Theory HOL-Algebra.Sylow
theory Sylow
imports Coset Exponent
begin
text ‹See also \<^cite>‹"Kammueller-Paulson:1999"›.›
text ‹The combinatorial argument is in theory ‹Exponent›.›
lemma le_extend_mult: "⟦0 < c; a ≤ b⟧ ⟹ a ≤ b * c" for c :: nat
using gr0_conv_Suc by fastforce
locale sylow = group +
fixes p and a and m and calM and RelM
assumes prime_p: "prime p"
and order_G: "order G = (p^a) * m"
and finite_G[iff]: "finite (carrier G)"
defines "calM ≡ {s. s ⊆ carrier G ∧ card s = p^a}"
and "RelM ≡ {(N1, N2). N1 ∈ calM ∧ N2 ∈ calM ∧ (∃g ∈ carrier G. N1 = N2 #> g)}"
begin
lemma RelM_subset: "RelM ⊆ calM × calM"
by (auto simp only: RelM_def)
lemma RelM_refl_on: "refl_on calM RelM"
by (auto simp: refl_on_def RelM_def calM_def) (blast intro!: coset_mult_one [symmetric])
lemma RelM_sym: "sym RelM"
unfolding sym_def RelM_def calM_def
using coset_mult_assoc coset_mult_one r_inv_ex
by (smt (verit, best) case_prod_conv mem_Collect_eq)
lemma RelM_trans: "trans RelM"
by (auto simp add: trans_def RelM_def calM_def coset_mult_assoc)
lemma RelM_equiv: "equiv calM RelM"
using RelM_subset RelM_refl_on RelM_sym RelM_trans by (intro equivI)
lemma M_subset_calM_prep: "M' ∈ calM // RelM ⟹ M' ⊆ calM"
unfolding RelM_def by (blast elim!: quotientE)
end
subsection ‹Main Part of the Proof›
locale sylow_central = sylow +
fixes H and M1 and M
assumes M_in_quot: "M ∈ calM // RelM"
and not_dvd_M: "¬ (p ^ Suc (multiplicity p m) dvd card M)"
and M1_in_M: "M1 ∈ M"
defines "H ≡ {g. g ∈ carrier G ∧ M1 #> g = M1}"
begin
lemma M_subset_calM: "M ⊆ calM"
by (simp add: M_in_quot M_subset_calM_prep)
lemma card_M1: "card M1 = p^a"
using M1_in_M M_subset_calM calM_def by blast
lemma exists_x_in_M1: "∃x. x ∈ M1"
using prime_p [THEN prime_gt_Suc_0_nat] card_M1 one_in_subset by fastforce
lemma M1_subset_G [simp]: "M1 ⊆ carrier G"
using M1_in_M M_subset_calM calM_def mem_Collect_eq subsetCE by blast
lemma M1_inj_H: "∃f ∈ H→M1. inj_on f H"
proof -
from exists_x_in_M1 obtain m1 where m1M: "m1 ∈ M1"..
show ?thesis
proof
have "m1 ∈ carrier G"
by (simp add: m1M M1_subset_G [THEN subsetD])
then show "inj_on (λz∈H. m1 ⊗ z) H"
by (simp add: H_def inj_on_def)
show "restrict ((⊗) m1) H ∈ H → M1"
using H_def m1M rcosI by auto
qed
qed
end
subsection ‹Discharging the Assumptions of ‹sylow_central››
context sylow
begin
lemma EmptyNotInEquivSet: "{} ∉ calM // RelM"
using RelM_equiv in_quotient_imp_non_empty by blast
lemma existsM1inM: "M ∈ calM // RelM ⟹ ∃M1. M1 ∈ M"
using RelM_equiv equiv_Eps_in by blast
lemma zero_less_o_G: "0 < order G"
by (simp add: order_def card_gt_0_iff carrier_not_empty)
lemma zero_less_m: "m > 0"
using zero_less_o_G by (simp add: order_G)
lemma card_calM: "card calM = (p^a) * m choose p^a"
by (simp add: calM_def n_subsets order_G [symmetric] order_def)
lemma zero_less_card_calM: "card calM > 0"
by (simp add: card_calM zero_less_binomial le_extend_mult zero_less_m)
lemma max_p_div_calM: "¬ (p ^ Suc (multiplicity p m) dvd card calM)"
proof
assume "p ^ Suc (multiplicity p m) dvd card calM"
with zero_less_card_calM prime_p
have "Suc (multiplicity p m) ≤ multiplicity p (card calM)"
by (intro multiplicity_geI) auto
then show False
by (simp add: card_calM const_p_fac prime_p zero_less_m)
qed
lemma finite_calM: "finite calM"
unfolding calM_def by (rule finite_subset [where B = "Pow (carrier G)"]) auto
