|
| 1 | +/- |
| 2 | +Copyright (c) 2022 Floris van Doorn, Heather Macbeth. All rights reserved. |
| 3 | +Released under Apache 2.0 license as described in the file LICENSE. |
| 4 | +Authors: Floris van Doorn, Heather Macbeth |
| 5 | +-/ |
| 6 | +import geometry.manifold.cont_mdiff |
| 7 | + |
| 8 | +/-! # Smooth vector bundles |
| 9 | +
|
| 10 | +This file will eventually contain the definition of a smooth vector bundle. For now, it contains |
| 11 | +preliminaries regarding an associated `structure_groupoid`, the groupoid of |
| 12 | +`smooth_fibrewise_linear` functions. When a (topological) vector bundle is smooth, then the |
| 13 | +composition of charts associated to the vector bundle belong to this groupoid. |
| 14 | +-/ |
| 15 | + |
| 16 | +noncomputable theory |
| 17 | + |
| 18 | +open set topological_space |
| 19 | +open_locale manifold topology |
| 20 | + |
| 21 | +/-! ### The groupoid of smooth, fibrewise-linear maps -/ |
| 22 | + |
| 23 | +variables {𝕜 B F : Type*} [topological_space B] |
| 24 | +variables [nontrivially_normed_field 𝕜] [normed_add_comm_group F] [normed_space 𝕜 F] |
| 25 | + |
| 26 | +namespace fiberwise_linear |
| 27 | + |
| 28 | +variables {φ φ' : B → F ≃L[𝕜] F} {U U' : set B} |
| 29 | + |
| 30 | +/-- For `B` a topological space and `F` a `𝕜`-normed space, a map from `U : set B` to `F ≃L[𝕜] F` |
| 31 | +determines a local homeomorphism from `B × F` to itself by its action fibrewise. -/ |
| 32 | +def local_homeomorph (φ : B → F ≃L[𝕜] F) (hU : is_open U) |
| 33 | + (hφ : continuous_on (λ x, φ x : B → F →L[𝕜] F) U) |
| 34 | + (h2φ : continuous_on (λ x, (φ x).symm : B → F →L[𝕜] F) U) : |
| 35 | + local_homeomorph (B × F) (B × F) := |
| 36 | +{ to_fun := λ x, (x.1, φ x.1 x.2), |
| 37 | + inv_fun := λ x, (x.1, (φ x.1).symm x.2), |
| 38 | + source := U ×ˢ univ, |
| 39 | + target := U ×ˢ univ, |
| 40 | + map_source' := λ x hx, mk_mem_prod hx.1 (mem_univ _), |
| 41 | + map_target' := λ x hx, mk_mem_prod hx.1 (mem_univ _), |
| 42 | + left_inv' := λ x _, prod.ext rfl (continuous_linear_equiv.symm_apply_apply _ _), |
| 43 | + right_inv' := λ x _, prod.ext rfl (continuous_linear_equiv.apply_symm_apply _ _), |
| 44 | + open_source := hU.prod is_open_univ, |
| 45 | + open_target := hU.prod is_open_univ, |
| 46 | + continuous_to_fun := begin |
| 47 | + have : continuous_on (λ p : B × F, ((φ p.1 : F →L[𝕜] F), p.2)) (U ×ˢ univ), |
| 48 | + { exact hφ.prod_map continuous_on_id }, |
| 49 | + exact continuous_on_fst.prod (is_bounded_bilinear_map_apply.continuous.comp_continuous_on this), |
| 50 | + end, |
| 51 | + continuous_inv_fun := begin |
| 52 | + have : continuous_on (λ p : B × F, (((φ p.1).symm : F →L[𝕜] F), p.2)) (U ×ˢ univ), |
