/- Copyright (c) 2015 Ulrik Buchholtz, Egbert Rijke and Floris van Doorn. All rights reserved. Released under Apache 2.0 license as described in the file LICENSE. Authors: Ulrik Buchholtz, Egbert Rijke, Floris van Doorn Formalization of the higher groups paper -/ import .homotopy.EM algebra.category.constructions.pullback open eq is_conn pointed is_trunc trunc equiv is_equiv trunc_index susp nat algebra prod.ops sigma sigma.ops category EM namespace higher_group set_option pp.binder_types true universe variable u /- Results not necessarily about higher groups which we repeat here, because they are mentioned in the higher group paper -/ namespace hide open pushout definition connect_intro_pequiv {k : ℕ} {B : Type*} (A : Type*) (H : is_conn k B) : ppmap B (connect k A) ≃* ppmap B A := is_conn.connect_intro_pequiv A H definition is_conn_fun_prod_of_wedge (n m : ℕ) (A B : Type*) [cA : is_conn n A] [cB : is_conn m B] : is_conn_fun (m + n) (@prod_of_wedge A B) := is_conn_fun_prod_of_wedge n m A B definition is_trunc_ppi_of_is_conn (k n : ℕ) (X : Type*) (H : is_conn (k.-1) X) (Y : X → Type*) (H3 : Πx, is_trunc (k + n) (Y x)) : is_trunc n (Π*(x : X), Y x) := is_conn.is_trunc_ppi_of_is_conn _ (k.-2) H _ _ (le_of_eq (sub_one_add_plus_two_sub_one k n)⁻¹) _ H3 end hide /- The k-groupal n-types. We require that the carrier has a point (preserved by the equivalence) -/ structure GType (n k : ℕ) : Type := /- (n,k)GType, denoted here as [n;k]GType -/ (car : ptrunctype.{u} n) (B : pconntype.{u} (k.-1)) /- this is Bᵏ -/ (e : car ≃* Ω[k] B) structure InfGType (k : ℕ) : Type := /- (∞,k)GType, denoted here as [∞;k]GType -/ (car : pType.{u}) (B : pconntype.{u} (k.-1)) /- this is Bᵏ -/ (e : car ≃* Ω[k] B) structure ωGType (n : ℕ) := /- (n,ω)GType, denoted here as [n;ω]GType -/ (B : Π(k : ℕ), (n+k)-Type*[k.-1]) (e : Π(k : ℕ), B k ≃* Ω (B (k+1))) attribute InfGType.car GType.car [coercion] variables {n k k' l : ℕ} notation `[`:95 n:0 `; ` k `]GType`:0 := GType n k notation `[∞; `:95 k:0 `]GType`:0 := InfGType k notation `[`:95 n:0 `;ω]GType`:0 := ωGType n open GType open InfGType (renaming B→iB e→ie) open ωGType (renaming B→oB e→oe) /- some basic properties -/ lemma is_trunc_B' (G : [n;k]GType) : is_trunc (k+n) (B G) := begin apply is_trunc_of_is_trunc_loopn, exact is_trunc_equiv_closed _ (e G) _, exact _ end lemma is_trunc_B (G : [n;k]GType) : is_trunc (n+k) (B G) := transport (λm, is_trunc m (B G)) (add.comm k n) (is_trunc_B' G) local attribute [instance] is_trunc_B definition GType.sigma_char (n k : ℕ) : GType.{u} n k ≃ Σ(B : pconntype.{u} (k.-1)), Σ(X : ptrunctype.{u} n), X ≃* Ω[k] B := begin fapply equiv.MK, { intro G, exact ⟨B G, G, e G⟩ }, { intro v, exact GType.mk v.2.1 v.1 v.2.2 }, { intro v, induction v with v₁ v₂, induction v₂, reflexivity }, { intro G, induction G, reflexivity }, end definition GType_equiv (n k : ℕ) : [n;k]GType ≃ (n+k)-Type*[k.