Datasets:
Tasks:
Text Generation
Modalities:
Text
Sub-tasks:
language-modeling
Languages:
English
Size:
100K - 1M
License:
/- | |
Copyright (c) 2019 Johan Commelin. All rights reserved. | |
Released under Apache 2.0 license as described in the file LICENSE. | |
Authors: Johan Commelin, Fabian Glöckle | |
-/ | |
import linear_algebra.finite_dimensional | |
import linear_algebra.projection | |
import linear_algebra.sesquilinear_form | |
import ring_theory.finiteness | |
import linear_algebra.free_module.finite.rank | |
/-! | |
# Dual vector spaces | |
The dual space of an R-module M is the R-module of linear maps `M → R`. | |
## Main definitions | |
* `dual R M` defines the dual space of M over R. | |
* Given a basis for an `R`-module `M`, `basis.to_dual` produces a map from `M` to `dual R M`. | |
* Given families of vectors `e` and `ε`, `dual_pair e ε` states that these families have the | |
characteristic properties of a basis and a dual. | |
* `dual_annihilator W` is the submodule of `dual R M` where every element annihilates `W`. | |
## Main results | |
* `to_dual_equiv` : the linear equivalence between the dual module and primal module, | |
given a finite basis. | |
* `dual_pair.basis` and `dual_pair.eq_dual`: if `e` and `ε` form a dual pair, `e` is a basis and | |
`ε` is its dual basis. | |
* `quot_equiv_annihilator`: the quotient by a subspace is isomorphic to its dual annihilator. | |
## Notation | |
We sometimes use `V'` as local notation for `dual K V`. | |
## TODO | |
Erdös-Kaplansky theorem about the dimension of a dual vector space in case of infinite dimension. | |
-/ | |
noncomputable theory | |
namespace module | |
variables (R : Type*) (M : Type*) | |
variables [comm_semiring R] [add_comm_monoid M] [module R M] | |
/-- The dual space of an R-module M is the R-module of linear maps `M → R`. -/ | |
@[derive [add_comm_monoid, module R]] def dual := M →ₗ[R] R | |
instance {S : Type*} [comm_ring S] {N : Type*} [add_comm_group N] [module S N] : | |
add_comm_group (dual S N) := linear_map.add_comm_group | |
instance : linear_map_class (dual R M) R M R := | |
linear_map.semilinear_map_class | |
/-- The canonical pairing of a vector space and its algebraic dual. -/ | |
def dual_pairing (R M) [comm_semiring R] [add_comm_monoid M] [module R M] : | |
module.dual R M →ₗ[R] M →ₗ[R] R := linear_map.id | |
@[simp] lemma dual_pairing_apply (v x) : dual_pairing R M v x = v x := rfl | |
namespace dual | |
instance : inhabited (dual R M) := linear_map.inhabited | |
instance : has_coe_to_fun (dual R M) (λ _, M → R) := ⟨linear_map.to_fun⟩ | |
/-- Maps a module M to the dual of the dual of M. See `module.erange_coe` and | |
`module.eval_equiv`. -/ | |
def eval : M →ₗ[R] (dual R (dual R M)) := linear_map.flip linear_map.id | |
@[simp] lemma eval_apply (v : M) (a : dual R M) : eval R M v a = a v := | |
begin | |
dunfold eval, | |
rw [linear_map.flip_apply, linear_map.id_apply] | |
end | |
variables {R M} {M' : Type*} [add_comm_monoid M'] [module R M'] | |
/-- The transposition of linear maps, as a linear map from `M →ₗ[R] M'` to | |
`dual R M' →ₗ[R] dual R M`. -/ | |
def transpose : (M →ₗ[R] M') →ₗ[R] (dual R M' →ₗ[R] dual R M) := | |
(linear_map.llcomp R M M' R).flip | |
lemma transpose_apply (u : M →ₗ[R] M') (l : dual R M') : transpose u l = l.comp u := rfl | |
variables {M'' : Type*} [add_comm_monoid M''] [module R M''] | |
lemma transpose_comp (u : M' →ₗ[R] M'') (v : M →ₗ[R] M') : | |
transpose (u.comp v) = (transpose v).comp (transpose u) := rfl | |
end dual | |
end module | |
namespace basis | |
universes u v w | |
open module module.dual submodule linear_map cardinal function | |
open_locale big_operators | |
variables {R M K V ι : Type*} | |
section comm_semiring | |
variables [comm_semiring R] [add_comm_monoid M] [module R M] [decidable_eq ι] | |
variables (b : basis ι R M) | |
/-- The linear map from a vector space equipped with basis to its dual vector space, | |
