have "a = 1" "is_unit b"

note eq
also from ab(1) a = 1 have "cr * cg = to_fract b" by simp
also have "smult  (fract_poly (q' * r')) = fract_poly (smult b (q' * r'))" by simp
finally have "p = ([:b:] * q') * r'" by (simp del: fract_poly_smult)
from p and this have "([:b:] * q') dvd 1  r' dvd 1" by (rule irreducibleD)
hence "q' dvd 1  r' dvd 1" by (auto dest: dvd_mult_right simp del: mult_pCons_left)
hence  by (auto simp: fract_poly_is_unit)
with q r show "is_unit q  is_unit r"
by (auto simp add: is_unit_smult_iff dvd_field_iff nz)
qed

next

assume irred:  and primitive: "content p = 1"
show "irreducible p"
proof (rule irreducibleI)
from irred show "p  0" by auto
next
from irred show "¬p dvd 1"
by (auto simp: irreducible_def dest: fract_poly_is_unit)
next
fix q r assume qr: "p = q * r"
hence  by simp
from irred and this have
by (rule irreducibleD)
with primitive qr show "q dvd 1  r dvd 1"
by (auto simp:  content_prod_eq_1_iff is_unit_fract_poly_iff)
qed
qed

lemma irreducible_imp_prime_poly:
fixes p ::
assumes "irreducible p"
shows   "prime_elem p"
proof (cases "degree p = 0")
case True
with assms show ?thesis
by (auto simp: prime_elem_const_poly_iff irreducible_const_poly_iff
intro!: irreducible_imp_prime_elem elim!: degree_eq_zeroE)
next
case False
from assms False have irred:  and primitive: "content p = 1"
from irred have prime: "prime_elem (fract_poly p)" by (rule field_poly_irreducible_imp_prime)
show ?thesis
proof (rule prime_elemI)
fix q r assume "p dvd q * r"
hence "fract_poly p dvd fract_poly (q * r)" by (rule fract_poly_dvd)
hence  by simp
from prime and this have
by (rule prime_elem_dvd_multD)
with primitive show "p dvd q  p dvd r" by (auto dest: fract_poly_dvdD)
qed (insert assms, auto simp: irreducible_def)
qed

lemma degree_primitive_part_fract [simp]:

proof -
have
also have
by (auto simp: degree_map_poly)
finally show ?thesis ..
qed

lemma irreducible_primitive_part_fract:
fixes p ::
assumes "irreducible p"
shows
proof -
from assms have deg:
by (intro notI)
(auto elim!: degree_eq_zeroE simp: irreducible_def is_unit_poly_iff dvd_field_iff)
hence [simp]: "p  0" by auto

note irreducible p
also have
also have
by (intro irreducible_mult_unit_left) (simp_all add: is_unit_poly_iff dvd_field_iff)
finally show ?thesis using deg
qed

lemma prime_elem_primitive_part_fract:
fixes p ::
shows
by (intro irreducible_imp_prime_poly irreducible_primitive_part_fract)

lemma irreducible_linear_field_poly:
fixes a b :: "'a::field"
assumes "b  0"
shows "irreducible [:a,b:]"
proof (rule irreducibleI)
fix p q assume pq: "[:a,b:] = p * q"
also from pq assms have "degree  = degree p + degree q"
by (intro degree_mult_eq) auto
finally have  using assms by auto
with assms pq show "is_unit p  is_unit q"
by (auto simp: is_unit_const_poly_iff dvd_field_iff elim!: degree_eq_zeroE)
qed (insert assms, auto simp: is_unit_poly_iff)

lemma prime_elem_linear_field_poly:
"(b :: 'a :: field)  0  prime_elem [:a,b:]"
by (rule field_poly_irreducible_imp_prime, rule irreducible_linear_field_poly)

lemma irreducible_linear_poly:
fixes a b ::
shows "b  0  coprime a b  irreducible [:a,b:]"
by (auto intro!: irreducible_linear_field_poly
simp:   nonconst_poly_irreducible_iff content_def map_poly_pCons)

lemma prime_elem_linear_poly:
fixes a b ::
shows "b  0  coprime a b  prime_elem [:a,b:]"
by (rule irreducible_imp_prime_poly, rule irreducible_linear_poly)

subsection ‹Prime factorisation of polynomials›

lemma poly_prime_factorization_exists_content_1:
fixes p ::
assumes "p  0" "content p = 1"
shows   "A. (p. p ∈# A  prime_elem p)  prod_mset A = normalize p"
proof -
let ?P =
define c where
define c' where "c' = c * to_fract (lead_coeff p)"
define e where
define A where
have
by (simp add: e_def content_prod_mset multiset.map_comp o_def)
also have "image_mset (λx. content (primitive_part_fract x)) ?P = image_mset (λ_. 1) ?P"
by (intro image_mset_cong content_primitive_part_fract) auto
finally have content_e: "content e = 1"
by simp

