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/- | |
Copyright (c) 2022 Antoine Labelle. All rights reserved. | |
Released under Apache 2.0 license as described in the file LICENSE. | |
Authors: Antoine Labelle | |
-/ | |
import representation_theory.basic | |
import representation_theory.Rep | |
/-! | |
# Subspace of invariants a group representation | |
This file introduces the subspace of invariants of a group representation | |
and proves basic results about it. | |
The main tool used is the average of all elements of the group, seen as an element of | |
`monoid_algebra k G`. The action of this special element gives a projection onto the | |
subspace of invariants. | |
In order for the definition of the average element to make sense, we need to assume for most of the | |
results that the order of `G` is invertible in `k` (e. g. `k` has characteristic `0`). | |
-/ | |
open_locale big_operators | |
open monoid_algebra | |
open representation | |
namespace group_algebra | |
variables (k G : Type*) [comm_semiring k] [group G] | |
variables [fintype G] [invertible (fintype.card G : k)] | |
/-- | |
The average of all elements of the group `G`, considered as an element of `monoid_algebra k G`. | |
-/ | |
noncomputable def average : monoid_algebra k G := | |
⅟(fintype.card G : k) • ∑ g : G, of k G g | |
/-- | |
`average k G` is invariant under left multiplication by elements of `G`. | |
-/ | |
@[simp] | |
theorem mul_average_left (g : G) : | |
(finsupp.single g 1 * average k G : monoid_algebra k G) = average k G := | |
begin | |
simp only [mul_one, finset.mul_sum, algebra.mul_smul_comm, average, monoid_algebra.of_apply, | |
finset.sum_congr, monoid_algebra.single_mul_single], | |
set f : G → monoid_algebra k G := λ x, finsupp.single x 1, | |
show ⅟ ↑(fintype.card G) • ∑ (x : G), f (g * x) = ⅟ ↑(fintype.card G) • ∑ (x : G), f x, | |
rw function.bijective.sum_comp (group.mul_left_bijective g) _, | |
end | |
/-- | |
`average k G` is invariant under right multiplication by elements of `G`. | |
-/ | |
@[simp] | |
theorem mul_average_right (g : G) : | |
average k G * finsupp.single g 1 = average k G := | |
begin | |
simp only [mul_one, finset.sum_mul, algebra.smul_mul_assoc, average, monoid_algebra.of_apply, | |
finset.sum_congr, monoid_algebra.single_mul_single], | |
set f : G → monoid_algebra k G := λ x, finsupp.single x 1, | |
show ⅟ ↑(fintype.card G) • ∑ (x : G), f (x * g) = ⅟ ↑(fintype.card G) • ∑ (x : G), f x, | |
rw function.bijective.sum_comp (group.mul_right_bijective g) _, | |
end | |
end group_algebra | |
namespace representation | |
section invariants | |
open group_algebra | |
variables {k G V : Type*} [comm_semiring k] [group G] [add_comm_monoid V] [module k V] | |
variables (ρ : representation k G V) | |
/-- | |
The subspace of invariants, consisting of the vectors fixed by all elements of `G`. | |
-/ | |
def invariants : submodule k V := | |
{ carrier := set_of (λ v, ∀ (g : G), ρ g v = v), | |
zero_mem' := λ g, by simp only [map_zero], | |
add_mem' := λ v w hv hw g, by simp only [hv g, hw g, map_add], | |
smul_mem' := λ r v hv g, by simp only [hv g, linear_map.map_smulₛₗ, ring_hom.id_apply]} | |
@[simp] | |
lemma mem_invariants (v : V) : v ∈ invariants ρ ↔ ∀ (g: G), ρ g v = v := by refl | |
lemma invariants_eq_inter : | |
(invariants ρ).carrier = ⋂ g : G, function.fixed_points (ρ g) := | |
by {ext, simp [function.is_fixed_pt]} | |
variables [fintype G] [invertible (fintype.card G : k)] | |
/-- | |
The action of `average k G` gives a projection map onto the subspace of invariants. | |
-/ | |
@[simp] | |
noncomputable def average_map : V →ₗ[k] V := as_algebra_hom ρ (average k G) | |
/-- | |
The `average_map` sends elements of `V` to the subspace of invariants. | |
-/ | |
theorem average_map_invariant (v : V) : average_map ρ v ∈ invariants ρ := | |
λ g, by rw [average_map, ←as_algebra_hom_single_one, ←linear_map.mul_apply, | |
←map_mul (as_algebra_hom ρ), mul_average_left] | |
/-- | |
The `average_map` acts as the identity on the subspace of invariants. | |
-/ | |
theorem average_map_id (v : V) (hv : v ∈ invariants ρ) : average_map ρ v = v := | |
begin | |
rw mem_invariants at hv, | |
simp [average, map_sum, hv, finset.card_univ, nsmul_eq_smul_cast k _ v, smul_smul], | |
end | |
theorem is_proj_average_map : linear_map.is_proj ρ.invariants ρ.average_map := | |
⟨ρ.average_map_invariant, ρ.average_map_id⟩ | |
end invariants | |
namespace lin_hom | |
universes u | |
open category_theory Action | |
variables {k : Type u} [comm_ring k] {G : Group.{u}} | |
lemma mem_invariants_iff_comm {X Y : Rep k G} (f : X.V →ₗ[k] Y.V) (g : G) : | |
(lin_hom X.ρ Y.ρ) g f = f ↔ f.comp (X.ρ g) = (Y.ρ g).comp f := | |
begin | |
dsimp, | |
erw [←ρ_Aut_apply_inv], | |
rw [←linear_map.comp_assoc, ←Module.comp_def, ←Module.comp_def, iso.inv_comp_eq, ρ_Aut_apply_hom], | |
exact comm, | |
end | |
/-- The invariants of the representation `lin_hom X.ρ Y.ρ` correspond to the the representation | |
homomorphisms from `X` to `Y` -/ | |
@[simps] | |
def invariants_equiv_Rep_hom (X Y : Rep k G) : (lin_hom X.ρ Y.ρ).invariants ≃ₗ[k] (X ⟶ Y) := | |
{ to_fun := λ f, ⟨f.val, λ g, (mem_invariants_iff_comm _ g).1 (f.property g)⟩, | |
map_add' := λ _ _, rfl, | |
map_smul' := λ _ _, rfl, | |
inv_fun := λ f, ⟨f.hom, λ g, (mem_invariants_iff_comm _ g).2 (f.comm g)⟩, | |
left_inv := λ _, by { ext, refl }, | |
right_inv := λ _, by { ext, refl } } | |
end lin_hom | |
end representation | |