taking basis elements to corresponding dual basis elements. -/ | |
def to_dual : M →ₗ[R] module.dual R M := | |
b.constr ℕ $ λ v, b.constr ℕ $ λ w, if w = v then (1 : R) else 0 | |
lemma to_dual_apply (i j : ι) : | |
b.to_dual (b i) (b j) = if i = j then 1 else 0 := | |
by { erw [constr_basis b, constr_basis b], ac_refl } | |
@[simp] lemma to_dual_total_left (f : ι →₀ R) (i : ι) : | |
b.to_dual (finsupp.total ι M R b f) (b i) = f i := | |
begin | |
rw [finsupp.total_apply, finsupp.sum, linear_map.map_sum, linear_map.sum_apply], | |
simp_rw [linear_map.map_smul, linear_map.smul_apply, to_dual_apply, smul_eq_mul, | |
mul_boole, finset.sum_ite_eq'], | |
split_ifs with h, | |
{ refl }, | |
{ rw finsupp.not_mem_support_iff.mp h } | |
end | |
@[simp] lemma to_dual_total_right (f : ι →₀ R) (i : ι) : | |
b.to_dual (b i) (finsupp.total ι M R b f) = f i := | |
begin | |
rw [finsupp.total_apply, finsupp.sum, linear_map.map_sum], | |
simp_rw [linear_map.map_smul, to_dual_apply, smul_eq_mul, mul_boole, finset.sum_ite_eq], | |
split_ifs with h, | |
{ refl }, | |
{ rw finsupp.not_mem_support_iff.mp h } | |
end | |
lemma to_dual_apply_left (m : M) (i : ι) : b.to_dual m (b i) = b.repr m i := | |
by rw [← b.to_dual_total_left, b.total_repr] | |
lemma to_dual_apply_right (i : ι) (m : M) : b.to_dual (b i) m = b.repr m i := | |
by rw [← b.to_dual_total_right, b.total_repr] | |
lemma coe_to_dual_self (i : ι) : b.to_dual (b i) = b.coord i := | |
by { ext, apply to_dual_apply_right } | |
/-- `h.to_dual_flip v` is the linear map sending `w` to `h.to_dual w v`. -/ | |
def to_dual_flip (m : M) : (M →ₗ[R] R) := b.to_dual.flip m | |
lemma to_dual_flip_apply (m₁ m₂ : M) : b.to_dual_flip m₁ m₂ = b.to_dual m₂ m₁ := rfl | |
lemma to_dual_eq_repr (m : M) (i : ι) : b.to_dual m (b i) = b.repr m i := | |
b.to_dual_apply_left m i | |
lemma to_dual_eq_equiv_fun [fintype ι] (m : M) (i : ι) : b.to_dual m (b i) = b.equiv_fun m i := | |
by rw [b.equiv_fun_apply, to_dual_eq_repr] | |
lemma to_dual_inj (m : M) (a : b.to_dual m = 0) : m = 0 := | |
begin | |
rw [← mem_bot R, ← b.repr.ker, mem_ker, linear_equiv.coe_coe], | |
apply finsupp.ext, | |
intro b, | |
rw [← to_dual_eq_repr, a], | |
refl | |
end | |
theorem to_dual_ker : b.to_dual.ker = ⊥ := | |
ker_eq_bot'.mpr b.to_dual_inj | |
theorem to_dual_range [fin : fintype ι] : b.to_dual.range = ⊤ := | |
begin | |
rw eq_top_iff', | |
intro f, | |
rw linear_map.mem_range, | |
let lin_comb : ι →₀ R := finsupp.on_finset fin.elems (λ i, f.to_fun (b i)) _, | |
{ use finsupp.total ι M R b lin_comb, | |
apply b.ext, | |
{ intros i, | |
rw [b.to_dual_eq_repr _ i, repr_total b], | |
{ refl } } }, | |
{ intros a _, | |
apply fin.complete } | |
end | |
end comm_semiring | |
section | |
variables [comm_semiring R] [add_comm_monoid M] [module R M] [fintype ι] | |
variables (b : basis ι R M) | |
@[simp] lemma sum_dual_apply_smul_coord (f : module.dual R M) : ∑ x, f (b x) • b.coord x = f := | |
begin | |
ext m, | |
simp_rw [linear_map.sum_apply, linear_map.smul_apply, smul_eq_mul, mul_comm (f _), ←smul_eq_mul, | |
←f.map_smul, ←f.map_sum, basis.coord_apply, basis.sum_repr], | |
end | |
end | |
section comm_ring | |
variables [comm_ring R] [add_comm_group M] [module R M] [decidable_eq ι] | |
variables (b : basis ι R M) | |
/-- A vector space is linearly equivalent to its dual space. -/ | |
@[simps] | |
def to_dual_equiv [fintype ι] : M ≃ₗ[R] (dual R M) := | |
linear_equiv.of_bijective b.to_dual | |
(ker_eq_bot.mp b.to_dual_ker) (range_eq_top.mp b.to_dual_range) | |
/-- Maps a basis for `V` to a basis for the dual space. -/ | |
def dual_basis [fintype ι] : basis ι R (dual R M) := | |
b.map b.to_dual_equiv | |
-- We use `j = i` to match `basis.repr_self` | |
lemma dual_basis_apply_self [fintype ι] (i j : ι) : | |
b.dual_basis i (b j) = if j = i then 1 else 0 := | |
by { convert b.to_dual_apply i j using 2, rw _ j i } | |
lemma total_dual_basis [fintype ι] (f : ι →₀ R) (i : ι) : | |
