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/-
Copyright (c) 2020 Oliver Nash. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Oliver Nash, Antoine Labelle
-/
import linear_algebra.dual
import linear_algebra.matrix.to_lin
import linear_algebra.tensor_product_basis
import linear_algebra.free_module.finite.rank
/-!
# Contractions
Given modules $M, N$ over a commutative ring $R$, this file defines the natural linear maps:
$M^* \otimes M \to R$, $M \otimes M^* \to R$, and $M^* \otimes N β Hom(M, N)$, as well as proving
some basic properties of these maps.
## Tags
contraction, dual module, tensor product
-/
variables {ΞΉ : Type*} (R M N P Q : Type*)
local attribute [ext] tensor_product.ext
section contraction
open tensor_product linear_map matrix module
open_locale tensor_product big_operators
section comm_semiring
variables [comm_semiring R]
variables [add_comm_monoid M] [add_comm_monoid N] [add_comm_monoid P] [add_comm_monoid Q]
variables [module R M] [module R N] [module R P] [module R Q]
variables [decidable_eq ΞΉ] [fintype ΞΉ] (b : basis ΞΉ R M)
/-- The natural left-handed pairing between a module and its dual. -/
def contract_left : (module.dual R M) β M ββ[R] R := (uncurry _ _ _ _).to_fun linear_map.id
/-- The natural right-handed pairing between a module and its dual. -/
def contract_right : M β (module.dual R M) ββ[R] R :=
(uncurry _ _ _ _).to_fun (linear_map.flip linear_map.id)
/-- The natural map associating a linear map to the tensor product of two modules. -/
def dual_tensor_hom : (module.dual R M) β N ββ[R] M ββ[R] N :=
let M' := module.dual R M in
(uncurry R M' N (M ββ[R] N) : _ β M' β N ββ[R] M ββ[R] N) linear_map.smul_rightβ
variables {R M N P Q}
@[simp] lemma contract_left_apply (f : module.dual R M) (m : M) :
contract_left R M (f ββ m) = f m := by apply uncurry_apply
@[simp] lemma contract_right_apply (f : module.dual R M) (m : M) :
contract_right R M (m ββ f) = f m := by apply uncurry_apply
@[simp] lemma dual_tensor_hom_apply (f : module.dual R M) (m : M) (n : N) :
dual_tensor_hom R M N (f ββ n) m = (f m) β’ n :=
by { dunfold dual_tensor_hom, rw uncurry_apply, refl, }
@[simp] lemma transpose_dual_tensor_hom (f : module.dual R M) (m : M) :
dual.transpose (dual_tensor_hom R M M (f ββ m)) = dual_tensor_hom R _ _ (dual.eval R M m ββ f) :=
by { ext f' m', simp only [dual.transpose_apply, coe_comp, function.comp_app, dual_tensor_hom_apply,
linear_map.map_smulββ, ring_hom.id_apply, algebra.id.smul_eq_mul, dual.eval_apply, smul_apply],
exact mul_comm _ _ }
@[simp] lemma dual_tensor_hom_prod_map_zero (f : module.dual R M) (p : P) :
((dual_tensor_hom R M P) (f ββ[R] p)).prod_map (0 : N ββ[R] Q) =
dual_tensor_hom R (M Γ N) (P Γ Q) ((f ββ fst R M N) ββ inl R P Q p) :=
by {ext; simp only [coe_comp, coe_inl, function.comp_app, prod_map_apply, dual_tensor_hom_apply,
fst_apply, prod.smul_mk, zero_apply, smul_zero]}
@[simp] lemma zero_prod_map_dual_tensor_hom (g : module.dual R N) (q : Q) :
