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/- | |
Copyright (c) 2018 Patrick Massot. All rights reserved. | |
Released under Apache 2.0 license as described in the file LICENSE. | |
Authors: Patrick Massot, Johannes Hölzl | |
-/ | |
import topology.algebra.group_completion | |
import topology.algebra.ring | |
/-! | |
# Completion of topological rings: | |
This files endows the completion of a topological ring with a ring structure. | |
More precisely the instance `uniform_space.completion.ring` builds a ring structure | |
on the completion of a ring endowed with a compatible uniform structure in the sense of | |
`uniform_add_group`. There is also a commutative version when the original ring is commutative. | |
The last part of the file builds a ring structure on the biggest separated quotient of a ring. | |
## Main declarations: | |
Beyond the instances explained above (that don't have to be explicitly invoked), | |
the main constructions deal with continuous ring morphisms. | |
* `uniform_space.completion.extension_hom`: extends a continuous ring morphism from `R` | |
to a complete separated group `S` to `completion R`. | |
* `uniform_space.completion.map_ring_hom` : promotes a continuous ring morphism | |
from `R` to `S` into a continuous ring morphism from `completion R` to `completion S`. | |
-/ | |
open classical set filter topological_space add_comm_group | |
open_locale classical | |
noncomputable theory | |
universes u | |
namespace uniform_space.completion | |
open dense_inducing uniform_space function | |
variables (α : Type*) [ring α] [uniform_space α] | |
instance : has_one (completion α) := ⟨(1:α)⟩ | |
instance : has_mul (completion α) := | |
⟨curry $ (dense_inducing_coe.prod dense_inducing_coe).extend (coe ∘ uncurry (*))⟩ | |
@[norm_cast] lemma coe_one : ((1 : α) : completion α) = 1 := rfl | |
variables {α} [topological_ring α] | |
@[norm_cast] | |
lemma coe_mul (a b : α) : ((a * b : α) : completion α) = a * b := | |
((dense_inducing_coe.prod dense_inducing_coe).extend_eq | |
((continuous_coe α).comp (@continuous_mul α _ _ _)) (a, b)).symm | |
variables [uniform_add_group α] | |
lemma continuous_mul : continuous (λ p : completion α × completion α, p.1 * p.2) := | |
begin | |
let m := (add_monoid_hom.mul : α →+ α →+ α).compr₂ to_compl, | |
have : continuous (λ p : α × α, m p.1 p.2), | |
from (continuous_coe α).comp continuous_mul, | |
have di : dense_inducing (to_compl : α → completion α), | |
from dense_inducing_coe, | |
convert di.extend_Z_bilin di this, | |
ext ⟨x, y⟩, | |
refl | |
end | |
lemma continuous.mul {β : Type*} [topological_space β] {f g : β → completion α} | |
(hf : continuous f) (hg : continuous g) : continuous (λb, f b * g b) := | |
continuous_mul.comp (hf.prod_mk hg : _) | |
instance : ring (completion α) := | |
{ one_mul := assume a, completion.induction_on a | |
(is_closed_eq (continuous.mul continuous_const continuous_id) continuous_id) | |
(assume a, by rw [← coe_one, ← coe_mul, one_mul]), | |
mul_one := assume a, completion.induction_on a | |
(is_closed_eq (continuous.mul continuous_id continuous_const) continuous_id) | |
(assume a, by rw [← coe_one, ← coe_mul, mul_one]), | |
mul_assoc := assume a b c, completion.induction_on₃ a b c | |
(is_closed_eq | |
(continuous.mul (continuous.mul continuous_fst (continuous_fst.comp continuous_snd)) | |
(continuous_snd.comp continuous_snd)) | |
(continuous.mul continuous_fst | |
(continuous.mul (continuous_fst.comp continuous_snd) (continuous_snd.comp continuous_snd)))) | |
(assume a b c, by rw [← coe_mul, ← coe_mul, ← coe_mul, ← coe_mul, mul_assoc]), | |
left_distrib := assume a b c, completion.induction_on₃ a b c | |
(is_closed_eq | |
(continuous.mul continuous_fst (continuous.add | |
(continuous_fst.comp continuous_snd) | |
(continuous_snd.comp continuous_snd))) | |
(continuous.add | |
(continuous.mul continuous_fst (continuous_fst.comp continuous_snd)) | |
(continuous.mul continuous_fst (continuous_snd.comp continuous_snd)))) | |
(assume a b c, by rw [← coe_add, ← coe_mul, ← coe_mul, ← coe_mul, ←coe_add, mul_add]), | |
right_distrib := assume a b c, completion.induction_on₃ a b c | |
(is_closed_eq | |
(continuous.mul (continuous.add continuous_fst | |
(continuous_fst.comp continuous_snd)) (continuous_snd.comp continuous_snd)) | |
(continuous.add | |
(continuous.mul continuous_fst (continuous_snd.comp continuous_snd)) | |