finsupp.total ι (dual R M) R b.dual_basis f (b i) = f i := | |
begin | |
rw [finsupp.total_apply, finsupp.sum_fintype, linear_map.sum_apply], | |
{ simp_rw [linear_map.smul_apply, smul_eq_mul, dual_basis_apply_self, mul_boole, | |
finset.sum_ite_eq, if_pos (finset.mem_univ i)] }, | |
{ intro, rw zero_smul }, | |
end | |
lemma dual_basis_repr [fintype ι] (l : dual R M) (i : ι) : | |
b.dual_basis.repr l i = l (b i) := | |
by rw [← total_dual_basis b, basis.total_repr b.dual_basis l] | |
lemma dual_basis_equiv_fun [fintype ι] (l : dual R M) (i : ι) : | |
b.dual_basis.equiv_fun l i = l (b i) := | |
by rw [basis.equiv_fun_apply, dual_basis_repr] | |
lemma dual_basis_apply [fintype ι] (i : ι) (m : M) : b.dual_basis i m = b.repr m i := | |
b.to_dual_apply_right i m | |
@[simp] lemma coe_dual_basis [fintype ι] : | |
⇑b.dual_basis = b.coord := | |
by { ext i x, apply dual_basis_apply } | |
@[simp] lemma to_dual_to_dual [fintype ι] : | |
b.dual_basis.to_dual.comp b.to_dual = dual.eval R M := | |
begin | |
refine b.ext (λ i, b.dual_basis.ext (λ j, _)), | |
rw [linear_map.comp_apply, to_dual_apply_left, coe_to_dual_self, ← coe_dual_basis, | |
dual.eval_apply, basis.repr_self, finsupp.single_apply, dual_basis_apply_self] | |
end | |
theorem eval_ker {ι : Type*} (b : basis ι R M) : | |
(dual.eval R M).ker = ⊥ := | |
begin | |
rw ker_eq_bot', | |
intros m hm, | |
simp_rw [linear_map.ext_iff, dual.eval_apply, zero_apply] at hm, | |
exact (basis.forall_coord_eq_zero_iff _).mp (λ i, hm (b.coord i)) | |
end | |
lemma eval_range {ι : Type*} [fintype ι] (b : basis ι R M) : | |
(eval R M).range = ⊤ := | |
begin | |
classical, | |
rw [← b.to_dual_to_dual, range_comp, b.to_dual_range, map_top, to_dual_range _], | |
apply_instance | |
end | |
/-- A module with a basis is linearly equivalent to the dual of its dual space. -/ | |
def eval_equiv {ι : Type*} [fintype ι] (b : basis ι R M) : M ≃ₗ[R] dual R (dual R M) := | |
linear_equiv.of_bijective (eval R M) | |
(ker_eq_bot.mp b.eval_ker) (range_eq_top.mp b.eval_range) | |
@[simp] lemma eval_equiv_to_linear_map {ι : Type*} [fintype ι] (b : basis ι R M) : | |
(b.eval_equiv).to_linear_map = dual.eval R M := rfl | |
section | |
open_locale classical | |
variables [finite R M] [free R M] [nontrivial R] | |
instance dual_free : free R (dual R M) := free.of_basis (free.choose_basis R M).dual_basis | |
instance dual_finite : finite R (dual R M) := finite.of_basis (free.choose_basis R M).dual_basis | |
end | |
end comm_ring | |
/-- `simp` normal form version of `total_dual_basis` -/ | |
@[simp] lemma total_coord [comm_ring R] [add_comm_group M] [module R M] [fintype ι] | |
(b : basis ι R M) (f : ι →₀ R) (i : ι) : | |
finsupp.total ι (dual R M) R b.coord f (b i) = f i := | |
by { haveI := classical.dec_eq ι, rw [← coe_dual_basis, total_dual_basis] } | |
-- TODO(jmc): generalize to rings, once `module.rank` is generalized | |
theorem dual_dim_eq [field K] [add_comm_group V] [module K V] [fintype ι] (b : basis ι K V) : | |
cardinal.lift (module.rank K V) = module.rank K (dual K V) := | |
begin | |
classical, | |
have := linear_equiv.lift_dim_eq b.to_dual_equiv, | |
simp only [cardinal.lift_umax] at this, | |
rw [this, ← cardinal.lift_umax], | |
apply cardinal.lift_id, | |
end | |
end basis | |
namespace module | |
variables {K V : Type*} | |
variables [field K] [add_comm_group V] [module K V] | |
open module module.dual submodule linear_map cardinal basis finite_dimensional | |
theorem eval_ker : (eval K V).ker = ⊥ := | |
by { classical, exact (basis.of_vector_space K V).eval_ker } | |
-- TODO(jmc): generalize to rings, once `module.rank` is generalized | |
theorem dual_dim_eq [finite_dimensional K V] : | |
cardinal.lift (module.rank K V) = module.rank K (dual K V) := | |
(basis.of_vector_space K V).dual_dim_eq | |
lemma erange_coe [finite_dimensional K V] : (eval K V).range = ⊤ := | |
begin | |
letI : is_noetherian K V := is_noetherian.iff_fg.2 infer_instance, | |