from p  0 have "fract_poly p = [:lead_coeff (fract_poly p):] *
smult (inverse (lead_coeff (fract_poly p))) (fract_poly p)"
by simp
by (simp add: monom_0 degree_map_poly coeff_map_poly)
also from assms have "smult (inverse (lead_coeff (fract_poly p))) (fract_poly p) = prod_mset ?P"
by (subst field_poly_prod_mset_prime_factorization) simp_all
also have " = prod_mset (image_mset id ?P)" by simp
also have "image_mset id ?P =
image_mset (λx. [:fract_content x:] * fract_poly (primitive_part_fract x)) ?P"
by (intro image_mset_cong) (auto simp: content_times_primitive_part_fract)
also have "prod_mset  = smult c (fract_poly e)"
by (subst prod_mset.distrib) (simp_all add: prod_mset_fract_poly prod_mset_const_poly c_def e_def)
also have "[:to_fract (lead_coeff p):] *  = smult c' (fract_poly e)"
finally have eq: "fract_poly p = smult c' (fract_poly e)" .
also obtain b where b: "c' = to_fract b" "is_unit b"
proof -
from fract_poly_smult_eqE[OF eq]
obtain a b where ab:
"c' = to_fract b / to_fract a"
"smult a p = smult b e"
"coprime a b"
"normalize a = a" .
from ab(2) have "content (smult a p) = content (smult b e)" by (simp only: )
with assms content_e have "a = normalize b" by (simp add: ab(4))
with ab have ab': "a = 1" "is_unit b"
with ab ab' have "c' = to_fract b" by auto
from this and is_unit b show ?thesis by (rule that)
qed
hence "smult c' (fract_poly e) = fract_poly (smult b e)" by simp
finally have "p = smult b e" by (simp only: fract_poly_eq_iff)
hence "p = [:b:] * e" by simp
with b have
by (simp only: normalize_mult) (simp add: is_unit_normalize is_unit_poly_iff)
also have
by (simp add: multiset.map_comp e_def A_def normalize_prod_mset)
finally have  ..

have "prime_elem p" if "p ∈# A" for p
using that by (auto simp: A_def prime_elem_primitive_part_fract prime_elem_imp_irreducible
dest!: field_poly_in_prime_factorization_imp_prime )
from this and  show ?thesis
by (intro exI[of _ A]) blast
qed

lemma poly_prime_factorization_exists:
fixes p ::
assumes "p  0"
shows   "A. (p. p ∈# A  prime_elem p)  normalize (prod_mset A) = normalize p"
proof -
define B where "B = image_mset (λx. [:x:]) (prime_factorization (content p))"
have "A. (p. p ∈# A  prime_elem p)  prod_mset A = normalize (primitive_part p)"
by (rule poly_prime_factorization_exists_content_1) (insert assms, simp_all)
then obtain A where A: "p. p ∈# A  prime_elem p"
by blast
have
by simp
also from assms have "normalize (prod_mset B) = normalize [:content p:]"
by (simp add: prod_mset_const_poly normalize_const_poly prod_mset_prime_factorization_weak B_def)
also have
using A by simp
finally have "normalize (prod_mset (A + B)) = normalize (primitive_part p * [:content p:])"
by simp
moreover have "p. p ∈# B  prime_elem p"
by (auto simp: B_def intro!: lift_prime_elem_poly dest: in_prime_factors_imp_prime)
ultimately show ?thesis using A by (intro exI[of _ "A  B"]) (auto)
qed

end

subsection ‹Typeclass instances›

instance poly :: () factorial_semiring
by standard (rule poly_prime_factorization_exists)

instantiation poly :: () factorial_ring_gcd
begin

definition gcd_poly :: "'a poly  'a poly  'a poly" where
[code del]: "gcd_poly = gcd_factorial"

definition lcm_poly :: "'a poly  'a poly  'a poly" where
[code del]: "lcm_poly = lcm_factorial"

definition Gcd_poly :: "'a poly set  'a poly" where
[code del]: "Gcd_poly = Gcd_factorial"

definition Lcm_poly :: "'a poly set  'a poly" where
[code del]: "Lcm_poly = Lcm_factorial"

instance by standard (simp_all add: gcd_poly_def lcm_poly_def Gcd_poly_def Lcm_poly_def)

end

instance poly :: () semiring_gcd_mult_normalize ..

instance poly :: ()
..

instance poly :: () euclidean_ring_gcd
by (rule euclidean_ring_gcd_class.intro, rule factorial_euclidean_semiring_gcdI) standard

instance poly :: () factorial_semiring_multiplicative ..