(0 : M ββ[R] P).prod_map ((dual_tensor_hom R N Q) (g ββ[R] q)) =
dual_tensor_hom R (M Γ N) (P Γ Q) ((g ββ snd R M N) ββ inr R P Q q) :=
by {ext; simp only [coe_comp, coe_inr, function.comp_app, prod_map_apply, dual_tensor_hom_apply,
snd_apply, prod.smul_mk, zero_apply, smul_zero]}
lemma map_dual_tensor_hom (f : module.dual R M) (p : P) (g : module.dual R N) (q : Q) :
tensor_product.map (dual_tensor_hom R M P (f ββ[R] p)) (dual_tensor_hom R N Q (g ββ[R] q)) =
dual_tensor_hom R (M β[R] N) (P β[R] Q) (dual_distrib R M N (f ββ g) ββ[R] (p ββ[R] q)) :=
begin
ext m n, simp only [comprβ_apply, mk_apply, map_tmul, dual_tensor_hom_apply,
dual_distrib_apply, βsmul_tmul_smul],
end
@[simp] lemma comp_dual_tensor_hom (f : module.dual R M) (n : N) (g : module.dual R N) (p : P) :
(dual_tensor_hom R N P (g ββ[R] p)) ββ (dual_tensor_hom R M N (f ββ[R] n)) =
g n β’ dual_tensor_hom R M P (f ββ p) :=
begin
ext m, simp only [coe_comp, function.comp_app, dual_tensor_hom_apply, linear_map.map_smul,
ring_hom.id_apply, smul_apply], rw smul_comm,
end
/-- As a matrix, `dual_tensor_hom` evaluated on a basis element of `M* β N` is a matrix with a
single one and zeros elsewhere -/
theorem to_matrix_dual_tensor_hom
{m : Type*} {n : Type*} [fintype m] [fintype n] [decidable_eq m] [decidable_eq n]
(bM : basis m R M) (bN : basis n R N) (j : m) (i : n) :
to_matrix bM bN (dual_tensor_hom R M N (bM.coord j ββ bN i)) = std_basis_matrix i j 1 :=
begin
ext i' j',
by_cases hij : (i = i' β§ j = j');
simp [linear_map.to_matrix_apply, finsupp.single_eq_pi_single, hij],
rw [and_iff_not_or_not, not_not] at hij, cases hij; simp [hij],
end
end comm_semiring
section comm_ring
variables [comm_ring R]
variables [add_comm_group M] [add_comm_group N] [add_comm_group P] [add_comm_group Q]
variables [module R M] [module R N] [module R P] [module R Q]
variables [decidable_eq ΞΉ] [fintype ΞΉ] (b : basis ΞΉ R M)
variables {R M N P Q}
/-- If `M` is free, the natural linear map $M^* β N β Hom(M, N)$ is an equivalence. This function
provides this equivalence in return for a basis of `M`. -/
@[simps apply]
noncomputable def dual_tensor_hom_equiv_of_basis :
(module.dual R M) β[R] N ββ[R] M ββ[R] N :=
linear_equiv.of_linear
(dual_tensor_hom R M N)
(β i, (tensor_product.mk R _ N (b.dual_basis i)) ββ linear_map.applyβ (b i))
(begin
ext f m,
simp only [applyβ_apply_apply, coe_fn_sum, dual_tensor_hom_apply, mk_apply, id_coe, id.def,
fintype.sum_apply, function.comp_app, basis.coe_dual_basis, coe_comp,
basis.coord_apply, β f.map_smul, (dual_tensor_hom R M N).map_sum, β f.map_sum, b.sum_repr],
end)
(begin
ext f m,
simp only [applyβ_apply_apply, coe_fn_sum, dual_tensor_hom_apply, mk_apply, id_coe, id.def,
fintype.sum_apply, function.comp_app, basis.coe_dual_basis, coe_comp,
comprβ_apply, tmul_smul, smul_tmul', β sum_tmul, basis.sum_dual_apply_smul_coord],
end)
@[simp] lemma dual_tensor_hom_equiv_of_basis_to_linear_map :
(dual_tensor_hom_equiv_of_basis b : (module.dual R M) β[R] N ββ[R] M ββ[R] N).to_linear_map =
