# Documentation

Mathlib.Algebra.Algebra.Basic

# Algebras over commutative semirings #

In this file we define associative unital Algebras over commutative (semi)rings.

• algebra homomorphisms AlgHom are defined in Mathlib.Algebra.Algebra.Hom;

• algebra equivalences AlgEquiv are defined in Mathlib.Algebra.Algebra.Equiv;

• Subalgebras are defined in Mathlib.Algebra.Algebra.Subalgebra;

• The category AlgebraCat R of R-algebras is defined in the file Mathlib.Algebra.Category.Algebra.Basic.

See the implementation notes for remarks about non-associative and non-unital algebras.

## Main definitions: #

• Algebra R A: the algebra typeclass.
• algebraMap R A : R →+* A: the canonical map from R to A, as a RingHom. This is the preferred spelling of this map, it is also available as:
• Algebra.linearMap R A : R →ₗ[R] A, a LinearMap.
• algebra.ofId R A : R →ₐ[R] A, an AlgHom (defined in a later file).
• Instances of Algebra in this file:
• Algebra.id
• algebraNat
• algebraInt
• algebraRat
• Module.End.instAlgebra

## Implementation notes #

Given a commutative (semi)ring R, there are two ways to define an R-algebra structure on a (possibly noncommutative) (semi)ring A:

• By endowing A with a morphism of rings R →+* A denoted algebraMap R A which lands in the center of A.
• By requiring A be an R-module such that the action associates and commutes with multiplication as r • (a₁ * a₂) = (r • a₁) * a₂ = a₁ * (r • a₂).

We define Algebra R A in a way that subsumes both definitions, by extending SMul R A and requiring that this scalar action r • x must agree with left multiplication by the image of the structure morphism algebraMap R A r * x.

As a result, there are two ways to talk about an R-algebra A when A is a semiring:

1. variables [CommSemiring R] [Semiring A]
variables [Algebra R A]

2. variables [CommSemiring R] [Semiring A]
variables [Module R A] [SMulCommClass R A A] [IsScalarTower R A A]


The first approach implies the second via typeclass search; so any lemma stated with the second set of arguments will automatically apply to the first set. Typeclass search does not know that the second approach implies the first, but this can be shown with:

example {R A : Type*} [CommSemiring R] [Semiring A]
[Module R A] [SMulCommClass R A A] [IsScalarTower R A A] : Algebra R A :=
Algebra.ofModule smul_mul_assoc mul_smul_comm


The advantage of the first approach is that algebraMap R A is available, and AlgHom R A B and Subalgebra R A can be used. For concrete R and A, algebraMap R A is often definitionally convenient.

The advantage of the second approach is that CommSemiring R, Semiring A, and Module R A can all be relaxed independently; for instance, this allows us to:

• Replace Semiring A with NonUnitalNonAssocSemiring A in order to describe non-unital and/or non-associative algebras.
• Replace CommSemiring R and Module R A with CommGroup R' and DistribMulAction R' A, which when R' = Rˣ lets us talk about the "algebra-like" action of Rˣ on an R-algebra A.

While AlgHom R A B cannot be used in the second approach, NonUnitalAlgHom R A B still can.

You should always use the first approach when working with associative unital algebras, and mimic the second approach only when you need to weaken a condition on either R or A.

class Algebra (R : Type u) (A : Type v) [] [] extends , :
Type (max u v)
• smul : RAA
• toFun : RA
• map_one' : OneHom.toFun (Algebra.toRingHom) 1 = 1
• map_mul' : ∀ (x y : R), OneHom.toFun (Algebra.toRingHom) (x * y) = OneHom.toFun (Algebra.toRingHom) x * OneHom.toFun (Algebra.toRingHom) y
• map_zero' : OneHom.toFun (Algebra.toRingHom) 0 = 0
• map_add' : ∀ (x y : R), OneHom.toFun (Algebra.toRingHom) (x + y) = OneHom.toFun (Algebra.toRingHom) x + OneHom.toFun (Algebra.toRingHom) y
• commutes' : ∀ (r : R) (x : (fun x => A) r), Algebra.toRingHom r * x = x * Algebra.toRingHom r
• smul_def' : ∀ (r : R) (x : (fun x => A) r), r x = Algebra.toRingHom r * x

An associative unital R-algebra is a semiring A equipped with a map into its center R → A.

See the implementation notes in this file for discussion of the details of this definition.

Instances
def algebraMap (R : Type u) (A : Type v) [] [] [Algebra R A] :
R →+* A

Embedding R →+* A given by Algebra structure.

Instances For
def Algebra.cast {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] :
RA

Coercion from a commutative semiring to an algebra over this semiring.