exact (basis.of_vector_space K V).eval_range | |
end | |
variables (K V) | |
/-- A vector space is linearly equivalent to the dual of its dual space. -/ | |
def eval_equiv [finite_dimensional K V] : V ≃ₗ[K] dual K (dual K V) := | |
linear_equiv.of_bijective (eval K V) | |
(ker_eq_bot.mp eval_ker) (range_eq_top.mp erange_coe) | |
variables {K V} | |
@[simp] lemma eval_equiv_to_linear_map [finite_dimensional K V] : | |
(eval_equiv K V).to_linear_map = dual.eval K V := rfl | |
end module | |
section dual_pair | |
open module | |
variables {R M ι : Type*} | |
variables [comm_semiring R] [add_comm_monoid M] [module R M] [decidable_eq ι] | |
/-- `e` and `ε` have characteristic properties of a basis and its dual -/ | |
@[nolint has_inhabited_instance] | |
structure dual_pair (e : ι → M) (ε : ι → (dual R M)) := | |
(eval : ∀ i j : ι, ε i (e j) = if i = j then 1 else 0) | |
(total : ∀ {m : M}, (∀ i, ε i m = 0) → m = 0) | |
[finite : ∀ m : M, fintype {i | ε i m ≠ 0}] | |
end dual_pair | |
namespace dual_pair | |
open module module.dual linear_map function | |
variables {R M ι : Type*} | |
variables [comm_ring R] [add_comm_group M] [module R M] | |
variables {e : ι → M} {ε : ι → dual R M} | |
/-- The coefficients of `v` on the basis `e` -/ | |
def coeffs [decidable_eq ι] (h : dual_pair e ε) (m : M) : ι →₀ R := | |
{ to_fun := λ i, ε i m, | |
support := by { haveI := h.finite m, exact {i : ι | ε i m ≠ 0}.to_finset }, | |
mem_support_to_fun := by {intro i, rw set.mem_to_finset, exact iff.rfl } } | |
@[simp] lemma coeffs_apply [decidable_eq ι] (h : dual_pair e ε) (m : M) (i : ι) : | |
h.coeffs m i = ε i m := rfl | |
/-- linear combinations of elements of `e`. | |
This is a convenient abbreviation for `finsupp.total _ M R e l` -/ | |
def lc {ι} (e : ι → M) (l : ι →₀ R) : M := l.sum (λ (i : ι) (a : R), a • (e i)) | |
lemma lc_def (e : ι → M) (l : ι →₀ R) : lc e l = finsupp.total _ _ _ e l := rfl | |
variables [decidable_eq ι] (h : dual_pair e ε) | |
include h | |
lemma dual_lc (l : ι →₀ R) (i : ι) : ε i (dual_pair.lc e l) = l i := | |
begin | |
erw linear_map.map_sum, | |
simp only [h.eval, map_smul, smul_eq_mul], | |
rw finset.sum_eq_single i, | |
{ simp }, | |
{ intros q q_in q_ne, | |
simp [q_ne.symm] }, | |
{ intro p_not_in, | |
simp [finsupp.not_mem_support_iff.1 p_not_in] }, | |
end | |
@[simp] | |
lemma coeffs_lc (l : ι →₀ R) : h.coeffs (dual_pair.lc e l) = l := | |
by { ext i, rw [h.coeffs_apply, h.dual_lc] } | |
/-- For any m : M n, \sum_{p ∈ Q n} (ε p m) • e p = m -/ | |
@[simp] | |
lemma lc_coeffs (m : M) : dual_pair.lc e (h.coeffs m) = m := | |
begin | |
refine eq_of_sub_eq_zero (h.total _), | |
intros i, | |
simp [-sub_eq_add_neg, linear_map.map_sub, h.dual_lc, sub_eq_zero] | |
end | |
/-- `(h : dual_pair e ε).basis` shows the family of vectors `e` forms a basis. -/ | |
@[simps] | |
def basis : basis ι R M := | |
basis.of_repr | |
{ to_fun := coeffs h, | |
inv_fun := lc e, | |
left_inv := lc_coeffs h, | |
right_inv := coeffs_lc h, | |
map_add' := λ v w, by { ext i, exact (ε i).map_add v w }, | |
map_smul' := λ c v, by { ext i, exact (ε i).map_smul c v } } | |
@[simp] lemma coe_basis : ⇑h.basis = e := | |
by { ext i, rw basis.apply_eq_iff, ext j, | |
rw [h.basis_repr_apply, coeffs_apply, h.eval, finsupp.single_apply], | |
convert if_congr eq_comm rfl rfl } -- `convert` to get rid of a `decidable_eq` mismatch | |
lemma mem_of_mem_span {H : set ι} {x : M} (hmem : x ∈ submodule.span R (e '' H)) : | |
∀ i : ι, ε i x ≠ 0 → i ∈ H := | |
begin | |
intros i hi, | |
rcases (finsupp.mem_span_image_iff_total _).mp hmem with ⟨l, supp_l, rfl⟩, | |
apply not_imp_comm.mp ((finsupp.mem_supported' _ _).mp supp_l i), | |
rwa [← lc_def, h.dual_lc] at hi | |
end | |
lemma coe_dual_basis [fintype ι] : ⇑h.basis.dual_basis = ε := | |
funext (λ i, h.basis.ext (λ j, by rw [h.basis.dual_basis_apply_self, h.coe_basis, h.eval, | |
if_congr eq_comm rfl rfl])) | |
end dual_pair | |
namespace submodule | |
universes u v w | |