| 53 | + { exact h2φ.prod_map continuous_on_id }, |
| 54 | + exact continuous_on_fst.prod (is_bounded_bilinear_map_apply.continuous.comp_continuous_on this), |
| 55 | + end, } |
| 56 | + |
| 57 | +/-- Compute the composition of two local homeomorphisms induced by fiberwise linear |
| 58 | +equivalences. -/ |
| 59 | +lemma trans_local_homeomorph_apply |
| 60 | + (hU : is_open U) |
| 61 | + (hφ : continuous_on (λ x, φ x : B → F →L[𝕜] F) U) |
| 62 | + (h2φ : continuous_on (λ x, (φ x).symm : B → F →L[𝕜] F) U) |
| 63 | + (hU' : is_open U') |
| 64 | + (hφ' : continuous_on (λ x, φ' x : B → F →L[𝕜] F) U') |
| 65 | + (h2φ' : continuous_on (λ x, (φ' x).symm : B → F →L[𝕜] F) U') |
| 66 | + (b : B) (v : F) : |
| 67 | + (fiberwise_linear.local_homeomorph φ hU hφ h2φ ≫ₕ |
| 68 | + fiberwise_linear.local_homeomorph φ' hU' hφ' h2φ') ⟨b, v⟩ = ⟨b, φ' b (φ b v)⟩ := |
| 69 | +rfl |
| 70 | + |
| 71 | +/-- Compute the source of the composition of two local homeomorphisms induced by fiberwise linear |
| 72 | +equivalences. -/ |
| 73 | +lemma source_trans_local_homeomorph |
| 74 | + (hU : is_open U) |
| 75 | + (hφ : continuous_on (λ x, φ x : B → F →L[𝕜] F) U) |
| 76 | + (h2φ : continuous_on (λ x, (φ x).symm : B → F →L[𝕜] F) U) |
| 77 | + (hU' : is_open U') |
| 78 | + (hφ' : continuous_on (λ x, φ' x : B → F →L[𝕜] F) U') |
| 79 | + (h2φ' : continuous_on (λ x, (φ' x).symm : B → F →L[𝕜] F) U') : |
| 80 | + (fiberwise_linear.local_homeomorph φ hU hφ h2φ ≫ₕ |
| 81 | + fiberwise_linear.local_homeomorph φ' hU' hφ' h2φ').source = (U ∩ U') ×ˢ univ := |
| 82 | +by { dsimp [fiberwise_linear.local_homeomorph], mfld_set_tac } |
| 83 | + |
| 84 | +/-- Compute the target of the composition of two local homeomorphisms induced by fiberwise linear |
| 85 | +equivalences. -/ |
| 86 | +lemma target_trans_local_homeomorph |
| 87 | + (hU : is_open U) |
| 88 | + (hφ : continuous_on (λ x, φ x : B → F →L[𝕜] F) U) |
| 89 | + (h2φ : continuous_on (λ x, (φ x).symm : B → F →L[𝕜] F) U) |
| 90 | + (hU' : is_open U') |
| 91 | + (hφ' : continuous_on (λ x, φ' x : B → F →L[𝕜] F) U') |
| 92 | + (h2φ' : continuous_on (λ x, (φ' x).symm : B → F →L[𝕜] F) U') : |
| 93 | + (fiberwise_linear.local_homeomorph φ hU hφ h2φ ≫ₕ |
| 94 | + fiberwise_linear.local_homeomorph φ' hU' hφ' h2φ').target = (U ∩ U') ×ˢ univ := |
| 95 | +by { dsimp [fiberwise_linear.local_homeomorph], mfld_set_tac } |
| 96 | + |
| 97 | +end fiberwise_linear |
| 98 | + |
| 99 | +variables {EB : Type*} [normed_add_comm_group EB] [normed_space 𝕜 EB] |
| 100 | + {HB : Type*} [topological_space HB] [charted_space HB B] {IB : model_with_corners 𝕜 EB HB} |
| 101 | + |
