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-
-(*************************************************************)
-(* This file is distributed under the terms of the *)
-(* GNU Lesser General Public License Version 2.1 *)
-(*************************************************************)
-(* Benjamin.Gregoire@inria.fr Laurent.Thery@inria.fr *)
-(*************************************************************)
-
-(**********************************************************************
- Lagrange.v
-
- Proof of Lagrange theorem:
- the oder of a subgroup divides the order of a group
-
- Definition: lagrange
- **********************************************************************)
-Require Import List.
-Require Import UList.
-Require Import ListAux.
-Require Import ZArith Znumtheory.
-Require Import NatAux.
-Require Import FGroup.
-
-Open Scope Z_scope.
-
-Section Lagrange.
-
-Variable A: Set.
-
-Variable A_dec: forall a b: A, {a = b} + {~ a = b}.
-
-Variable op: A -> A -> A.
-
-Variable G: (FGroup op).
-
-Variable H:(FGroup op).
-
-Hypothesis G_in_H: (incl G.(s) H.(s)).
-
-(**************************************
- A group and a subgroup have the same neutral element
- **************************************)
-
-Theorem same_e_for_H_and_G: H.(e) = G.(e).
-apply trans_equal with (op H.(e) H.(e)); sauto.
-apply trans_equal with (op H.(e) (op G.(e) (H.(i) G.(e)))); sauto.
-eq_tac; sauto.
-apply trans_equal with (op G.(e) (op G.(e) (H.(i) G.(e)))); sauto.
-repeat rewrite H.(assoc); sauto.
-eq_tac; sauto.
-apply trans_equal with G.(e); sauto.
-apply trans_equal with (op G.(e) H.(e)); sauto.
-eq_tac; sauto.
-Qed.
-
-(**************************************
- The proof works like this.
- If G = {e, g1, g2, g3, .., gn} and {e, h1, h2, h3, ..., hm}
- we construct the list mkGH
- {e, g1, g2, g3, ...., gn
- hi*e, hi * g1, hi * g2, ..., hi * gn if hi does not appear before
- ....
- hk*e, hk * g1, hk * g2, ..., hk * gn if hk does not appear before
- }
- that contains all the element of H.
- We show that this list does not contain double (ulist).
- **************************************)
-
-Fixpoint mkList (base l: (list A)) { struct l} : (list A) :=
- match l with
- nil => nil
- | cons a l1 => let r1 := mkList base l1 in
- if (In_dec A_dec a r1) then r1 else
- (map (op a) base) ++ r1
- end.
-
-Definition mkGH := mkList G.(s) H.(s).
-
-Theorem mkGH_length: divide (length G.(s)) (length mkGH).
-unfold mkGH; elim H.(s); simpl.
-exists 0%nat; auto with arith.
-intros a l1 (c, H1); case (In_dec A_dec a (mkList G.(s) l1)); intros H2.
-exists c; auto.
-exists (1 + c)%nat; rewrite ListAux.length_app; rewrite ListAux.length_map; rewrite H1; ring.
-Qed.
-
-Theorem mkGH_incl: incl H.(s) mkGH.
-assert (H1: forall l, incl l H.(s) -> incl l (mkList G.(s) l)).
-intros l; elim l; simpl; auto with datatypes.
-intros a l1 H1 H2.
-case (In_dec A_dec a (mkList (s G) l1)); auto with datatypes.
-intros H3; assert (H4: incl l1 (mkList (s G) l1)).
-apply H1; auto with datatypes.
-intros b H4; apply H2; auto with datatypes.
-intros b; simpl; intros [H5 | H5]; subst; auto.
-intros _ b; simpl; intros [H3 | H3]; subst; auto.
-apply in_or_app; left.
-cut (In H.(e) G.(s)).
-elim (s G); simpl; auto.
-intros c l2 Hl2 [H3 | H3]; subst; sauto.
-assert (In b H.(s)); sauto.
-apply (H2 b); auto with datatypes.