Instances For
def algebraMap.coeHTCT (R : Type u_1) (A : Type u_2) [] [] [Algebra R A] :
Instances For
@[simp]
theorem algebraMap.coe_zero {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] :
0 = 0
@[simp]
theorem algebraMap.coe_one {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] :
1 = 1
theorem algebraMap.coe_add {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] (a : R) (b : R) :
↑(a + b) = a + b
theorem algebraMap.coe_mul {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] (a : R) (b : R) :
↑(a * b) = a * b
theorem algebraMap.coe_pow {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] (a : R) (n : ) :
↑(a ^ n) = a ^ n
theorem algebraMap.coe_neg {R : Type u_1} {A : Type u_2} [] [Ring A] [Algebra R A] (x : R) :
↑(-x) = -x
theorem algebraMap.coe_prod {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] {ι : Type u_3} {s : } (a : ιR) :
↑(Finset.prod s fun i => a i) = Finset.prod s fun i => ↑(a i)
theorem algebraMap.coe_sum {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] {ι : Type u_3} {s : } (a : ιR) :
↑(Finset.sum s fun i => a i) = Finset.sum s fun i => ↑(a i)
@[simp]
theorem algebraMap.coe_inj {R : Type u_1} {A : Type u_2} [] [] [] [Algebra R A] {a : R} {b : R} :
a = b a = b
@[simp]
theorem algebraMap.lift_map_eq_zero_iff {R : Type u_1} {A : Type u_2} [] [] [] [Algebra R A] (a : R) :
a = 0 a = 0
theorem algebraMap.coe_inv {R : Type u_1} (A : Type u_2) [] [] [Algebra R A] (r : R) :
r⁻¹ = (r)⁻¹
theorem algebraMap.coe_div {R : Type u_1} (A : Type u_2) [] [] [Algebra R A] (r : R) (s : R) :
↑(r / s) = r / s
theorem algebraMap.coe_zpow {R : Type u_1} (A : Type u_2) [] [] [Algebra R A] (r : R) (z : ) :
↑(r ^ z) = r ^ z
theorem algebraMap.coe_ratCast (R : Type u_1) (A : Type u_2) [] [] [Algebra R A] (q : ) :
q = q
def RingHom.toAlgebra' {R : Type u_1} {S : Type u_2} [] [] (i : R →+* S) (h : ∀ (c : R) (x : (fun x => S) c), i c * x = x * i c) :

Creating an algebra from a morphism to the center of a semiring.

Instances For
def RingHom.toAlgebra {R : Type u_1} {S : Type u_2} [] [] (i : R →+* S) :

Creating an algebra from a morphism to a commutative semiring.

Instances For
theorem RingHom.algebraMap_toAlgebra {R : Type u_1} {S : Type u_2} [] [] (i : R →+* S) :
= i
@[reducible]
def Algebra.ofModule' {R : Type u} {A : Type w} [] [] [Module R A] (h₁ : ∀ (r : R) (x : A), r 1 * x = r x) (h₂ : ∀ (r : R) (x : A), x * r 1 = r x) :

Let R be a commutative semiring, let A be a semiring with a Module R structure. If (r • 1) * x = x * (r • 1) = r • x for all r : R and x : A, then A is an Algebra over R.

See note [reducible non-instances].

Instances For
@[reducible]
def Algebra.ofModule {R : Type u} {A : Type w} [] [] [Module R A] (h₁ : ∀ (r : R) (x y : A), r x * y = r (x * y)) (h₂ : ∀ (r : R) (x y : A), x * r y = r (x * y)) :

Let R be a commutative semiring, let A be a semiring with a Module R structure. If (r • x) * y = x * (r • y) = r • (x * y) for all r : R and x y : A, then A is an Algebra over R.

See note [reducible non-instances].

Instances For
theorem Algebra.algebra_ext {R : Type u_2} [] {A : Type u_3} [] (P : Algebra R A) (Q : Algebra R A) (h : ∀ (r : R), ↑() r = ↑() r) :
P = Q

To prove two algebra structures on a fixed [CommSemiring R] [Semiring A] agree, it suffices to check the algebraMaps agree.

instance Algebra.toModule {R : Type u} {A : Type w} [] [] [Algebra R A] :
Module R A
theorem Algebra.smul_def {R : Type u} {A : Type w} [] [] [Algebra R A] (r : R) (x : A) :
r x = ↑() r * x
theorem Algebra.algebraMap_eq_smul_one {R : Type u} {A : Type w} [] [] [Algebra R A] (r : R) :
↑() r = r 1
theorem Algebra.algebraMap_eq_smul_one' {R : Type u} {A : Type w} [] [] [Algebra R A] :
↑() = fun r => r 1
theorem Algebra.commutes {R : Type u} {A : Type w} [] [] [Algebra R A] (r : R) (x : A) :
↑() r * x = x * ↑() r

mul_comm for Algebras when one element is from the base ring.