| 102 | +/-- Let `e` be a local homeomorphism of `B × F`. Suppose that at every point `p` in the source of |
| 103 | +`e`, there is some neighbourhood `s` of `p` on which `e` is equal to a bi-smooth fibrewise linear |
| 104 | +local homeomorphism. |
| 105 | +
|
| 106 | +Then the source of `e` is of the form `U ×ˢ univ`, for some set `U` in `B`, and, at any point `x` in |
| 107 | +`U`, admits a neighbourhood `u` of `x` such that `e` is equal on `u ×ˢ univ` to some bi-smooth |
| 108 | +fibrewise linear local homeomorphism. -/ |
| 109 | +lemma smooth_fibrewise_linear.locality_aux₁ (e : local_homeomorph (B × F) (B × F)) |
| 110 | + (h : ∀ p ∈ e.source, ∃ s : set (B × F), is_open s ∧ p ∈ s ∧ |
| 111 | + ∃ (φ : B → (F ≃L[𝕜] F)) (u : set B) (hu : is_open u) |
| 112 | + (hφ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, (φ x : F →L[𝕜] F)) u) |
| 113 | + (h2φ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, ((φ x).symm : F →L[𝕜] F)) u), |
| 114 | + (e.restr s).eq_on_source |
| 115 | + (fiberwise_linear.local_homeomorph φ hu hφ.continuous_on h2φ.continuous_on)) : |
| 116 | + ∃ (U : set B) (hU : e.source = U ×ˢ univ), |
| 117 | + ∀ x ∈ U, ∃ (φ : B → (F ≃L[𝕜] F)) (u : set B) (hu : is_open u) (huU : u ⊆ U) (hux : x ∈ u) |
| 118 | + (hφ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, (φ x : F →L[𝕜] F)) u) |
| 119 | + (h2φ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, ((φ x).symm : F →L[𝕜] F)) u), |
| 120 | + (e.restr (u ×ˢ univ)).eq_on_source |
| 121 | + (fiberwise_linear.local_homeomorph φ hu hφ.continuous_on h2φ.continuous_on) := |
| 122 | +begin |
| 123 | + rw set_coe.forall' at h, |
| 124 | + choose s hs hsp φ u hu hφ h2φ heφ using h, |
| 125 | + have hesu : ∀ p : e.source, e.source ∩ s p = u p ×ˢ univ, |
| 126 | + { intros p, |
| 127 | + rw ← e.restr_source' (s _) (hs _), |
| 128 | + exact (heφ p).1 }, |
| 129 | + have hu' : ∀ p : e.source, (p : B × F).fst ∈ u p, |
| 130 | + { intros p, |
| 131 | + have : (p : B × F) ∈ e.source ∩ s p := ⟨p.prop, hsp p⟩, |
| 132 | + simpa only [hesu, mem_prod, mem_univ, and_true] using this }, |
| 133 | + have heu : ∀ p : e.source, ∀ q : B × F, q.fst ∈ u p → q ∈ e.source, |
| 134 | + { intros p q hq, |
| 135 | + have : q ∈ u p ×ˢ (univ : set F) := ⟨hq, trivial⟩, |
| 136 | + rw ← hesu p at this, |
| 137 | + exact this.1 }, |
| 138 | + have he : e.source = (prod.fst '' e.source) ×ˢ (univ : set F), |
| 139 | + { apply has_subset.subset.antisymm, |
| 140 | + { intros p hp, |
| 141 | + exact ⟨⟨p, hp, rfl⟩, trivial⟩ }, |
| 142 | + { rintros ⟨x, v⟩ ⟨⟨p, hp, rfl : p.fst = x⟩, -⟩, |
| 143 | + exact heu ⟨p, hp⟩ (p.fst, v) (hu' ⟨p, hp⟩) } }, |
| 144 | + refine ⟨prod.fst '' e.source, he, _⟩, |
