feat: port LinearAlgebra.Multilinear.Basis (#3330)

Author: Ruben-VandeVelde

Mathlib.lean

@@ -1236,6 +1236,7 @@ import Mathlib.LinearAlgebra.Matrix.Orthogonal
 import Mathlib.LinearAlgebra.Matrix.Symmetric
 import Mathlib.LinearAlgebra.Matrix.Trace
 import Mathlib.LinearAlgebra.Multilinear.Basic
+import Mathlib.LinearAlgebra.Multilinear.Basis
 import Mathlib.LinearAlgebra.Multilinear.TensorProduct
 import Mathlib.LinearAlgebra.Pi
 import Mathlib.LinearAlgebra.Prod

Mathlib/LinearAlgebra/Multilinear/Basis.lean

@@ -0,0 +1,70 @@
+/-
+Copyright (c) 2021 Joseph Myers. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Joseph Myers
+
+! This file was ported from Lean 3 source module linear_algebra.multilinear.basis
+! leanprover-community/mathlib commit ce11c3c2a285bbe6937e26d9792fda4e51f3fe1a
+! Please do not edit these lines, except to modify the commit id
+! if you have ported upstream changes.
+-/
+import Mathlib.LinearAlgebra.Basis
+import Mathlib.LinearAlgebra.Multilinear.Basic
+
+/-!
+# Multilinear maps in relation to bases.
+
+This file proves lemmas about the action of multilinear maps on basis vectors.
+
+## TODO
+
+ * Refactor the proofs in terms of bases of tensor products, once there is an equivalent of
+   `Basis.tensorProduct` for `pi_tensor_product`.
+
+-/
+
+
+open MultilinearMap
+
+variable {R : Type _} {ι : Type _} {n : ℕ} {M : Fin n → Type _} {M₂ : Type _} {M₃ : Type _}
+
+variable [CommSemiring R] [AddCommMonoid M₂] [AddCommMonoid M₃] [∀ i, AddCommMonoid (M i)]
+
+variable [∀ i, Module R (M i)] [Module R M₂] [Module R M₃]
+
+/-- Two multilinear maps indexed by `Fin n` are equal if they are equal when all arguments are
+basis vectors. -/
+theorem Basis.ext_multilinear_fin {f g : MultilinearMap R M M₂} {ι₁ : Fin n → Type _}
+    (e : ∀ i, Basis (ι₁ i) R (M i))
+    (h : ∀ v : ∀ i, ι₁ i, (f fun i => e i (v i)) = g fun i => e i (v i)) : f = g := by
+  induction' n with m hm
+  · ext x
+    convert h finZeroElim <;>
+    -- Porting note: added below
+    · rename_i x
+      apply finZeroElim x
+  · apply Function.LeftInverse.injective uncurry_curryLeft
+    refine' Basis.ext (e 0) _
+    intro i
+    apply hm (Fin.tail e)
+    intro j
+    convert h (Fin.cons i j)
+    iterate 2
+      rw [curryLeft_apply]
+      congr 1 with x
+      refine' Fin.cases rfl (fun x => _) x
+      dsimp [Fin.tail]
+      rw [Fin.cons_succ, Fin.cons_succ]
+#align basis.ext_multilinear_fin Basis.ext_multilinear_fin
+
+/-- Two multilinear maps indexed by a `Fintype` are equal if they are equal when all arguments
+are basis vectors. Unlike `Basis.ext_multilinear_fin`, this only uses a single basis; a
+dependently-typed version would still be true, but the proof would need a dependently-typed
+version of `dom_dom_congr`. -/
+theorem Basis.ext_multilinear [Finite ι] {f g : MultilinearMap R (fun _ : ι => M₂) M₃} {ι₁ : Type _}
+    (e : Basis ι₁ R M₂) (h : ∀ v : ι → ι₁, (f fun i => e (v i)) = g fun i => e (v i)) : f = g := by
+  cases nonempty_fintype ι
+  exact
+    (domDomCongr_eq_iff (Fintype.equivFin ι) f g).mp
+      (Basis.ext_multilinear_fin (fun _ => e) fun i => h (i ∘ _))
+#align basis.ext_multilinear Basis.ext_multilinear