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/-
Copyright (c) 2022 Heather Macbeth. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Heather Macbeth
-/
import to_mathlib.analysis.cont_diff
import analysis.special_functions.trigonometric.deriv
/-! # Rotation about an axis, considered as a function in that axis -/
noncomputable theory
variables (E : Type*) [inner_product_space โ„ E] [finite_dimensional โ„ E]
/-- The identification of a finite-dimensional inner product space with its algebraic dual. -/
def to_dual : E โ‰ƒโ‚—[โ„] (E โ†’โ‚—[โ„] โ„) :=
(inner_product_space.to_dual โ„ E).to_linear_equiv โ‰ชโ‰ซโ‚— linear_map.to_continuous_linear_map.symm
variables {E} (ฮฉ : alternating_map โ„ E โ„ (fin 3))
include E ฮฉ
/-- Linear map from `E` to `E โ†’โ‚—[โ„] E` constructed from a 3-form `ฮฉ` on `E` and an identification of
`E` with its dual. Effectively, the Hodge star operation. (Under appropriate hypotheses it turns
out that the image of this map is in `๐”ฐ๐”ฌ(E)`, the skew-symmetric operators, which can be identified
with `ฮ›ยฒE`.) -/
def A : E โ†’โ‚—[โ„] (E โ†’โ‚—[โ„] E) :=
begin
let z : alternating_map โ„ E โ„ (fin 0) โ‰ƒโ‚—[โ„] โ„ :=
alternating_map.const_linear_equiv_of_is_empty.symm,
let y : alternating_map โ„ E โ„ (fin 1) โ†’โ‚—[โ„] E โ†’โ‚—[โ„] โ„ :=
(linear_map.llcomp โ„ E (alternating_map โ„ E โ„ (fin 0)) โ„ z) โˆ˜โ‚—
alternating_map.curry_left_linear_map,
let y' : alternating_map โ„ E โ„ (fin 1) โ†’โ‚—[โ„] E :=
(linear_map.llcomp โ„ (alternating_map โ„ E โ„ (fin 1)) (E โ†’โ‚—[โ„] โ„) E (to_dual E).symm) y,
let x : alternating_map โ„ E โ„ (fin 2) โ†’โ‚—[โ„] E โ†’โ‚—[โ„] E :=
(linear_map.llcomp โ„ E (alternating_map โ„ E โ„ (fin 1)) _ y') โˆ˜โ‚—
alternating_map.curry_left_linear_map,
exact x โˆ˜โ‚— ฮฉ.curry_left_linear_map,
end
lemma A_apply_self (v : E) : A ฮฉ v v = 0 := by simp [A]
attribute [irreducible] A
/-- The map `A`, upgraded from linear to continuous-linear; useful for calculus. -/
def A' : E โ†’L[โ„] (E โ†’L[โ„] E) :=
(โ†‘(linear_map.to_continuous_linear_map : (E โ†’โ‚—[โ„] E) โ‰ƒโ‚—[โ„] (E โ†’L[โ„] E))
โˆ˜โ‚— (A ฮฉ)).to_continuous_linear_map
@[simp] lemma A'_apply (v : E) : A' ฮฉ v = (A ฮฉ v).to_continuous_linear_map := rfl
/-- A family of endomorphisms of `E`, parametrized by `E ร— โ„`. The idea is that for nonzero `v : E`
and `t : โ„` this endomorphism should be the rotation by the angle `t` about the axis spanned by `v`,
although this definition does not itself impose enough conditions to ensure that meaning. -/
def rot (p : E ร— โ„) : E โ†’L[โ„] E :=
(โ„ โˆ™ p.1).subtypeL โˆ˜L (orthogonal_projection (โ„ โˆ™ p.1) : E โ†’L[โ„] (โ„ โˆ™ p.1))
+ real.cos p.2 โ€ข (โ„ โˆ™ p.1)แ—ฎ.subtypeL โˆ˜L (orthogonal_projection (โ„ โˆ™ p.1)แ—ฎ : E โ†’L[โ„] (โ„ โˆ™ p.1)แ—ฎ)
+ real.sin p.2 โ€ข (A ฮฉ p.1).to_continuous_linear_map
/-- Alternative form of the construction `rot`, convenient for the smoothness calculation. -/
def rot_aux (p : E ร— โ„) : E โ†’L[โ„] E :=
real.cos p.2 โ€ข continuous_linear_map.id โ„ E +
((1 - real.cos p.2) โ€ข (โ„ โˆ™ p.1).subtypeL โˆ˜L (orthogonal_projection (โ„ โˆ™ p.1) : E โ†’L[โ„] (โ„ โˆ™ p.1))
+ real.sin p.2 โ€ข (A' ฮฉ p.1))
lemma rot_eq_aux : rot ฮฉ = rot_aux ฮฉ :=
begin
ext1 p,
dsimp [rot, rot_aux],
rw id_eq_sum_orthogonal_projection_self_orthogonal_complement (โ„ โˆ™ p.1),
simp only [smul_add, sub_smul, one_smul],
abel,
end
/-- The map `rot` is smooth on `(E \ {0}) ร— โ„`. -/
lemma cont_diff_rot {p : E ร— โ„} (hp : p.1 โ‰  0) : cont_diff_at โ„ โŠค (rot ฮฉ) p :=
begin
simp only [rot_eq_aux],
refine (cont_diff_at_snd.cos.smul cont_diff_at_const).add _,
refine ((cont_diff_at_const.sub cont_diff_at_snd.cos).smul _).add
(cont_diff_at_snd.sin.smul _),
{ exact (cont_diff_at_orthogonal_projection_singleton hp).comp _ cont_diff_at_fst },
{ exact (A' ฮฉ).cont_diff.cont_diff_at.comp _ cont_diff_at_fst },
end
/-- The map `rot` sends `E ร— {0}` to the identity. -/
lemma rot_zero (v : E) : rot ฮฉ (v, 0) = continuous_linear_map.id โ„ E :=
begin
ext w,
simpa [rot] using (eq_sum_orthogonal_projection_self_orthogonal_complement (โ„ โˆ™ v) w).symm,
end
/-- The map `rot` sends `(v, ฯ€)` to a transformation which on `(โ„ โˆ™ v)แ—ฎ` acts as the negation. -/
lemma rot_pi (v : E) {w : E} (hw : w โˆˆ (โ„ โˆ™ v)แ—ฎ) : rot ฮฉ (v, real.pi) w = - w :=
by simp [rot, orthogonal_projection_eq_self_iff.mpr hw,
orthogonal_projection_mem_subspace_orthogonal_complement_eq_zero hw]
/-- The map `rot` sends `(v, t)` to a transformation preserving `v`. -/
lemma rot_self (p : E ร— โ„) : rot ฮฉ p p.1 = p.1 :=
begin
have H : โ†‘(orthogonal_projection (โ„ โˆ™ p.1) p.1) = p.1 :=
orthogonal_projection_eq_self_iff.mpr (submodule.mem_span_singleton_self p.1),
simp [rot, A_apply_self, orthogonal_projection_orthogonal_complement_singleton_eq_zero, H],
end