| 145 | + rintros x ⟨p, hp, rfl⟩, |
| 146 | + refine ⟨φ ⟨p, hp⟩, u ⟨p, hp⟩, hu ⟨p, hp⟩, _, hu' _, hφ ⟨p, hp⟩, h2φ ⟨p, hp⟩, _⟩, |
| 147 | + { intros y hy, refine ⟨(y, 0), heu ⟨p, hp⟩ ⟨_, _⟩ hy, rfl⟩ }, |
| 148 | + { rw [← hesu, e.restr_source_inter], exact heφ ⟨p, hp⟩ }, |
| 149 | +end |
| 150 | + |
| 151 | +/-- Let `e` be a local homeomorphism of `B × F` whose source is `U ×ˢ univ`, for some set `U` in |
| 152 | +`B`, and which, at any point `x` in `U`, admits a neighbourhood `u` of `x` such that `e` is equal on |
| 153 | +`u ×ˢ univ` to some bi-smooth fibrewise linear local homeomorphism. Then `e` itself is equal to |
| 154 | +some bi-smooth fibrewise linear local homeomorphism. |
| 155 | +
|
| 156 | +This is the key mathematical point of the `locality` condition in the construction of the |
| 157 | +`structure_groupoid` of bi-smooth fibrewise linear local homeomorphisms. The proof is by gluing |
| 158 | +together the various bi-smooth fibrewise linear local homeomorphism which exist locally. |
| 159 | +
|
| 160 | +The `U` in the conclusion is the same `U` as in the hypothesis. We state it like this, because this |
| 161 | +is exactly what we need for `smooth_fiberwise_linear`. -/ |
| 162 | +lemma smooth_fibrewise_linear.locality_aux₂ (e : local_homeomorph (B × F) (B × F)) |
| 163 | + (U : set B) (hU : e.source = U ×ˢ univ) |
| 164 | + (h : ∀ x ∈ U, ∃ (φ : B → (F ≃L[𝕜] F)) (u : set B) (hu : is_open u) (hUu : u ⊆ U) (hux : x ∈ u) |
| 165 | + (hφ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, (φ x : F →L[𝕜] F)) u) |
| 166 | + (h2φ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, ((φ x).symm : F →L[𝕜] F)) u), |
| 167 | + (e.restr (u ×ˢ univ)).eq_on_source |
| 168 | + (fiberwise_linear.local_homeomorph φ hu hφ.continuous_on h2φ.continuous_on)) : |
| 169 | + ∃ (Φ : B → (F ≃L[𝕜] F)) (U : set B) (hU₀ : is_open U) |
| 170 | + (hΦ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, (Φ x : F →L[𝕜] F)) U) |
| 171 | + (h2Φ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, ((Φ x).symm : F →L[𝕜] F)) U), |
| 172 | + e.eq_on_source (fiberwise_linear.local_homeomorph Φ hU₀ hΦ.continuous_on h2Φ.continuous_on) := |
| 173 | +begin |
| 174 | + classical, |
| 175 | + rw set_coe.forall' at h, |
| 176 | + choose! φ u hu hUu hux hφ h2φ heφ using h, |
| 177 | + have heuφ : ∀ x : U, eq_on e (λ q, (q.1, φ x q.1 q.2)) (u x ×ˢ univ), |
| 178 | + { intros x p hp, |
| 179 | + refine (heφ x).2 _, |
| 180 | + rw (heφ x).1, |
| 181 | + exact hp }, |
| 182 | + have huφ : ∀ (x x' : U) (y : B) (hyx : y ∈ u x) (hyx' : y ∈ u x'), φ x y = φ x' y, |
| 183 | + { intros p p' y hyp hyp', |
| 184 | + ext v, |