(continuous.mul (continuous_fst.comp continuous_snd) (continuous_snd.comp continuous_snd)))) | |
(assume a b c, by rw [← coe_add, ← coe_mul, ← coe_mul, ← coe_mul, ←coe_add, add_mul]), | |
.. add_monoid_with_one.unary, | |
..completion.add_comm_group, ..completion.has_mul α, ..completion.has_one α } | |
/-- The map from a uniform ring to its completion, as a ring homomorphism. -/ | |
def coe_ring_hom : α →+* completion α := | |
⟨coe, coe_one α, assume a b, coe_mul a b, coe_zero, assume a b, coe_add a b⟩ | |
lemma continuous_coe_ring_hom : continuous (coe_ring_hom : α → completion α) := | |
continuous_coe α | |
variables {β : Type u} [uniform_space β] [ring β] [uniform_add_group β] [topological_ring β] | |
(f : α →+* β) (hf : continuous f) | |
/-- The completion extension as a ring morphism. -/ | |
def extension_hom [complete_space β] [separated_space β] : | |
completion α →+* β := | |
have hf' : continuous (f : α →+ β), from hf, -- helping the elaborator | |
have hf : uniform_continuous f, from uniform_continuous_add_monoid_hom_of_continuous hf', | |
{ to_fun := completion.extension f, | |
map_zero' := by rw [← coe_zero, extension_coe hf, f.map_zero], | |
map_add' := assume a b, completion.induction_on₂ a b | |
(is_closed_eq | |
(continuous_extension.comp continuous_add) | |
((continuous_extension.comp continuous_fst).add | |
(continuous_extension.comp continuous_snd))) | |
(assume a b, | |
by rw [← coe_add, extension_coe hf, extension_coe hf, extension_coe hf, | |
f.map_add]), | |
map_one' := by rw [← coe_one, extension_coe hf, f.map_one], | |
map_mul' := assume a b, completion.induction_on₂ a b | |
(is_closed_eq | |
(continuous_extension.comp continuous_mul) | |
((continuous_extension.comp continuous_fst).mul (continuous_extension.comp continuous_snd))) | |
(assume a b, | |
by rw [← coe_mul, extension_coe hf, extension_coe hf, extension_coe hf, f.map_mul]) } | |
instance top_ring_compl : topological_ring (completion α) := | |
{ continuous_add := continuous_add, | |
continuous_mul := continuous_mul } | |
/-- The completion map as a ring morphism. -/ | |
def map_ring_hom (hf : continuous f) : completion α →+* completion β := | |
extension_hom (coe_ring_hom.comp f) (continuous_coe_ring_hom.comp hf) | |
variables (R : Type*) [comm_ring R] [uniform_space R] [uniform_add_group R] [topological_ring R] | |
instance : comm_ring (completion R) := | |
{ mul_comm := assume a b, completion.induction_on₂ a b | |
(is_closed_eq (continuous_fst.mul continuous_snd) | |
(continuous_snd.mul continuous_fst)) | |
(assume a b, by rw [← coe_mul, ← coe_mul, mul_comm]), | |
..completion.ring } | |
end uniform_space.completion | |
namespace uniform_space | |
variables {α : Type*} | |
lemma ring_sep_rel (α) [comm_ring α] [uniform_space α] [uniform_add_group α] [topological_ring α] : | |
separation_setoid α = submodule.quotient_rel (ideal.closure ⊥) := | |
setoid.ext $ λ x y, (add_group_separation_rel x y).trans $ | |
iff.trans (by refl) (submodule.quotient_rel_r_def _).symm | |
lemma ring_sep_quot | |
(α : Type u) [r : comm_ring α] [uniform_space α] [uniform_add_group α] [topological_ring α] : | |
quotient (separation_setoid α) = (α ⧸ (⊥ : ideal α).closure) := | |
by rw [@ring_sep_rel α r]; refl | |
/-- Given a topological ring `α` equipped with a uniform structure that makes subtraction uniformly | |
continuous, get an equivalence between the separated quotient of `α` and the quotient ring | |
corresponding to the closure of zero. -/ | |
def sep_quot_equiv_ring_quot (α) | |
[r : comm_ring α] [uniform_space α] [uniform_add_group α] [topological_ring α] : | |
quotient (separation_setoid α) ≃ (α ⧸ (⊥ : ideal α).closure) := | |
quotient.congr_right $ λ x y, (add_group_separation_rel x y).trans $ | |
iff.trans (by refl) (submodule.quotient_rel_r_def _).symm | |
/- TODO: use a form of transport a.k.a. lift definition a.k.a. transfer -/ | |
instance comm_ring [comm_ring α] [uniform_space α] [uniform_add_group α] [topological_ring α] : | |
comm_ring (quotient (separation_setoid α)) := | |
by rw ring_sep_quot α; apply_instance | |
instance topological_ring | |
[comm_ring α] [uniform_space α] [uniform_add_group α] [topological_ring α] : | |
topological_ring (quotient (separation_setoid α)) := | |
begin | |
convert topological_ring_quotient (⊥ : ideal α).closure; try {apply ring_sep_rel}, | |
simp [uniform_space.comm_ring] | |
end | |
end uniform_space | |