-1] := GType.sigma_char n k ⬝e sigma_equiv_of_is_embedding_left_contr ptruncconntype.to_pconntype (is_embedding_ptruncconntype_to_pconntype (n+k) (k.-1)) begin intro X, apply is_trunc_equiv_closed_rev -2, { apply sigma_equiv_sigma_right, intro B', refine _ ⬝e (ptrunctype_eq_equiv B' (ptrunctype.mk (Ω[k] X) !is_trunc_loopn_nat pt))⁻¹ᵉ, assert lem : Π(A : n-Type*) (B : Type*) (H : is_trunc n B), (A ≃* B) ≃ (A ≃* (ptrunctype.mk B H pt)), { intro A B'' H, induction B'', reflexivity }, apply lem }, exact _ end begin intro B' H, apply fiber.mk (ptruncconntype.mk B' (is_trunc_B (GType.mk H.1 B' H.2)) pt _), induction B' with G' B' e', reflexivity end definition GType_equiv_pequiv {n k : ℕ} (G : [n;k]GType) : GType_equiv n k G ≃* B G := by reflexivity definition GType_eq_equiv {n k : ℕ} (G H : [n;k]GType) : (G = H :> [n;k]GType) ≃ (B G ≃* B H) := eq_equiv_fn_eq (GType_equiv n k) _ _ ⬝e !ptruncconntype_eq_equiv definition GType_eq {n k : ℕ} {G H : [n;k]GType} (e : B G ≃* B H) : G = H := (GType_eq_equiv G H)⁻¹ᵉ e /- similar properties for [∞;k]GType -/ definition InfGType.sigma_char (k : ℕ) : InfGType.{u} k ≃ Σ(B : pconntype.{u} (k.-1)), Σ(X : pType.{u}), X ≃* Ω[k] B := begin fapply equiv.MK, { intro G, exact ⟨iB G, G, ie G⟩ }, { intro v, exact InfGType.mk v.2.1 v.1 v.2.2 }, { intro v, induction v with v₁ v₂, induction v₂, reflexivity }, { intro G, induction G, reflexivity }, end definition InfGType_equiv (k : ℕ) : [∞;k]GType ≃ Type*[k.-1] := InfGType.sigma_char k ⬝e @sigma_equiv_of_is_contr_right _ _ (λX, is_trunc_equiv_closed_rev -2 (sigma_equiv_sigma_right (λB', !pType_eq_equiv⁻¹ᵉ)) _) definition InfGType_equiv_pequiv {k : ℕ} (G : [∞;k]GType) : InfGType_equiv k G ≃* iB G := by reflexivity definition InfGType_eq_equiv {k : ℕ} (G H : [∞;k]GType) : (G = H :> [∞;k]GType) ≃ (iB G ≃* iB H) := eq_equiv_fn_eq (InfGType_equiv k) _ _ ⬝e !pconntype_eq_equiv definition InfGType_eq {k : ℕ} {G H : [∞;k]GType} (e : iB G ≃* iB H) : G = H := (InfGType_eq_equiv G H)⁻¹ᵉ e /- alternative constructor for ωGType -/ definition ωGType.mk_le {n : ℕ} (k₀ : ℕ) (C : Π⦃k : ℕ⦄, k₀ ≤ k → ((n+k)-Type*[k.-1] : Type.{u+1})) (e : Π⦃k : ℕ⦄ (H : k₀ ≤ k), C H ≃* Ω (C (le.step H))) : ([n;ω]GType : Type.{u+1}) := begin fconstructor, { apply rec_down_le _ k₀ C, intro n' D, refine (ptruncconntype.mk (Ω D) _ pt _), apply is_trunc_loop, apply is_trunc_ptruncconntype, apply is_conn_loop, apply is_conn_ptruncconntype }, { intro n', apply rec_down_le_univ, exact e, intro n D, reflexivity } end definition ωGType.mk_le_beta {n : ℕ} {k₀ : ℕ} (C : Π⦃k : ℕ⦄, k₀ ≤ k → ((n+k)-Type*[k.-1] : Type.