| 185 | + have h1 : e (y, v) = (y, φ p y v) := heuφ _ ⟨(id hyp : (y, v).fst ∈ u p), trivial⟩, |
| 186 | + have h2 : e (y, v) = (y, φ p' y v) := heuφ _ ⟨(id hyp' : (y, v).fst ∈ u p'), trivial⟩, |
| 187 | + exact congr_arg prod.snd (h1.symm.trans h2) }, |
| 188 | + have hUu' : U = ⋃ i, u i, |
| 189 | + { ext x, |
| 190 | + rw mem_Union, |
| 191 | + refine ⟨λ h, ⟨⟨x, h⟩, hux _⟩, _⟩, |
| 192 | + rintros ⟨x, hx⟩, |
| 193 | + exact hUu x hx }, |
| 194 | + have hU' : is_open U, |
| 195 | + { rw hUu', |
| 196 | + apply is_open_Union hu }, |
| 197 | + let Φ₀ : U → F ≃L[𝕜] F := Union_lift u (λ x, (φ x) ∘ coe) huφ U hUu'.le, |
| 198 | + let Φ : B → F ≃L[𝕜] F := λ y, if hy : y ∈ U then Φ₀ ⟨y, hy⟩ else continuous_linear_equiv.refl 𝕜 F, |
| 199 | + have hΦ : ∀ (y) (hy : y ∈ U), Φ y = Φ₀ ⟨y, hy⟩ := λ y hy, dif_pos hy, |
| 200 | + have hΦφ : ∀ x : U, ∀ y ∈ u x, Φ y = φ x y, |
| 201 | + { intros x y hyu, |
| 202 | + refine (hΦ y (hUu x hyu)).trans _, |
| 203 | + exact Union_lift_mk ⟨y, hyu⟩ _ }, |
| 204 | + have hΦ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ y, (Φ y : F →L[𝕜] F)) U, |
| 205 | + { apply cont_mdiff_on_of_locally_cont_mdiff_on, |
| 206 | + intros x hx, |
| 207 | + refine ⟨u ⟨x, hx⟩, hu ⟨x, hx⟩, hux _, _⟩, |
| 208 | + refine (cont_mdiff_on.congr (hφ ⟨x, hx⟩) _).mono (inter_subset_right _ _), |
| 209 | + intros y hy, |
| 210 | + rw hΦφ ⟨x, hx⟩ y hy }, |
| 211 | + have h2Φ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ y, ((Φ y).symm : F →L[𝕜] F)) U, |
| 212 | + { apply cont_mdiff_on_of_locally_cont_mdiff_on, |
| 213 | + intros x hx, |
| 214 | + refine ⟨u ⟨x, hx⟩, hu ⟨x, hx⟩, hux _, _⟩, |
| 215 | + refine (cont_mdiff_on.congr (h2φ ⟨x, hx⟩) _).mono (inter_subset_right _ _), |
| 216 | + intros y hy, |
| 217 | + rw hΦφ ⟨x, hx⟩ y hy }, |
| 218 | + refine ⟨Φ, U, hU', hΦ, h2Φ, hU, λ p hp, _⟩, |
| 219 | + rw [hU] at hp, |
| 220 | + -- using rw on the next line seems to cause a timeout in kernel type-checking |
| 221 | + refine (heuφ ⟨p.fst, hp.1⟩ ⟨hux _, hp.2⟩).trans _, |
| 222 | + congrm (_, _), |
| 223 | + rw hΦφ, |
| 224 | + apply hux |
| 225 | +end |
| 226 | + |
| 227 | +variables (F B IB) |
| 228 | +/-- For `B` a manifold and `F` a normed space, the groupoid on `B × F` consisting of local |
| 229 | +homeomorphisms which are bi-smooth and fibrewise linear, and induce the identity on `B`. |
| 230 | +When a (topological) vector bundle is smooth, then the composition of charts associated |
| 231 | +to the vector bundle belong to this groupoid. -/ |
| 232 | +def smooth_fiberwise_linear : structure_groupoid (B × F) := |
| 233 | +{ members := ⋃ (φ : B → F ≃L[𝕜] F) (U : set B) (hU : is_open U) |
| 234 | + (hφ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, φ x : B → F →L[𝕜] F) U) |