lemma lemma_A1: "∃M ∈ calM // RelM. ¬ (p ^ Suc (multiplicity p m) dvd card M)"
using RelM_equiv equiv_imp_dvd_card finite_calM max_p_div_calM by blast
end
subsubsection ‹Introduction and Destruct Rules for ‹H››
context sylow_central
begin
lemma H_I: "⟦g ∈ carrier G; M1 #> g = M1⟧ ⟹ g ∈ H"
by (simp add: H_def)
lemma H_into_carrier_G: "x ∈ H ⟹ x ∈ carrier G"
by (simp add: H_def)
lemma in_H_imp_eq: "g ∈ H ⟹ M1 #> g = M1"
by (simp add: H_def)
lemma H_m_closed: "⟦x ∈ H; y ∈ H⟧ ⟹ x ⊗ y ∈ H"
by (simp add: H_def coset_mult_assoc [symmetric])
lemma H_not_empty: "H ≠ {}"
by (force simp add: H_def intro: exI [of _ 𝟭])
lemma H_is_subgroup: "subgroup H G"
proof (rule subgroupI)
show "H ⊆ carrier G"
using H_into_carrier_G by blast
show "⋀a. a ∈ H ⟹ inv a ∈ H"
by (metis H_I H_into_carrier_G M1_subset_G coset_mult_assoc coset_mult_one in_H_imp_eq inv_closed r_inv)
show "⋀a b. ⟦a ∈ H; b ∈ H⟧ ⟹ a ⊗ b ∈ H"
by (blast intro: H_m_closed)
qed (use H_not_empty in auto)
lemma rcosetGM1g_subset_G: "⟦g ∈ carrier G; x ∈ M1 #> g⟧ ⟹ x ∈ carrier G"
by (blast intro: M1_subset_G [THEN r_coset_subset_G, THEN subsetD])
lemma finite_M1: "finite M1"
by (rule finite_subset [OF M1_subset_G finite_G])
lemma finite_rcosetGM1g: "g ∈ carrier G ⟹ finite (M1 #> g)"
using rcosetGM1g_subset_G finite_G M1_subset_G cosets_finite rcosetsI by blast
lemma M1_cardeq_rcosetGM1g: "g ∈ carrier G ⟹ card (M1 #> g) = card M1"
by (metis M1_subset_G card_rcosets_equal rcosetsI)
lemma M1_RelM_rcosetGM1g:
assumes "g ∈ carrier G"
shows "(M1, M1 #> g) ∈ RelM"
proof -
have "M1 #> g ⊆ carrier G"
by (simp add: assms r_coset_subset_G)
moreover have "card (M1 #> g) = p ^ a"
using assms by (simp add: card_M1 M1_cardeq_rcosetGM1g)
moreover have "∃h∈carrier G. M1 = M1 #> g #> h"
by (metis assms M1_subset_G coset_mult_assoc coset_mult_one r_inv_ex)
ultimately show ?thesis
by (simp add: RelM_def calM_def card_M1)
qed
end
subsection ‹Equal Cardinalities of ‹M› and the Set of Cosets›
text ‹Injections between \<^term>‹M› and \<^term>‹rcosets⇘G⇙ H› show that
their cardinalities are equal.›
lemma ElemClassEquiv: "⟦equiv A r; C ∈ A // r⟧ ⟹ ∀x ∈ C. ∀y ∈ C. (x, y) ∈ r"
unfolding equiv_def quotient_def sym_def trans_def by blast
context sylow_central
begin
lemma M_elem_map: "M2 ∈ M ⟹ ∃g. g ∈ carrier G ∧ M1 #> g = M2"
using M1_in_M M_in_quot [THEN RelM_equiv [THEN ElemClassEquiv]]
by (simp add: RelM_def) (blast dest!: bspec)
lemmas M_elem_map_carrier = M_elem_map [THEN someI_ex, THEN conjunct1]
lemmas M_elem_map_eq = M_elem_map [THEN someI_ex, THEN conjunct2]
lemma M_funcset_rcosets_H:
"(λx∈M. H #> (SOME g. g ∈ carrier G ∧ M1 #> g = x)) ∈ M → rcosets H"
by (metis (lifting) H_is_subgroup M_elem_map_carrier rcosetsI restrictI subgroup.subset)
lemma inj_M_GmodH: "∃f ∈ M → rcosets H. inj_on f M"
proof
let ?inv = "λx. SOME g. g ∈ carrier G ∧ M1 #> g = x"
show "inj_on (λx∈M. H #> ?inv x) M"
proof (rule inj_onI, simp)
fix x y
assume eq: "H #> ?inv x = H #> ?inv y" and xy: "x ∈ M" "y ∈ M"
have "x = M1 #> ?inv x"
by (simp add: M_elem_map_eq ‹x ∈ M›)
also have "… = M1 #> ?inv y"
proof (rule coset_mult_inv1 [OF in_H_imp_eq [OF coset_join1]])
show "H #> ?inv x ⊗ inv (?inv y) = H"
by (simp add: H_into_carrier_G M_elem_map_carrier xy coset_mult_inv2 eq subsetI)
qed (simp_all add: H_is_subgroup M_elem_map_carrier xy)
also have "… = y"
using M_elem_map_eq ‹y ∈ M› by simp
finally show "x=y" .