{u+1})) (e : Π⦃k : ℕ⦄ (H : k₀ ≤ k), C H ≃* Ω (C (le.step H))) (k : ℕ) (H : k₀ ≤ k) : oB (ωGType.mk_le k₀ C e) k ≃* C H := ptruncconntype_eq_equiv _ _ !rec_down_le_beta_ge definition GType_hom (G H : [n;k]GType) : Type := B G →* B H definition ωGType_hom (G H : [n;ω]GType) : Type* := pointed.MK (Σ(f : Πn, oB G n →* oB H n), Πn, psquare (f n) (Ω→ (f (n+1))) (oe G n) (oe H n)) ⟨λn, pconst (oB G n) (oB H n), λn, !phconst_square ⬝vp* !ap1_pconst⟩ /- Constructions on higher groups -/ definition Decat (G : [n+1;k]GType) : [n;k]GType := GType.mk (ptrunctype.mk (ptrunc n G) _ pt) (pconntype.mk (ptrunc (n + k) (B G)) _ pt) abstract begin refine ptrunc_pequiv_ptrunc n (e G) ⬝e* _, symmetry, exact !loopn_ptrunc_pequiv_nat end end definition Disc (G : [n;k]GType) : [n+1;k]GType := GType.mk (ptrunctype.mk G (show is_trunc (n.+1) G, from _) pt) (B G) (e G) definition Decat_adjoint_Disc (G : [n+1;k]GType) (H : [n;k]GType) : ppmap (B (Decat G)) (B H) ≃* ppmap (B G) (B (Disc H)) := pmap_ptrunc_pequiv (n + k) (B G) (B H) definition Decat_adjoint_Disc_natural {G G' : [n+1;k]GType} {H H' : [n;k]GType} (g : B G' →* B G) (h : B H →* B H') : psquare (Decat_adjoint_Disc G H) (Decat_adjoint_Disc G' H') (ppcompose_left h ∘* ppcompose_right (ptrunc_functor _ g)) (ppcompose_left h ∘* ppcompose_right g) := pmap_ptrunc_pequiv_natural (n + k) g h definition Decat_Disc (G : [n;k]GType) : Decat (Disc G) = G := GType_eq !ptrunc_pequiv definition InfDecat (n : ℕ) (G : [∞;k]GType) : [n;k]GType := GType.mk (ptrunctype.mk (ptrunc n G) _ pt) (pconntype.mk (ptrunc (n + k) (iB G)) _ pt) abstract begin refine ptrunc_pequiv_ptrunc n (ie G) ⬝e* _, symmetry, exact !loopn_ptrunc_pequiv_nat end end definition InfDisc (n : ℕ) (G : [n;k]GType) : [∞;k]GType := InfGType.mk G (B G) (e G) definition InfDecat_adjoint_InfDisc (G : [∞;k]GType) (H : [n;k]GType) : ppmap (B (InfDecat n G)) (B H) ≃* ppmap (iB G) (iB (InfDisc n H)) := pmap_ptrunc_pequiv (n + k) (iB G) (B H) definition InfDecat_adjoint_InfDisc_natural {G G' : [∞;k]GType} {H H' : [n;k]GType} (g : iB G' →* iB G) (h : B H →* B H') : psquare (InfDecat_adjoint_InfDisc G H) (InfDecat_adjoint_InfDisc G' H') (ppcompose_left h ∘* ppcompose_right (ptrunc_functor _ g)) (ppcompose_left h ∘* ppcompose_right g) := pmap_ptrunc_pequiv_natural (n + k) g h definition InfDecat_InfDisc (G : [n;k]GType) : InfDecat n (InfDisc n G) = G := GType_eq !ptrunc_pequiv definition Deloop (G : [n;k+1]GType) : [n+1;k]GType := have is_conn k (B G), from is_conn_pconntype (B G), have is_trunc (n + (k + 1)) (B G), from is_trunc_B G, have is_trunc ((n + 1) + k) (B G), from transport (λ(n : ℕ), is_trunc n _) (succ_add n k)⁻¹ this, GType.mk (ptrunctype.mk (Ω[k] (B G)) !is_trunc_loopn_nat pt) (pconntype.mk (B G) !is_conn_of_is_conn_succ pt) (pequiv_of_equiv erfl idp) definition Loop (G : [n+1;k]GType) : [n;k+1]GType := GType.mk (ptrunctype.mk (Ω G) !is_trunc_loop_nat pt) (connconnect k (B G)) (loop_pequiv_loop (e G) ⬝e* (loopn_connect k (B G))⁻¹ᵉ*) definition Deloop_adjoint_Loop (G : [n;k+1]GType) (H : [n+1;k]GType) : ppmap (B (Deloop G)) (B H) ≃* ppmap (B G) (B (Loop H)) := (connect_intro_pequiv _ !is_conn_pconntype)⁻¹ᵉ* definition Deloop_adjoint_Loop_natural {G G' : [n;k+1]GType} {H H' : [n+1;k]GType} (g : B G' →* B G) (h : B H →* B H') : psquare (Deloop_adjoint_Loop G H) (Deloop_adjoint_Loop G' H') (ppcompose_left h ∘* ppcompose_right g) (ppcompose_left (connect_functor k h) ∘* ppcompose_right g) := (connect_intro_pequiv_natural g h _ _)⁻¹ʰ* definition Loop_Deloop (G : [n;k+1]GType) : Loop (Deloop G) = G := GType_eq (connect_pequiv (is_conn_pconntype (B G))) definition Forget (G : [n;k+1]GType) : [n;k]GType := have is_conn k (B G), from !is_conn_pconntype, GType.mk G (pconntype.mk (Ω (B G)) !is_conn_loop pt) abstract begin refine e G ⬝e* !loopn_succ_in end end definition Stabilize (G : [n;k]GType) : [n;k+1]GType := have is_conn k (susp (B G)), from !is_conn_susp, have Hconn : is_conn k (ptrunc (n + k + 1) (susp (B G))), from !is_conn_ptrunc, GType.mk (ptrunctype.mk (ptrunc n (Ω[k+1] (susp (B G)))) _ pt) (pconntype.mk (ptrunc (n+k+1) (susp (B G))) Hconn pt) abstract begin refine !loopn_ptrunc_pequiv⁻¹ᵉ* ⬝e* _, apply loopn_pequiv_loopn, exact ptrunc_change_index !of_nat_add_of_nat _ end end definition Stabilize_pequiv {G H : [n;k]GType} (e : B G ≃* B H) : B (Stabilize G) ≃* B (Stabilize H) := ptrunc_pequiv_ptrunc (n+k+1) (susp_pequiv e) definition Stabilize_adjoint_Forget (G : [n;k]GType) (H : [n;k+1]GType) : ppmap (B (Stabilize G)) (B H) ≃* ppmap (B G) (B (Forget H)) := have is_trunc (n + k + 1) (B H), from !is_trunc_B, pmap_ptrunc_pequiv (n + k + 1) (⅀ (B G)) (B H) ⬝e* susp_adjoint_loop (B G) (B H) definition Stabilize_adjoint_Forget_natural {G G' : [n;k]GType} {H H' : [n;k+1]GType} (g : B G' →* B G) (h : B H →* B H') : psquare (Stabilize_adjoint_Forget G H) (Stabilize_adjoint_Forget G' H') (ppcompose_left h ∘* ppcompose_right (ptrunc_functor (n+k+1) (⅀→ g))) (ppcompose_left (Ω→ h) ∘* ppcompose_right g) := begin have is_trunc (n + k + 1) (B H), from !is_trunc_B, have is_trunc (n + k + 1) (B H'), from !is_trunc_B, refine pmap_ptrunc_pequiv_natural (n+k+1) (⅀→ g) h ⬝h* _, exact susp_adjoint_loop_natural_left g ⬝v* susp_adjoint_loop_natural_right h end definition Forget_Stabilize (H : k ≥ n + 2) (G : [n;k]GType) : B (Forget (Stabilize G)) ≃* B G := loop_ptrunc_pequiv _ _ ⬝e* begin cases k with k, { cases H }, { have k ≥ succ n, from le_of_succ_le_succ H, assert this : n + succ k ≤ 2 * k, { rewrite [two_mul, add_succ, -succ_add], exact nat.add_le_add_right this k }, exact freudenthal_pequiv this (B G) } end⁻¹ᵉ* ⬝e* ptrunc_pequiv (n + k) _ definition Stabilize_Forget (H : k ≥ n + 1) (G : [n;k+1]GType) : B (Stabilize (Forget G)) ≃* B G := begin assert lem1 : n + succ k ≤ 2 * k, { rewrite [two_mul, add_succ, -succ_add], exact nat.add_le_add_right H k }, have is_conn k (B G), from !is_conn_pconntype, have