feat(data/complex/module): ![1, I] is a basis of C over R (#4713)

Co-authored-by: Johan Commelin <johan@commelin.net>

src/data/complex/module.lean

@@ -4,14 +4,17 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Alexander Bentkamp, Sébastien Gouëzel
 -/
 import data.complex.basic
-import algebra.algebra.basic
+import data.matrix.notation
+import field_theory.tower
+import linear_algebra.finite_dimensional
 
 /-!
 # Complex number as a vector space over `ℝ`
 
 This file contains three instances:
 * `ℂ` is an `ℝ` algebra;
 * any complex vector space is a real vector space;
+* any finite dimensional complex vector space is a finite dimesional real vector space;
 * the space of `ℝ`-linear maps from a real vector space to a complex vector space is a complex
   vector space.
 
@@ -30,41 +33,88 @@ namespace complex
 
 instance algebra_over_reals : algebra ℝ ℂ := (complex.of_real).to_algebra
 
+@[simp] lemma coe_algebra_map : ⇑(algebra_map ℝ ℂ) = complex.of_real := rfl
+
+@[simp] lemma re_smul (a : ℝ) (z : ℂ) : re (a • z) = a * re z := by simp [algebra.smul_def]
+
+@[simp] lemma im_smul (a : ℝ) (z : ℂ) : im (a • z) = a * im z := by simp [algebra.smul_def]
+
+open submodule finite_dimensional
+
+lemma is_basis_one_I : is_basis ℝ ![1, I] :=
+begin
+  refine ⟨linear_independent_fin2.2 ⟨I_ne_zero, λ a, mt (congr_arg re) $ by simp⟩,
+    eq_top_iff'.2 $ λ z, _⟩,
+  suffices : ∃ a b, z = a • I + b • 1,
+    by simpa [mem_span_insert, mem_span_singleton, -set.singleton_one],
+  use [z.im, z.re],
+  simp [algebra.smul_def, add_comm]
+end
+
+instance : finite_dimensional ℝ ℂ := of_fintype_basis is_basis_one_I
+
+@[simp] lemma findim_real_complex : finite_dimensional.findim ℝ ℂ = 2 :=
+by rw [findim_eq_card_basis is_basis_one_I, fintype.card_fin]
+
+@[simp] lemma dim_real_complex : vector_space.dim ℝ ℂ = 2 :=
+by simp [← findim_eq_dim, findim_real_complex]
+
+lemma {u} dim_real_complex' : cardinal.lift.{0 u} (vector_space.dim ℝ ℂ) = 2 :=
+by simp [← findim_eq_dim, findim_real_complex, bit0]
+
 end complex
 
 /- Register as an instance (with low priority) the fact that a complex vector space is also a real
 vector space. -/
+@[priority 900]
 instance module.complex_to_real (E : Type*) [add_comm_group E] [module ℂ E] : module ℝ E :=
 semimodule.restrict_scalars' ℝ ℂ E
-attribute [instance, priority 900] module.complex_to_real
+
+instance module.real_complex_tower (E : Type*) [add_comm_group E] [module ℂ E] :
+  is_scalar_tower ℝ ℂ E :=
+semimodule.restrict_scalars.is_scalar_tower ℝ
 
 instance (E : Type*) [add_comm_group E] [module ℝ E]
   (F : Type*) [add_comm_group F] [module ℂ F] : module ℂ (E →ₗ[ℝ] F) :=
 linear_map.module_extend_scalars _ _ _ _
 
+@[priority 100]
+instance finite_dimensional.complex_to_real (E : Type*) [add_comm_group E] [module ℂ E]
+  [finite_dimensional ℂ E] : finite_dimensional ℝ E :=
+finite_dimensional.trans ℝ ℂ E
+
+lemma dim_real_of_complex (E : Type*) [add_comm_group E] [module ℂ E] :
+  vector_space.dim ℝ E = 2 * vector_space.dim ℂ E :=
+cardinal.lift_inj.1 $
+  by { rw [← dim_mul_dim' ℝ ℂ E, complex.dim_real_complex], simp [bit0] }
+
+lemma findim_real_of_complex (E : Type*) [add_comm_group E] [module ℂ E] :
+  finite_dimensional.findim ℝ E = 2 * finite_dimensional.findim ℂ E :=
+by rw [← finite_dimensional.findim_mul_findim ℝ ℂ E, complex.findim_real_complex]
+
 namespace complex
 
 /-- Linear map version of the real part function, from `ℂ` to `ℝ`. -/
 def linear_map.re : ℂ →ₗ[ℝ] ℝ :=
 { to_fun := λx, x.re,
-  map_add' := by simp,
-  map_smul' := λc x, by { change ((c : ℂ) * x).re = c * x.re, simp } }
+  map_add' := add_re,
+  map_smul' := re_smul }
 
 @[simp] lemma linear_map.coe_re : ⇑linear_map.re = re := rfl
 
 /-- Linear map version of the imaginary part function, from `ℂ` to `ℝ`. -/
 def linear_map.im : ℂ →ₗ[ℝ] ℝ :=
 { to_fun := λx, x.im,
-  map_add' := by simp,
-  map_smul' := λc x, by { change ((c : ℂ) * x).im = c * x.im, simp } }
+  map_add' := add_im,
+  map_smul' := im_smul }
 
 @[simp] lemma linear_map.coe_im : ⇑linear_map.im = im := rfl
 
 /-- Linear map version of the canonical embedding of `ℝ` in `ℂ`. -/
 def linear_map.of_real : ℝ →ₗ[ℝ] ℂ :=
 { to_fun := coe,
-  map_add' := by simp,
-  map_smul' := λc x, by { simp, refl } }
+  map_add' := of_real_add,
+  map_smul' := λc x, by simp [algebra.smul_def] }
 
 @[simp] lemma linear_map.coe_of_real : ⇑linear_map.of_real = coe := rfl
 

src/data/matrix/notation.lean

@@ -115,7 +115,14 @@ by { ext, simp [vec_tail] }
 
 @[simp] lemma cons_head_tail (u : fin m.succ → α) :
  vec_cons (vec_head u) (vec_tail u) = u :=
-by { ext i, refine fin.cases _ _ i; simp [vec_head, vec_tail] }
+fin.cons_self_tail _
+
+@[simp] lemma range_cons (x : α) (u : fin n → α) :
+  set.range (vec_cons x u) = {x} ∪ set.range u :=
+set.ext $ λ y, by simp [fin.exists_fin_succ, eq_comm]
+
+@[simp] lemma range_empty (u : fin 0 → α) : set.range u = ∅ :=
+set.range_eq_empty.2 $ λ ⟨k⟩, k.elim0
 
 /-- `![a, b, ...] 1` is equal to `b`.