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ring_theory.algebra_tower

Towers of algebras #

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We set up the basic theory of algebra towers. An algebra tower A/S/R is expressed by having instances of algebra A S, algebra R S, algebra R A and is_scalar_tower R S A, the later asserting the compatibility condition (r • s) • a = r • (s • a).

In field_theory/tower.lean we use this to prove the tower law for finite extensions, that if R and S are both fields, then [A:R] = [A:S] [S:A].

In this file we prepare the main lemma: if {bi | i ∈ I} is an R-basis of S and {cj | j ∈ J} is a S-basis of A, then {bi cj | i ∈ I, j ∈ J} is an R-basis of A. This statement does not require the base rings to be a field, so we also generalize the lemma to rings in this file.

def is_scalar_tower.invertible.algebra_tower (R : Type u) (S : Type v) (A : Type w) [comm_semiring R] [comm_semiring S] [semiring A] [algebra R S] [algebra S A] [algebra R A] [is_scalar_tower R S A] (r : R) [invertible ((algebra_map R S) r)] :

Suppose that R -> S -> A is a tower of algebras. If an element r : R is invertible in S, then it is invertible in A.

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A natural number that is invertible when coerced to R is also invertible when coerced to any R-algebra.

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@[simp]
theorem basis.algebra_map_coeffs_repr_apply_support_val {R : Type u} (A : Type w) {ι : Type u_1} {M : Type u_2} [comm_semiring R] [semiring A] [add_comm_monoid M] [algebra R A] [module A M] [module R M] [is_scalar_tower R A M] (b : basis ι R M) (h : function.bijective (algebra_map R A)) (ᾰ : M) :
noncomputable def basis.algebra_map_coeffs {R : Type u} (A : Type w) {ι : Type u_1} {M : Type u_2} [comm_semiring R] [semiring A] [add_comm_monoid M] [algebra R A] [module A M] [module R M] [is_scalar_tower R A M] (b : basis ι R M) (h : function.bijective (algebra_map R A)) :
basis ι A M

If R and A have a bijective algebra_map R A and act identically on M, then a basis for M as R-module is also a basis for M as R'-module.

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@[simp]
theorem basis.algebra_map_coeffs_repr_apply_to_fun {R : Type u} (A : Type w) {ι : Type u_1} {M : Type u_2} [comm_semiring R] [semiring A] [add_comm_monoid M] [algebra R A] [module A M] [module R M] [is_scalar_tower R A M] (b : basis ι R M) (h : function.bijective (algebra_map R A)) (ᾰ : M) (ᾰ_1 : ι) :
(((basis.algebra_map_coeffs A b h).repr) ᾰ) ᾰ_1 = (algebra_map R A) (((b.repr) ᾰ) ᾰ_1)
theorem basis.algebra_map_coeffs_apply {R : Type u} (A : Type w) {ι : Type u_1} {M : Type u_2} [comm_semiring R] [semiring A] [add_comm_monoid M] [algebra R A] [module A M] [module R M] [is_scalar_tower R A M] (b : basis ι R M) (h : function.bijective (algebra_map R A)) (i : ι) :
@[simp]
theorem basis.coe_algebra_map_coeffs {R : Type u} (A : Type w) {ι : Type u_1} {M : Type u_2} [comm_semiring R] [semiring A] [add_comm_monoid M] [algebra R A] [module A M] [module R M] [is_scalar_tower R A M] (b : basis ι R M) (h : function.bijective (algebra_map R A)) :
theorem linear_independent_smul {R : Type u} {S : Type v} {A : Type w} [comm_semiring R] [semiring S] [add_comm_monoid A] [algebra R S] [module S A] [module R A] [is_scalar_tower R S A] {ι : Type v₁} {b : ι S} {ι' : Type w₁} {c : ι' A} (hb : linear_independent R b) (hc : linear_independent S c) :
linear_independent R (λ (p : ι × ι'), b p.fst c p.snd)
noncomputable def basis.smul {R : Type u} {S : Type v} {A : Type w} [comm_semiring R] [semiring S] [add_comm_monoid A] [algebra R S] [module S A] [module R A] [is_scalar_tower R S A] {ι : Type v₁} {ι' : Type w₁} (b : basis ι R S) (c : basis ι' S A) :
basis × ι') R A

basis.smul (b : basis ι R S) (c : basis ι S A) is the R-basis on A where the (i, j)th basis vector is b i • c j.

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@[simp]
theorem basis.smul_repr {R : Type u} {S : Type v} {A : Type w} [comm_semiring R] [semiring S] [add_comm_monoid A] [algebra R S] [module S A] [module R A] [is_scalar_tower R S A] {ι : Type v₁} {ι' : Type w₁} (b : basis ι R S) (c : basis ι' S A) (x : A) (ij : ι × ι') :
(((b.smul c).repr) x) ij = ((b.repr) (((c.repr) x) ij.snd)) ij.fst
theorem basis.smul_repr_mk {R : Type u} {S : Type v} {A : Type w} [comm_semiring R] [semiring S] [add_comm_monoid A] [algebra R S] [module S A] [module R A] [is_scalar_tower R S A] {ι : Type v₁} {ι' : Type w₁} (b : basis ι R S) (c : basis ι' S A) (x : A) (i : ι) (j : ι') :
(((b.smul c).repr) x) (i, j) = ((b.repr) (((c.repr) x) j)) i
@[simp]
theorem basis.smul_apply {R : Type u} {S : Type v} {A : Type w} [comm_semiring R] [semiring S] [add_comm_monoid A] [algebra R S] [module S A] [module R A] [is_scalar_tower R S A] {ι : Type v₁} {ι' : Type w₁} (b : basis ι R S) (c : basis ι' S A) (ij : ι × ι') :
(b.smul c) ij = b ij.fst c ij.snd
theorem basis.algebra_map_injective {R : Type u} {S : Type v} [comm_ring R] [ring S] [algebra R S] {ι : Type u_1} [no_zero_divisors R] [nontrivial S] (b : basis ι R S) :
def alg_hom.restrict_domain {A : Type w} (B : Type u₁) {C : Type u_1} {D : Type u_2} [comm_semiring A] [comm_semiring C] [comm_semiring D] [algebra A C] [algebra A D] (f : C →ₐ[A] D) [comm_semiring B] [algebra A B] [algebra B C] [is_scalar_tower A B C] :

Restrict the domain of an alg_hom.

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def alg_hom.extend_scalars {A : Type w} (B : Type u₁) {C : Type u_1} {D : Type u_2} [comm_semiring A] [comm_semiring C] [comm_semiring D] [algebra A C] [algebra A D] (f : C →ₐ[A] D) [comm_semiring B] [algebra A B] [algebra B C] [is_scalar_tower A B C] :

Extend the scalars of an alg_hom.

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def alg_hom_equiv_sigma {A : Type w} {B : Type u₁} {C : Type u_1} {D : Type u_2} [comm_semiring A] [comm_semiring C] [comm_semiring D] [algebra A C] [algebra A D] [comm_semiring B] [algebra A B] [algebra B C] [is_scalar_tower A B C] :
(C →ₐ[A] D) Σ (f : B →ₐ[A] D), C →ₐ[B] D

alg_homs from the top of a tower are equivalent to a pair of alg_homs.

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