-rewrite same_e_for_H_and_G; sauto.
-apply in_or_app; right.
-apply H1; auto with datatypes.
-apply incl_tran with (2:= H2); auto with datatypes.
-unfold mkGH; apply H1; auto with datatypes.
-Qed.
-
-Theorem incl_mkGH: incl mkGH H.(s).
-assert (H1: forall l, incl l H.(s) -> incl (mkList G.(s) l) H.(s)).
-intros l; elim l; simpl; auto with datatypes.
-intros a l1 H1 H2.
-case (In_dec A_dec a (mkList (s G) l1)); intros H3; auto with datatypes.
-apply H1; apply incl_tran with (2 := H2); auto with datatypes.
-apply incl_app.
-intros b H4.
-case ListAux.in_map_inv with (1:= H4); auto.
-intros c (Hc1, Hc2); subst; sauto.
-apply internal; auto with datatypes.
-apply H1; apply incl_tran with (2 := H2); auto with datatypes.
-unfold mkGH; apply H1; auto with datatypes.
-Qed.
-
-Theorem ulist_mkGH: ulist mkGH.
-assert (H1: forall l, incl l H.(s) -> ulist (mkList G.(s) l)).
-intros l; elim l; simpl; auto with datatypes.
-intros a l1 H1 H2.
-case (In_dec A_dec a (mkList (s G) l1)); intros H3; auto with datatypes.
-apply H1; apply incl_tran with (2 := H2); auto with datatypes.
-apply ulist_app; auto.
-apply ulist_map; sauto.
-intros x y H4 H5 H6; apply g_cancel_l with (g:= H) (a := a); sauto.
-apply H2; auto with datatypes.
-apply H1; apply incl_tran with (2 := H2); auto with datatypes.
-intros b H4 H5.
-case ListAux.in_map_inv with (1:= H4); auto.
-intros c (Hc, Hc1); subst.
-assert (H6: forall l a b, In b G.(s) -> incl l H.(s) -> In a (mkList G.(s) l) -> In (op a b) (mkList G.(s) l)).
-intros ll u v; elim ll; simpl; auto with datatypes.
-intros w ll1 T0 T1 T2.
-case (In_dec A_dec w (mkList (s G) ll1)); intros T3 T4; auto with datatypes.
-apply T0; auto; apply incl_tran with (2:= T2); auto with datatypes.
-case in_app_or with (1 := T4); intros T5; auto with datatypes.
-apply in_or_app; left.
-case ListAux.in_map_inv with (1:= T5); auto.
-intros z (Hz1, Hz2); subst.
-replace (op (op w z) v) with (op w (op z v)); sauto.
-apply in_map; sauto.
-apply assoc with H; auto with datatypes.
-apply in_or_app; right; auto with datatypes.
-apply T0; try apply incl_tran with (2 := T2); auto with datatypes.
-case H3; replace a with (op (op a c) (G.(i) c)); auto with datatypes.
-apply H6; sauto.
-apply incl_tran with (2 := H2); auto with datatypes.
-apply trans_equal with (op a (op c (G.(i) c))); sauto.
-apply sym_equal; apply assoc with H; auto with datatypes.
-replace (op c (G.(i) c)) with (G.(e)); sauto.
-rewrite <- same_e_for_H_and_G.
-assert (In a H.(s)); sauto; apply (H2 a); auto with datatypes.
-unfold mkGH; apply H1; auto with datatypes.
-Qed.
-
-(**************************************
- Lagrange theorem
- **************************************)
-
-Theorem lagrange: (g_order G | (g_order H)).
-unfold g_order.
-rewrite Permutation.permutation_length with (l := H.(s)) (m:= mkGH).
-case mkGH_length; intros x H1; exists (Z_of_nat x).
-rewrite H1; rewrite Zmult_comm; apply inj_mult.
-apply ulist_incl2_permutation; auto.
-apply ulist_mkGH.
-apply mkGH_incl.
-apply incl_mkGH.
-Qed.
-
-End Lagrange.