variables {R : Type u} {M : Type v} [comm_semiring R] [add_comm_monoid M] [module R M] | |
variable {W : submodule R M} | |
/-- The `dual_restrict` of a submodule `W` of `M` is the linear map from the | |
dual of `M` to the dual of `W` such that the domain of each linear map is | |
restricted to `W`. -/ | |
def dual_restrict (W : submodule R M) : | |
module.dual R M →ₗ[R] module.dual R W := | |
linear_map.dom_restrict' W | |
@[simp] lemma dual_restrict_apply | |
(W : submodule R M) (φ : module.dual R M) (x : W) : | |
W.dual_restrict φ x = φ (x : M) := rfl | |
/-- The `dual_annihilator` of a submodule `W` is the set of linear maps `φ` such | |
that `φ w = 0` for all `w ∈ W`. -/ | |
def dual_annihilator {R : Type u} {M : Type v} [comm_semiring R] [add_comm_monoid M] | |
[module R M] (W : submodule R M) : submodule R $ module.dual R M := | |
W.dual_restrict.ker | |
@[simp] lemma mem_dual_annihilator (φ : module.dual R M) : | |
φ ∈ W.dual_annihilator ↔ ∀ w ∈ W, φ w = 0 := | |
begin | |
refine linear_map.mem_ker.trans _, | |
simp_rw [linear_map.ext_iff, dual_restrict_apply], | |
exact ⟨λ h w hw, h ⟨w, hw⟩, λ h w, h w.1 w.2⟩ | |
end | |
lemma dual_restrict_ker_eq_dual_annihilator (W : submodule R M) : | |
W.dual_restrict.ker = W.dual_annihilator := | |
rfl | |
lemma dual_annihilator_sup_eq_inf_dual_annihilator (U V : submodule R M) : | |
(U ⊔ V).dual_annihilator = U.dual_annihilator ⊓ V.dual_annihilator := | |
begin | |
ext φ, | |
rw [mem_inf, mem_dual_annihilator, mem_dual_annihilator, mem_dual_annihilator], | |
split; intro h, | |
{ refine ⟨_, _⟩; | |
intros x hx, | |
exact h x (mem_sup.2 ⟨x, hx, 0, zero_mem _, add_zero _⟩), | |
exact h x (mem_sup.2 ⟨0, zero_mem _, x, hx, zero_add _⟩) }, | |
{ simp_rw mem_sup, | |
rintro _ ⟨x, hx, y, hy, rfl⟩, | |
rw [linear_map.map_add, h.1 _ hx, h.2 _ hy, add_zero] } | |
end | |
/-- The pullback of a submodule in the dual space along the evaluation map. -/ | |
def dual_annihilator_comap (Φ : submodule R (module.dual R M)) : submodule R M := | |
Φ.dual_annihilator.comap (module.dual.eval R M) | |
lemma mem_dual_annihilator_comap_iff {Φ : submodule R (module.dual R M)} (x : M) : | |
x ∈ Φ.dual_annihilator_comap ↔ ∀ φ ∈ Φ, (φ x : R) = 0 := | |
by simp_rw [dual_annihilator_comap, mem_comap, mem_dual_annihilator, module.dual.eval_apply] | |
end submodule | |
namespace subspace | |
open submodule linear_map | |
universes u v w | |
-- We work in vector spaces because `exists_is_compl` only hold for vector spaces | |
variables {K : Type u} {V : Type v} [field K] [add_comm_group V] [module K V] | |
/-- Given a subspace `W` of `V` and an element of its dual `φ`, `dual_lift W φ` is | |
the natural extension of `φ` to an element of the dual of `V`. | |
That is, `dual_lift W φ` sends `w ∈ W` to `φ x` and `x` in the complement of `W` to `0`. -/ | |
noncomputable def dual_lift (W : subspace K V) : | |
module.dual K W →ₗ[K] module.dual K V := | |
let h := classical.indefinite_description _ W.exists_is_compl in | |
(linear_map.of_is_compl_prod h.2).comp (linear_map.inl _ _ _) | |
variable {W : subspace K V} | |
@[simp] lemma dual_lift_of_subtype {φ : module.dual K W} (w : W) : | |
W.dual_lift φ (w : V) = φ w := | |
by { erw of_is_compl_left_apply _ w, refl } | |
lemma dual_lift_of_mem {φ : module.dual K W} {w : V} (hw : w ∈ W) : | |
W.dual_lift φ w = φ ⟨w, hw⟩ := | |
by convert dual_lift_of_subtype ⟨w, hw⟩ | |
@[simp] lemma dual_restrict_comp_dual_lift (W : subspace K V) : | |
W.dual_restrict.comp W.dual_lift = 1 := | |
by { ext φ x, simp } | |
lemma dual_restrict_left_inverse (W : subspace K V) : | |
function.left_inverse W.dual_restrict W.dual_lift := | |
λ x, show W.dual_restrict.comp W.dual_lift x = x, | |
by { rw [dual_restrict_comp_dual_lift], refl } | |
lemma dual_lift_right_inverse (W : subspace K V) : | |
function.right_inverse W.dual_lift W.dual_restrict := | |
W.dual_restrict_left_inverse | |
lemma dual_restrict_surjective : | |