dual_tensor_hom R M N :=
rfl
@[simp] lemma dual_tensor_hom_equiv_of_basis_symm_cancel_left (x : (module.dual R M) β[R] N) :
(dual_tensor_hom_equiv_of_basis b).symm (dual_tensor_hom R M N x) = x :=
by rw [βdual_tensor_hom_equiv_of_basis_apply b, linear_equiv.symm_apply_apply]
@[simp] lemma dual_tensor_hom_equiv_of_basis_symm_cancel_right (x : M ββ[R] N) :
dual_tensor_hom R M N ((dual_tensor_hom_equiv_of_basis b).symm x) = x :=
by rw [βdual_tensor_hom_equiv_of_basis_apply b, linear_equiv.apply_symm_apply]
variables (R M N P Q)
variables [module.free R M] [module.finite R M] [nontrivial R]
open_locale classical
/-- If `M` is finite free, the natural map $M^* β N β Hom(M, N)$ is an
equivalence. -/
@[simp] noncomputable def dual_tensor_hom_equiv : (module.dual R M) β[R] N ββ[R] M ββ[R] N :=
dual_tensor_hom_equiv_of_basis (module.free.choose_basis R M)
end comm_ring
end contraction
section hom_tensor_hom
open_locale tensor_product
open module tensor_product linear_map
section comm_ring
variables [comm_ring R]
variables [add_comm_group M] [add_comm_group N] [add_comm_group P] [add_comm_group Q]
variables [module R M] [module R N] [module R P] [module R Q]
variables [free R M] [finite R M] [free R N] [finite R N] [nontrivial R]
/-- When `M` is a finite free module, the map `ltensor_hom_to_hom_ltensor` is an equivalence. Note
that `ltensor_hom_equiv_hom_ltensor` is not defined directly in terms of
`ltensor_hom_to_hom_ltensor`, but the equivalence between the two is given by
`ltensor_hom_equiv_hom_ltensor_to_linear_map` and `ltensor_hom_equiv_hom_ltensor_apply`. -/
noncomputable def ltensor_hom_equiv_hom_ltensor : P β[R] (M ββ[R] Q) ββ[R] (M ββ[R] P β[R] Q) :=
congr (linear_equiv.refl R P) (dual_tensor_hom_equiv R M Q).symm βͺβ«β
tensor_product.left_comm R P _ Q βͺβ«β dual_tensor_hom_equiv R M _
/-- When `M` is a finite free module, the map `rtensor_hom_to_hom_rtensor` is an equivalence. Note
that `rtensor_hom_equiv_hom_rtensor` is not defined directly in terms of
`rtensor_hom_to_hom_rtensor`, but the equivalence between the two is given by
`rtensor_hom_equiv_hom_rtensor_to_linear_map` and `rtensor_hom_equiv_hom_rtensor_apply`. -/
noncomputable def rtensor_hom_equiv_hom_rtensor : (M ββ[R] P) β[R] Q ββ[R] (M ββ[R] P β[R] Q) :=
congr (dual_tensor_hom_equiv R M P).symm (linear_equiv.refl R Q) βͺβ«β
tensor_product.assoc R _ P Q βͺβ«β dual_tensor_hom_equiv R M _
@[simp] lemma ltensor_hom_equiv_hom_ltensor_to_linear_map :
(ltensor_hom_equiv_hom_ltensor R M P Q).to_linear_map = ltensor_hom_to_hom_ltensor R M P Q :=
begin
let e := congr (linear_equiv.refl R P) (dual_tensor_hom_equiv R M Q),
have h : function.surjective e.to_linear_map := e.surjective,
refine (cancel_right h).1 _,
ext p f q m,
simp only [ltensor_hom_equiv_hom_ltensor, dual_tensor_hom_equiv, comprβ_apply, mk_apply, coe_comp,
linear_equiv.coe_to_linear_map, function.comp_app, map_tmul, linear_equiv.coe_coe,