| 235 | + (h2φ : smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ x, (φ x).symm : B → F →L[𝕜] F) U), |
| 236 | + {e | e.eq_on_source (fiberwise_linear.local_homeomorph φ hU hφ.continuous_on h2φ.continuous_on)}, |
| 237 | + trans' := begin |
| 238 | + simp_rw [mem_Union], |
| 239 | + rintros e e' ⟨φ, U, hU, hφ, h2φ, heφ⟩ ⟨φ', U', hU', hφ', h2φ', heφ'⟩, |
| 240 | + refine ⟨λ b, (φ b).trans (φ' b), _, hU.inter hU', _, _, setoid.trans (heφ.trans' heφ') ⟨_, _⟩⟩, |
| 241 | + { show smooth_on IB 𝓘(𝕜, F →L[𝕜] F) |
| 242 | + (λ (x : B), (φ' x).to_continuous_linear_map ∘L (φ x).to_continuous_linear_map) (U ∩ U'), |
| 243 | + exact (hφ'.mono $ inter_subset_right _ _).clm_comp (hφ.mono $ inter_subset_left _ _) }, |
| 244 | + { show smooth_on IB 𝓘(𝕜, F →L[𝕜] F) (λ (x : B), |
| 245 | + (φ x).symm.to_continuous_linear_map ∘L (φ' x).symm.to_continuous_linear_map) (U ∩ U'), |
| 246 | + exact (h2φ.mono $ inter_subset_left _ _).clm_comp (h2φ'.mono $ inter_subset_right _ _) }, |
| 247 | + { apply fiberwise_linear.source_trans_local_homeomorph }, |
| 248 | + { rintros ⟨b, v⟩ hb, apply fiberwise_linear.trans_local_homeomorph_apply } |
| 249 | + end, |
| 250 | + symm' := begin |
| 251 | + simp_rw [mem_Union], |
| 252 | + rintros e ⟨φ, U, hU, hφ, h2φ, heφ⟩, |
| 253 | + refine ⟨λ b, (φ b).symm, U, hU, h2φ, _, heφ.symm'⟩, |
| 254 | + simp_rw continuous_linear_equiv.symm_symm, |
| 255 | + exact hφ |
| 256 | + end, |
| 257 | + id_mem' := begin |
| 258 | + simp_rw [mem_Union], |
| 259 | + refine ⟨λ b, continuous_linear_equiv.refl 𝕜 F, univ, is_open_univ, _, _, ⟨_, λ b hb, _⟩⟩, |
| 260 | + { apply cont_mdiff_on_const }, |
| 261 | + { apply cont_mdiff_on_const }, |
| 262 | + { simp only [fiberwise_linear.local_homeomorph, local_homeomorph.refl_local_equiv, |
| 263 | + local_equiv.refl_source, univ_prod_univ] }, |
| 264 | + { simp only [fiberwise_linear.local_homeomorph, local_homeomorph.refl_apply, prod.mk.eta, |
| 265 | + id.def, continuous_linear_equiv.coe_refl', local_homeomorph.mk_coe, local_equiv.coe_mk] }, |
| 266 | + end, |
| 267 | + locality' := begin -- the hard work has been extracted to `locality_aux₁` and `locality_aux₂` |
| 268 | + simp_rw [mem_Union], |
| 269 | + intros e he, |
| 270 | + obtain ⟨U, hU, h⟩ := smooth_fibrewise_linear.locality_aux₁ e he, |
| 271 | + exact smooth_fibrewise_linear.locality_aux₂ e U hU h, |
| 272 | + end, |
| 273 | + eq_on_source' := begin |
| 274 | + simp_rw [mem_Union], |
| 275 | + rintros e e' ⟨φ, U, hU, hφ, h2φ, heφ⟩ hee', |
| 276 | + exact ⟨φ, U, hU, hφ, h2φ, setoid.trans hee' heφ⟩, |
| 277 | + end } |
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