qed
show "(λx∈M. H #> ?inv x) ∈ M → rcosets H"
by (rule M_funcset_rcosets_H)
qed
end
subsubsection ‹The Opposite Injection›
context sylow_central
begin
lemma H_elem_map: "H1 ∈ rcosets H ⟹ ∃g. g ∈ carrier G ∧ H #> g = H1"
by (auto simp: RCOSETS_def)
lemmas H_elem_map_carrier = H_elem_map [THEN someI_ex, THEN conjunct1]
lemmas H_elem_map_eq = H_elem_map [THEN someI_ex, THEN conjunct2]
lemma rcosets_H_funcset_M:
"(λC ∈ rcosets H. M1 #> (SOME g. g ∈ carrier G ∧ H #> g = C)) ∈ rcosets H → M"
using in_quotient_imp_closed [OF RelM_equiv M_in_quot _ M1_RelM_rcosetGM1g]
by (simp add: M1_in_M H_elem_map_carrier RCOSETS_def)
lemma inj_GmodH_M: "∃g ∈ rcosets H→M. inj_on g (rcosets H)"
proof
let ?inv = "λx. SOME g. g ∈ carrier G ∧ H #> g = x"
show "inj_on (λC∈rcosets H. M1 #> ?inv C) (rcosets H)"
proof (rule inj_onI, simp)
fix x y
assume eq: "M1 #> ?inv x = M1 #> ?inv y" and xy: "x ∈ rcosets H" "y ∈ rcosets H"
have "x = H #> ?inv x"
by (simp add: H_elem_map_eq ‹x ∈ rcosets H›)
also have "… = H #> ?inv y"
proof (rule coset_mult_inv1 [OF coset_join2])
show "?inv x ⊗ inv (?inv y) ∈ carrier G"
by (simp add: H_elem_map_carrier ‹x ∈ rcosets H› ‹y ∈ rcosets H›)
then show "(?inv x) ⊗ inv (?inv y) ∈ H"
by (simp add: H_I H_elem_map_carrier xy coset_mult_inv2 eq)
show "H ⊆ carrier G"
by (simp add: H_is_subgroup subgroup.subset)
qed (simp_all add: H_is_subgroup H_elem_map_carrier xy)
also have "… = y"
by (simp add: H_elem_map_eq ‹y ∈ rcosets H›)
finally show "x=y" .
qed
show "(λC∈rcosets H. M1 #> ?inv C) ∈ rcosets H → M"
using rcosets_H_funcset_M by blast
qed
lemma calM_subset_PowG: "calM ⊆ Pow (carrier G)"
by (auto simp: calM_def)
lemma finite_M: "finite M"
by (metis M_subset_calM finite_calM rev_finite_subset)
lemma cardMeqIndexH: "card M = card (rcosets H)"
using inj_M_GmodH inj_GmodH_M
by (metis H_is_subgroup card_bij finite_G finite_M finite_UnionD rcosets_part_G)
lemma index_lem: "card M * card H = order G"
by (simp add: cardMeqIndexH lagrange H_is_subgroup)
lemma card_H_eq: "card H = p^a"
proof (rule antisym)
show "p^a ≤ card H"
proof (rule dvd_imp_le)
have "p ^ (a + multiplicity p m) dvd card M * card H"
by (simp add: index_lem multiplicity_dvd order_G power_add)
then show "p ^ a dvd card H"
using div_combine not_dvd_M prime_p by blast
show "0 < card H"
by (blast intro: subgroup.finite_imp_card_positive H_is_subgroup)
qed
next
show "card H ≤ p^a"
using M1_inj_H card_M1 card_inj finite_M1 by fastforce
qed
end
lemma (in sylow) sylow_thm: "∃H. subgroup H G ∧ card H = p^a"
proof -
obtain M where M: "M ∈ calM // RelM" "¬ (p ^ Suc (multiplicity p m) dvd card M)"
using lemma_A1 by blast
then obtain M1 where "M1 ∈ M"
by (metis existsM1inM)
define H where "H ≡ {g. g ∈ carrier G ∧ M1 #> g = M1}"
with M ‹M1 ∈ M›
interpret sylow_central G p a m calM RelM H M1 M
by unfold_locales (auto simp add: H_def calM_def RelM_def)
show ?thesis
using H_is_subgroup card_H_eq by blast
qed
text ‹Needed because the locale's automatic definition refers to
\<^term>‹semigroup G› and \<^term>‹group_axioms G› rather than
simply to \<^term>‹group G›.›
lemma sylow_eq: "sylow G p a m ⟷ group G ∧ sylow_axioms G p a m"
by (simp add: sylow_def group_def)
subsection ‹Sylow's Theorem›
theorem sylow_thm:
"⟦prime p; group G; order G = (p^a) * m; finite (carrier G)⟧
⟹ ∃H. subgroup H G ∧ card H = p^a"
by (rule sylow.sylow_thm [of G p a m]) (simp add: sylow_eq sylow_axioms_def)
end