dual_tensor_hom_equiv_of_basis_apply, linear_equiv.trans_apply, congr_tmul,
linear_equiv.refl_apply, dual_tensor_hom_equiv_of_basis_symm_cancel_left, left_comm_tmul,
dual_tensor_hom_apply, ltensor_hom_to_hom_ltensor_apply, tmul_smul],
end
@[simp] lemma rtensor_hom_equiv_hom_rtensor_to_linear_map :
(rtensor_hom_equiv_hom_rtensor R M P Q).to_linear_map = rtensor_hom_to_hom_rtensor R M P Q :=
begin
let e := congr (dual_tensor_hom_equiv R M P) (linear_equiv.refl R Q),
have h : function.surjective e.to_linear_map := e.surjective,
refine (cancel_right h).1 _,
ext f p q m,
simp only [rtensor_hom_equiv_hom_rtensor, dual_tensor_hom_equiv, comprβ_apply, mk_apply, coe_comp,
linear_equiv.coe_to_linear_map, function.comp_app, map_tmul, linear_equiv.coe_coe,
dual_tensor_hom_equiv_of_basis_apply, linear_equiv.trans_apply, congr_tmul,
dual_tensor_hom_equiv_of_basis_symm_cancel_left, linear_equiv.refl_apply, assoc_tmul,
dual_tensor_hom_apply, rtensor_hom_to_hom_rtensor_apply, smul_tmul'],
end
variables {R M N P Q}
@[simp] lemma ltensor_hom_equiv_hom_ltensor_apply (x : P β[R] (M ββ[R] Q)) :
ltensor_hom_equiv_hom_ltensor R M P Q x = ltensor_hom_to_hom_ltensor R M P Q x :=
by rw [βlinear_equiv.coe_to_linear_map, ltensor_hom_equiv_hom_ltensor_to_linear_map]
@[simp] lemma rtensor_hom_equiv_hom_rtensor_apply (x : (M ββ[R] P) β[R] Q) :
rtensor_hom_equiv_hom_rtensor R M P Q x = rtensor_hom_to_hom_rtensor R M P Q x :=
by rw [βlinear_equiv.coe_to_linear_map, rtensor_hom_equiv_hom_rtensor_to_linear_map]
variables (R M N P Q)
/--
When `M` and `N` are free `R` modules, the map `hom_tensor_hom_map` is an equivalence. Note that
`hom_tensor_hom_equiv` is not defined directly in terms of `hom_tensor_hom_map`, but the equivalence
between the two is given by `hom_tensor_hom_equiv_to_linear_map` and `hom_tensor_hom_equiv_apply`.
-/
noncomputable
def hom_tensor_hom_equiv : (M ββ[R] P) β[R] (N ββ[R] Q) ββ[R] (M β[R] N ββ[R] P β[R] Q) :=
rtensor_hom_equiv_hom_rtensor R M P _ βͺβ«β
(linear_equiv.refl R M).arrow_congr (ltensor_hom_equiv_hom_ltensor R N _ Q) βͺβ«β
lift.equiv R M N _
@[simp]
lemma hom_tensor_hom_equiv_to_linear_map :
(hom_tensor_hom_equiv R M N P Q).to_linear_map = hom_tensor_hom_map R M N P Q :=
begin
ext f g m n,
simp only [hom_tensor_hom_equiv, comprβ_apply, mk_apply, linear_equiv.coe_to_linear_map,
linear_equiv.trans_apply, lift.equiv_apply, linear_equiv.arrow_congr_apply,
linear_equiv.refl_symm, linear_equiv.refl_apply, rtensor_hom_equiv_hom_rtensor_apply,
ltensor_hom_equiv_hom_ltensor_apply, ltensor_hom_to_hom_ltensor_apply,
rtensor_hom_to_hom_rtensor_apply, hom_tensor_hom_map_apply, map_tmul],
end
variables {R M N P Q}
@[simp]
lemma hom_tensor_hom_equiv_apply (x : (M ββ[R] P) β[R] (N ββ[R] Q)) :
hom_tensor_hom_equiv R M N P Q x = hom_tensor_hom_map R M N P Q x :=
by rw [βlinear_equiv.coe_to_linear_map, hom_tensor_hom_equiv_to_linear_map]
end comm_ring
end hom_tensor_hom
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