function.surjective W.dual_restrict := | |
W.dual_lift_right_inverse.surjective | |
lemma dual_lift_injective : function.injective W.dual_lift := | |
W.dual_restrict_left_inverse.injective | |
/-- The quotient by the `dual_annihilator` of a subspace is isomorphic to the | |
dual of that subspace. -/ | |
noncomputable def quot_annihilator_equiv (W : subspace K V) : | |
(module.dual K V ⧸ W.dual_annihilator) ≃ₗ[K] module.dual K W := | |
(quot_equiv_of_eq _ _ W.dual_restrict_ker_eq_dual_annihilator).symm.trans $ | |
W.dual_restrict.quot_ker_equiv_of_surjective dual_restrict_surjective | |
/-- The natural isomorphism forom the dual of a subspace `W` to `W.dual_lift.range`. -/ | |
noncomputable def dual_equiv_dual (W : subspace K V) : | |
module.dual K W ≃ₗ[K] W.dual_lift.range := | |
linear_equiv.of_injective _ dual_lift_injective | |
lemma dual_equiv_dual_def (W : subspace K V) : | |
W.dual_equiv_dual.to_linear_map = W.dual_lift.range_restrict := rfl | |
@[simp] lemma dual_equiv_dual_apply (φ : module.dual K W) : | |
W.dual_equiv_dual φ = ⟨W.dual_lift φ, mem_range.2 ⟨φ, rfl⟩⟩ := rfl | |
section | |
open_locale classical | |
open finite_dimensional | |
variables {V₁ : Type*} [add_comm_group V₁] [module K V₁] | |
instance [H : finite_dimensional K V] : finite_dimensional K (module.dual K V) := | |
by apply_instance | |
variables [finite_dimensional K V] [finite_dimensional K V₁] | |
@[simp] lemma dual_finrank_eq : | |
finrank K (module.dual K V) = finrank K V := | |
linear_equiv.finrank_eq (basis.of_vector_space K V).to_dual_equiv.symm | |
/-- The quotient by the dual is isomorphic to its dual annihilator. -/ | |
noncomputable def quot_dual_equiv_annihilator (W : subspace K V) : | |
(module.dual K V ⧸ W.dual_lift.range) ≃ₗ[K] W.dual_annihilator := | |
linear_equiv.quot_equiv_of_quot_equiv $ | |
linear_equiv.trans W.quot_annihilator_equiv W.dual_equiv_dual | |
/-- The quotient by a subspace is isomorphic to its dual annihilator. -/ | |
noncomputable def quot_equiv_annihilator (W : subspace K V) : | |
(V ⧸ W) ≃ₗ[K] W.dual_annihilator := | |
begin | |
refine _ ≪≫ₗ W.quot_dual_equiv_annihilator, | |
refine linear_equiv.quot_equiv_of_equiv _ (basis.of_vector_space K V).to_dual_equiv, | |
exact (basis.of_vector_space K W).to_dual_equiv.trans W.dual_equiv_dual | |
end | |
open finite_dimensional | |
@[simp] | |
lemma finrank_dual_annihilator_comap_eq {Φ : subspace K (module.dual K V)} : | |
finrank K Φ.dual_annihilator_comap = finrank K Φ.dual_annihilator := | |
begin | |
rw [submodule.dual_annihilator_comap, ← module.eval_equiv_to_linear_map], | |
exact linear_equiv.finrank_eq (linear_equiv.of_submodule' _ _), | |
end | |
lemma finrank_add_finrank_dual_annihilator_comap_eq | |
(W : subspace K (module.dual K V)) : | |
finrank K W + finrank K W.dual_annihilator_comap = finrank K V := | |
begin | |
rw [finrank_dual_annihilator_comap_eq, W.quot_equiv_annihilator.finrank_eq.symm, add_comm, | |
submodule.finrank_quotient_add_finrank, subspace.dual_finrank_eq], | |
end | |
end | |
end subspace | |
open module | |
section dual_map | |
variables {R : Type*} [comm_semiring R] {M₁ : Type*} {M₂ : Type*} | |
variables [add_comm_monoid M₁] [module R M₁] [add_comm_monoid M₂] [module R M₂] | |
/-- Given a linear map `f : M₁ →ₗ[R] M₂`, `f.dual_map` is the linear map between the dual of | |
`M₂` and `M₁` such that it maps the functional `φ` to `φ ∘ f`. -/ | |
def linear_map.dual_map (f : M₁ →ₗ[R] M₂) : dual R M₂ →ₗ[R] dual R M₁ := | |
linear_map.lcomp R R f | |
@[simp] lemma linear_map.dual_map_apply (f : M₁ →ₗ[R] M₂) (g : dual R M₂) (x : M₁) : | |
f.dual_map g x = g (f x) := | |
linear_map.lcomp_apply f g x | |
@[simp] lemma linear_map.dual_map_id : | |
(linear_map.id : M₁ →ₗ[R] M₁).dual_map = linear_map.id := | |
by { ext, refl } | |
lemma linear_map.dual_map_comp_dual_map {M₃ : Type*} [add_comm_group M₃] [module R M₃] | |
(f : M₁ →ₗ[R] M₂) (g : M₂ →ₗ[R] M₃) : | |
f.dual_map.comp g.dual_map = (g.comp f).dual_map := | |