Π(G' : [n;k+1]GType), is_trunc (n + k + 1) (B G'), from is_trunc_B, note z := is_conn_fun_loop_susp_counit (B G) (nat.le_refl (2 * k)), refine ptrunc_pequiv_ptrunc_of_le (of_nat_le_of_nat lem1) (@(ptrunc_pequiv_ptrunc_of_is_conn_fun _ _) z) ⬝e* !ptrunc_pequiv, end definition stabilization (H : k ≥ n + 2) : is_equiv (@Stabilize n k) := begin fapply adjointify, { exact Forget }, { intro G, apply GType_eq, exact Stabilize_Forget (le.trans !self_le_succ H) _ }, { intro G, apply GType_eq, exact Forget_Stabilize H G } end /- an incomplete formalization of ω-Stabilization -/ definition ωForget (k : ℕ) (G : [n;ω]GType) : [n;k]GType := have is_trunc (n + k) (oB G k), from _, have is_trunc n (Ω[k] (oB G k)), from !is_trunc_loopn_nat, GType.mk (ptrunctype.mk (Ω[k] (oB G k)) _ pt) (oB G k) (pequiv_of_equiv erfl idp) definition nStabilize (H : k ≤ l) (G : GType.{u} n k) : GType.{u} n l := begin induction H with l H IH, exact G, exact Stabilize IH end definition nStabilize_pequiv (H H' : k ≤ l) {G G' : [n;k]GType} (e : B G ≃* B G') : B (nStabilize H G) ≃* B (nStabilize H' G') := begin induction H with l H IH, { exact e ⬝e* pequiv_ap (λH, B (nStabilize H G')) (is_prop.elim (le.refl k) H') }, cases H' with l H'', { exfalso, exact not_succ_le_self H }, exact Stabilize_pequiv (IH H'') end definition nStabilize_pequiv_of_eq (H : k ≤ l) (p : k = l) (G : [n;k]GType) : B (nStabilize H G) ≃* B G := begin induction p, exact pequiv_ap (λH, B (nStabilize H G)) (is_prop.elim H (le.refl k)) end definition ωStabilize_of_le (H : k ≥ n + 2) (G : [n;k]GType) : [n;ω]GType := ωGType.mk_le k (λl H', GType_equiv n l (nStabilize H' G)) (λl H', (Forget_Stabilize (le.trans H H') (nStabilize H' G))⁻¹ᵉ*) definition ωStabilize_of_le_beta (H : k ≥ n + 2) (G : [n;k]GType) (H' : l ≥ k) : oB (ωStabilize_of_le H G) l ≃* GType_equiv n l (nStabilize H' G) := ptruncconntype_eq_equiv _ _ !rec_down_le_beta_ge definition ωStabilize_of_le_pequiv (H : k ≥ n + 2) (H' : k' ≥ n + 2) {G : [n;k]GType} {G' : [n;k']GType} (e : B G ≃* B G') (l : ℕ) (Hl : l ≥ k) (Hl' : l ≥ k') (p : k = k') : oB (ωStabilize_of_le H G) l ≃* oB (ωStabilize_of_le H' G') l := begin refine ωStabilize_of_le_beta H G Hl ⬝e* _ ⬝e* (ωStabilize_of_le_beta H' G' Hl')⁻¹ᵉ*, induction p, exact nStabilize_pequiv _ _ e end definition ωForget_ωStabilize_of_le (H : k ≥ n + 2) (G : [n;k]GType) : B (ωForget k (ωStabilize_of_le H G)) ≃* B G := ωStabilize_of_le_beta H _ (le.refl k) definition ωStabilize (G : [n;k]GType) : [n;ω]GType := ωStabilize_of_le !le_max_left (nStabilize !le_max_right G) definition ωForget_ωStabilize (H : k ≥ n + 2) (G : [n;k]GType) : B (ωForget k (ωStabilize G)) ≃* B G := begin refine _ ⬝e* ωForget_ωStabilize_of_le H G, esimp [ωForget, ωStabilize], have H' : max (n + 2) k = k, from max_eq_right H, exact ωStabilize_of_le_pequiv !le_max_left H (nStabilize_pequiv_of_eq _ H'⁻¹ _) k (le_of_eq H') (le.refl k) H' end /- definition ωStabilize_adjoint_ωForget (G : [n;k]GType) (H : [n;ω]GType) : ωGType_hom (ωStabilize G) H ≃* ppmap (B G) (B (ωForget k H)) := sorry definition ωStabilize_ωForget (G : [n;ω]GType) (l : ℕ) : oB (ωStabilize (ωForget k G)) l ≃* oB G l := begin exact sorry end definition ωstabilization (H : k ≥ n + 2) : is_equiv (@ωStabilize n k) := begin apply adjointify _ (ωForget k), { intro G', exact sorry }, { intro G, apply GType_eq, exact ωForget_ωStabilize H G } end -/ definition is_trunc_GType_hom (G H : [n;k]GType) : is_trunc n (GType_hom G H) := is_trunc_pmap_of_is_conn _ (k.-2) _ _ (k + n) _ (le_of_eq (sub_one_add_plus_two_sub_one k n)⁻¹) (is_trunc_B' H) definition is_set_GType_hom (G H : [0;k]GType) : is_set (GType_hom G H) := is_trunc_GType_hom G H definition is_trunc_GType (n k : ℕ) : is_trunc (n + 1) [n;k]GType := begin apply @is_trunc_equiv_closed_rev _ _ (n + 1) (GType_equiv n k), apply is_trunc_succ_intro, intros X Y, apply @is_trunc_equiv_closed_rev _ _ _ (ptruncconntype_eq_equiv X Y), apply @is_trunc_equiv_closed_rev _ _ _ (pequiv.sigma_char_pmap X Y), apply @is_trunc_subtype (X →* Y) (λ f, trunctype.mk' -1 (is_equiv f)), exact is_trunc_GType_hom ((GType_equiv n k)⁻¹ᵉ X) ((GType_equiv n k)⁻¹ᵉ Y) end local attribute [instance] is_set_GType_hom definition cGType [constructor] (k : ℕ) : Precategory := pb_Precategory (cptruncconntype' (k.-1)) (GType_equiv 0 k ⬝e ptruncconntype_equiv (ap of_nat (zero_add k)) idp ⬝e (ptruncconntype'_equiv_ptruncconntype (k.-1))⁻¹ᵉ) example (k : ℕ) : Precategory.carrier (cGType k) = [0;k]GType := by reflexivity example (k : ℕ) (G H : cGType k) : (G ⟶ H) = (B G →* B H) := by reflexivity definition cGType_equivalence_cType [constructor] (k : ℕ) : cGType k ≃c cType*[k.-1] := !pb_Precategory_equivalence_of_equiv definition cGType_equivalence_Grp [constructor] : cGType.{u} 1 ≃c Grp.{u} := equivalence.trans !pb_Precategory_equivalence_of_equiv (equivalence.trans (equivalence.symm Grp_equivalence_cptruncconntype') proof equivalence.refl _ qed) definition cGType_equivalence_AbGrp [constructor] (k : ℕ) : cGType.{u} (k+2) ≃c AbGrp.{u} := equivalence.trans !pb_Precategory_equivalence_of_equiv (equivalence.trans (equivalence.symm (AbGrp_equivalence_cptruncconntype' k)) proof equivalence.refl _ qed) /- print axioms GType_equiv print axioms InfGType_equiv print axioms Decat_adjoint_Disc print axioms Decat_adjoint_Disc_natural print axioms InfDecat_adjoint_InfDisc print axioms InfDecat_adjoint_InfDisc_natural print axioms Deloop_adjoint_Loop print axioms Deloop_adjoint_Loop_natural print axioms Stabilize_adjoint_Forget print axioms Stabilize_adjoint_Forget_natural print axioms stabilization print axioms is_trunc_GType print axioms cGType_equivalence_Grp print axioms cGType_equivalence_AbGrp -/ end higher_group