subsection ‹Polynomial GCD›

lemma gcd_poly_decompose:
fixes p q ::
shows "gcd p q =
smult (gcd (content p) (content q)) (gcd (primitive_part p) (primitive_part q))"
proof (rule sym, rule gcdI)
have "[:gcd (content p) (content q):] * gcd (primitive_part p) (primitive_part q) dvd
[:content p:] * primitive_part p" by (intro mult_dvd_mono) simp_all
thus "smult (gcd (content p) (content q)) (gcd (primitive_part p) (primitive_part q)) dvd p"
by simp
next
have "[:gcd (content p) (content q):] * gcd (primitive_part p) (primitive_part q) dvd
[:content q:] * primitive_part q" by (intro mult_dvd_mono) simp_all
thus "smult (gcd (content p) (content q)) (gcd (primitive_part p) (primitive_part q)) dvd q"
by simp
next
fix d assume "d dvd p" "d dvd q"
hence "[:content d:] * primitive_part d dvd
[:gcd (content p) (content q):] * gcd (primitive_part p) (primitive_part q)"
by (intro mult_dvd_mono) auto
thus "d dvd smult (gcd (content p) (content q)) (gcd (primitive_part p) (primitive_part q))"
by simp
qed (auto simp: normalize_smult)

lemma gcd_poly_pseudo_mod:
fixes p q ::
assumes nz: "q  0" and prim: "content p = 1" "content q = 1"
shows   "gcd p q = gcd q (primitive_part (pseudo_mod p q))"
proof -
define r s where "r = fst (pseudo_divmod p q)" and "s = snd (pseudo_divmod p q)"
define a where "a = [:coeff q (degree q) ^ (Suc (degree p) - degree q):]"
have [simp]:
by (simp add: a_def unit_factor_poly_def unit_factor_power monom_0)
from nz have [simp]: "a  0" by (auto simp: a_def)

have rs: "pseudo_divmod p q = (r, s)" by (simp add: r_def s_def)
have "gcd (q * r + s) q = gcd q s"
with pseudo_divmod(1)[OF nz rs]
have "gcd (p * a) q = gcd q s" by (simp add: a_def)
also from prim have "gcd (p * a) q = gcd p q"
by (subst gcd_poly_decompose)
(auto simp: primitive_part_mult gcd_mult_unit1 primitive_part_prim
simp del: mult_pCons_right )
also from prim have "gcd q s = gcd q (primitive_part s)"
by (subst gcd_poly_decompose) (simp_all add: primitive_part_prim)
also have "s = pseudo_mod p q" by (simp add: s_def pseudo_mod_def)
finally show ?thesis .
qed

lemma degree_pseudo_mod_less:
assumes "q  0" "pseudo_mod p q  0"
shows   "degree (pseudo_mod p q) < degree q"
using pseudo_mod(2)[of q p] assms by auto

function gcd_poly_code_aux :: "'a :: factorial_ring_gcd poly  'a poly  'a poly" where
"gcd_poly_code_aux p q =
(if q = 0 then normalize p else gcd_poly_code_aux q (primitive_part (pseudo_mod p q)))"
by auto
termination
by (relation "measure ((λp. if p = 0 then 0 else Suc (degree p))  snd)")
(auto simp: degree_pseudo_mod_less)

declare gcd_poly_code_aux.simps [simp del]

lemma gcd_poly_code_aux_correct:
assumes "content p = 1" "q = 0  content q = 1"
shows   "gcd_poly_code_aux p q = gcd p q"
using assms
proof (induction p q rule: gcd_poly_code_aux.induct)
case (1 p q)
show ?case
proof (cases "q = 0")
case True
thus ?thesis by (subst gcd_poly_code_aux.simps) auto
next
case False
hence
by (subst gcd_poly_code_aux.simps) simp_all
also from "1.prems" False
have
by (cases "pseudo_mod p q = 0") auto
with "1.prems" False
have
by (intro 1) simp_all
also from "1.prems" False
have " = gcd p q" by (intro gcd_poly_pseudo_mod [symmetric]) auto
finally show ?thesis .
qed
qed

definition gcd_poly_code
:: "'a :: factorial_ring_gcd poly  'a poly  'a poly"
where "gcd_poly_code p q =
(if p = 0 then normalize q else if q = 0 then normalize p else
smult (gcd (content p) (content q))
(gcd_poly_code_aux (primitive_part p) (primitive_part q)))"

lemma gcd_poly_code [code]: "gcd p q = gcd_poly_code p q"
by (simp add: gcd_poly_code_def gcd_poly_code_aux_correct gcd_poly_decompose [symmetric])

lemma lcm_poly_code [code]:
fixes p q ::
shows "lcm p q = normalize (p * q div gcd p q)"
by (fact lcm_gcd)

lemmas Gcd_poly_set_eq_fold [code] =
Gcd_set_eq_fold [where ?'a = ]
lemmas Lcm_poly_set_eq_fold [code] =
Lcm_set_eq_fold [where ?'a = ]

text ‹Example:
@{lemma "Lcm {[:1, 2, 3:], [:2, 3, 4:]} = [:[:2:], [:7:], [:16:], [:17:], [:12 :: int:]:]" by eval}

end