rfl | |
/-- The `linear_equiv` version of `linear_map.dual_map`. -/ | |
def linear_equiv.dual_map (f : M₁ ≃ₗ[R] M₂) : dual R M₂ ≃ₗ[R] dual R M₁ := | |
{ inv_fun := f.symm.to_linear_map.dual_map, | |
left_inv := | |
begin | |
intro φ, ext x, | |
simp only [linear_map.dual_map_apply, linear_equiv.coe_to_linear_map, | |
linear_map.to_fun_eq_coe, linear_equiv.apply_symm_apply] | |
end, | |
right_inv := | |
begin | |
intro φ, ext x, | |
simp only [linear_map.dual_map_apply, linear_equiv.coe_to_linear_map, | |
linear_map.to_fun_eq_coe, linear_equiv.symm_apply_apply] | |
end, | |
.. f.to_linear_map.dual_map } | |
@[simp] lemma linear_equiv.dual_map_apply (f : M₁ ≃ₗ[R] M₂) (g : dual R M₂) (x : M₁) : | |
f.dual_map g x = g (f x) := | |
linear_map.lcomp_apply f g x | |
@[simp] lemma linear_equiv.dual_map_refl : | |
(linear_equiv.refl R M₁).dual_map = linear_equiv.refl R (dual R M₁) := | |
by { ext, refl } | |
@[simp] lemma linear_equiv.dual_map_symm {f : M₁ ≃ₗ[R] M₂} : | |
(linear_equiv.dual_map f).symm = linear_equiv.dual_map f.symm := rfl | |
lemma linear_equiv.dual_map_trans {M₃ : Type*} [add_comm_group M₃] [module R M₃] | |
(f : M₁ ≃ₗ[R] M₂) (g : M₂ ≃ₗ[R] M₃) : | |
g.dual_map.trans f.dual_map = (f.trans g).dual_map := | |
rfl | |
end dual_map | |
namespace linear_map | |
variables {R : Type*} [comm_semiring R] {M₁ : Type*} {M₂ : Type*} | |
variables [add_comm_monoid M₁] [module R M₁] [add_comm_monoid M₂] [module R M₂] | |
variable (f : M₁ →ₗ[R] M₂) | |
lemma ker_dual_map_eq_dual_annihilator_range : | |
f.dual_map.ker = f.range.dual_annihilator := | |
begin | |
ext φ, split; intro hφ, | |
{ rw mem_ker at hφ, | |
rw submodule.mem_dual_annihilator, | |
rintro y ⟨x, rfl⟩, | |
rw [← dual_map_apply, hφ, zero_apply] }, | |
{ ext x, | |
rw dual_map_apply, | |
rw submodule.mem_dual_annihilator at hφ, | |
exact hφ (f x) ⟨x, rfl⟩ } | |
end | |
lemma range_dual_map_le_dual_annihilator_ker : | |
f.dual_map.range ≤ f.ker.dual_annihilator := | |
begin | |
rintro _ ⟨ψ, rfl⟩, | |
simp_rw [submodule.mem_dual_annihilator, mem_ker], | |
rintro x hx, | |
rw [dual_map_apply, hx, map_zero] | |
end | |
section finite_dimensional | |
variables {K : Type*} [field K] {V₁ : Type*} {V₂ : Type*} | |
variables [add_comm_group V₁] [module K V₁] [add_comm_group V₂] [module K V₂] | |
open finite_dimensional | |
variable [finite_dimensional K V₂] | |
@[simp] lemma finrank_range_dual_map_eq_finrank_range (f : V₁ →ₗ[K] V₂) : | |
finrank K f.dual_map.range = finrank K f.range := | |
begin | |
have := submodule.finrank_quotient_add_finrank f.range, | |
rw [(subspace.quot_equiv_annihilator f.range).finrank_eq, | |
← ker_dual_map_eq_dual_annihilator_range] at this, | |
conv_rhs at this { rw ← subspace.dual_finrank_eq }, | |
refine add_left_injective (finrank K f.dual_map.ker) _, | |
change _ + _ = _ + _, | |
rw [finrank_range_add_finrank_ker f.dual_map, add_comm, this], | |
end | |
lemma range_dual_map_eq_dual_annihilator_ker [finite_dimensional K V₁] (f : V₁ →ₗ[K] V₂) : | |
f.dual_map.range = f.ker.dual_annihilator := | |
begin | |
refine eq_of_le_of_finrank_eq f.range_dual_map_le_dual_annihilator_ker _, | |
have := submodule.finrank_quotient_add_finrank f.ker, | |
rw (subspace.quot_equiv_annihilator f.ker).finrank_eq at this, | |
refine add_left_injective (finrank K f.ker) _, | |
simp_rw [this, finrank_range_dual_map_eq_finrank_range], | |
exact finrank_range_add_finrank_ker f, | |
end | |
end finite_dimensional | |
section field | |
variables {K V : Type*} | |
variables [field K] [add_comm_group V] [module K V] | |
lemma dual_pairing_nondegenerate : (dual_pairing K V).nondegenerate := | |
begin | |
refine ⟨separating_left_iff_ker_eq_bot.mpr ker_id, _⟩, | |
intros x, | |
contrapose, | |
rintros hx : x ≠ 0, | |
rw [not_forall], | |
let f : V →ₗ[K] K := classical.some (linear_pmap.mk_span_singleton x 1 hx).to_fun.exists_extend, | |
use [f], | |
refine ne_zero_of_eq_one _, | |
have h : f.comp (K ∙ x).subtype = (linear_pmap.mk_span_singleton x 1 hx).to_fun := | |
classical.some_spec (linear_pmap.mk_span_singleton x (1 : K) hx).to_fun.exists_extend, | |
exact (fun_like.congr_fun h _).trans (linear_pmap.mk_span_singleton_apply _ hx _), | |
end | |
end field | |
end linear_map | |
namespace tensor_product | |
variables (R : Type*) (M : Type*) (N : Type*) | |
variables {ι κ : Type*} | |
variables [decidable_eq ι] [decidable_eq κ] | |
variables [fintype ι] [fintype κ] | |
open_locale big_operators | |
open_locale tensor_product | |
local attribute [ext] tensor_product.ext | |
open tensor_product | |
open linear_map | |
section | |
variables [comm_semiring R] [add_comm_monoid M] [add_comm_monoid N] | |
variables [module R M] [module R N] | |
/-- | |
The canonical linear map from `dual M ⊗ dual N` to `dual (M ⊗ N)`, | |
sending `f ⊗ g` to the composition of `tensor_product.map f g` with | |
the natural isomorphism `R ⊗ R ≃ R`. | |
-/ | |
def dual_distrib : (dual R M) ⊗[R] (dual R N) →ₗ[R] dual R (M ⊗[R] N) := | |
(comp_right ↑(tensor_product.lid R R)) ∘ₗ hom_tensor_hom_map R M N R R | |
variables {R M N} | |
@[simp] | |
lemma dual_distrib_apply (f : dual R M) (g : dual R N) (m : M) (n : N) : | |
dual_distrib R M N (f ⊗ₜ g) (m ⊗ₜ n) = f m * g n := | |
by simp only [dual_distrib, coe_comp, function.comp_app, hom_tensor_hom_map_apply, | |
comp_right_apply, linear_equiv.coe_coe, map_tmul, lid_tmul, algebra.id.smul_eq_mul] | |
end | |
variables {R M N} | |
variables [comm_ring R] [add_comm_group M] [add_comm_group N] | |
variables [module R M] [module R N] | |
/-- | |
An inverse to `dual_tensor_dual_map` given bases. | |
-/ | |
noncomputable | |
def dual_distrib_inv_of_basis (b : basis ι R M) (c : basis κ R N) : | |
dual R (M ⊗[R] N) →ₗ[R] (dual R M) ⊗[R] (dual R N) := | |
∑ i j, (ring_lmap_equiv_self R ℕ _).symm (b.dual_basis i ⊗ₜ c.dual_basis j) | |
∘ₗ applyₗ (c j) ∘ₗ applyₗ (b i) ∘ₗ (lcurry R M N R) | |
@[simp] | |
lemma dual_distrib_inv_of_basis_apply (b : basis ι R M) (c : basis κ R N) | |
(f : dual R (M ⊗[R] N)) : dual_distrib_inv_of_basis b c f = | |
∑ i j, (f (b i ⊗ₜ c j)) • (b.dual_basis i ⊗ₜ c.dual_basis j) := | |
by simp [dual_distrib_inv_of_basis] | |
/-- | |
A linear equivalence between `dual M ⊗ dual N` and `dual (M ⊗ N)` given bases for `M` and `N`. | |
It sends `f ⊗ g` to the composition of `tensor_product.map f g` with the natural | |
isomorphism `R ⊗ R ≃ R`. | |
-/ | |
@[simps] | |
noncomputable def dual_distrib_equiv_of_basis (b : basis ι R M) (c : basis κ R N) : | |
(dual R M) ⊗[R] (dual R N) ≃ₗ[R] dual R (M ⊗[R] N) := | |
begin | |
refine linear_equiv.of_linear | |
(dual_distrib R M N) (dual_distrib_inv_of_basis b c) _ _, | |
{ ext f m n, | |
have h : ∀ (r s : R), r • s = s • r := is_commutative.comm, | |
simp only [compr₂_apply, mk_apply, comp_apply, id_apply, dual_distrib_inv_of_basis_apply, | |
linear_map.map_sum, map_smul, sum_apply, smul_apply, dual_distrib_apply, h (f _) _, | |
← f.map_smul, ←f.map_sum, ←smul_tmul_smul, ←tmul_sum, ←sum_tmul, basis.coe_dual_basis, | |
basis.coord_apply, basis.sum_repr] }, | |
{ ext f g, | |
simp only [compr₂_apply, mk_apply, comp_apply, id_apply, dual_distrib_inv_of_basis_apply, | |
dual_distrib_apply, ←smul_tmul_smul, ←tmul_sum, ←sum_tmul, basis.coe_dual_basis, | |
basis.sum_dual_apply_smul_coord] } | |
end | |
variables (R M N) | |
variables [module.finite R M] [module.finite R N] [module.free R M] [module.free R N] | |
variables [nontrivial R] | |
open_locale classical | |
/-- | |
A linear equivalence between `dual M ⊗ dual N` and `dual (M ⊗ N)` when `M` and `N` are finite free | |
modules. It sends `f ⊗ g` to the composition of `tensor_product.map f g` with the natural | |
isomorphism `R ⊗ R ≃ R`. | |
-/ | |
@[simp] | |
noncomputable | |
def dual_distrib_equiv : (dual R M) ⊗[R] (dual R N) ≃ₗ[R] dual R (M ⊗[R] N) := | |
dual_distrib_equiv_of_basis (module.free.choose_basis R M) (module.free.choose_basis R N) | |
end tensor_product | |