theorem Algebra.left_comm {R : Type u} {A : Type w} [] [] [Algebra R A] (x : A) (r : R) (y : A) :
x * (↑() r * y) = ↑() r * (x * y)

mul_left_comm for Algebras when one element is from the base ring.

theorem Algebra.right_comm {R : Type u} {A : Type w} [] [] [Algebra R A] (x : A) (r : R) (y : A) :
x * ↑() r * y = x * y * ↑() r

mul_right_comm for Algebras when one element is from the base ring.

instance IsScalarTower.right {R : Type u} {A : Type w} [] [] [Algebra R A] :
@[simp]
theorem Algebra.mul_smul_comm {R : Type u} {A : Type w} [] [] [Algebra R A] (s : R) (x : A) (y : A) :
x * s y = s (x * y)

This is just a special case of the global mul_smul_comm lemma that requires less typeclass search (and was here first).

@[simp]
theorem Algebra.smul_mul_assoc {R : Type u} {A : Type w} [] [] [Algebra R A] (r : R) (x : A) (y : A) :
r x * y = r (x * y)

This is just a special case of the global smul_mul_assoc lemma that requires less typeclass search (and was here first).

@[simp]
theorem smul_algebraMap {R : Type u} {A : Type w} [] [] [Algebra R A] {α : Type u_2} [] [] [] (a : α) (r : R) :
a ↑() r = ↑() r
def Algebra.linearMap (R : Type u) (A : Type w) [] [] [Algebra R A] :

The canonical ring homomorphism algebraMap R A : R →* A for any R-algebra A, packaged as an R-linear map.

Instances For
@[simp]
theorem Algebra.linearMap_apply (R : Type u) (A : Type w) [] [] [Algebra R A] (r : R) :
↑() r = ↑() r
theorem Algebra.coe_linearMap (R : Type u) (A : Type w) [] [] [Algebra R A] :
↑() = ↑()
instance Algebra.id (R : Type u) [] :
@[simp]
theorem Algebra.id.map_eq_id {R : Type u} [] :
=
theorem Algebra.id.map_eq_self {R : Type u} [] (x : R) :
↑() x = x
@[simp]
theorem Algebra.id.smul_eq_mul {R : Type u} [] (x : R) (y : R) :
x y = x * y
instance PUnit.algebra {R : Type u} [] :
@[simp]
theorem Algebra.algebraMap_pUnit {R : Type u} [] (r : R) :
↑() r = PUnit.unit
instance ULift.algebra {R : Type u} {A : Type w} [] [] [Algebra R A] :
Algebra R ()
theorem ULift.algebraMap_eq {R : Type u} {A : Type w} [] [] [Algebra R A] (r : R) :
↑(algebraMap R ()) r = { down := ↑() r }
@[simp]
theorem ULift.down_algebraMap {R : Type u} {A : Type w} [] [] [Algebra R A] (r : R) :
(↑(algebraMap R ()) r).down = ↑() r
instance Algebra.ofSubsemiring {R : Type u} {A : Type w} [] [] [Algebra R A] (S : ) :
Algebra { x // x S } A

Algebra over a subsemiring. This builds upon Subsemiring.module.

theorem Algebra.algebraMap_ofSubsemiring {R : Type u} [] (S : ) :
algebraMap { x // x S } R =
theorem Algebra.coe_algebraMap_ofSubsemiring {R : Type u} [] (S : ) :
↑(algebraMap { x // x S } R) = Subtype.val
theorem Algebra.algebraMap_ofSubsemiring_apply {R : Type u} [] (S : ) (x : { x // x S }) :
↑(algebraMap { x // x S } R) x = x
instance Algebra.ofSubring {R : Type u_2} {A : Type u_3} [] [Ring A] [Algebra R A] (S : ) :
Algebra { x // x S } A

Algebra over a subring. This builds upon Subring.module.

theorem Algebra.algebraMap_ofSubring {R : Type u_2} [] (S : ) :
algebraMap { x // x S } R =
theorem Algebra.coe_algebraMap_ofSubring {R : Type u_2} [] (S : ) :
↑(algebraMap { x // x S } R) = Subtype.val
theorem Algebra.algebraMap_ofSubring_apply {R : Type u_2} [] (S : ) (x : { x // x S }) :
↑(algebraMap { x // x S } R) x = x
def Algebra.algebraMapSubmonoid {R : Type u} [] (S : Type u_2) [] [Algebra R S] (M : ) :

Explicit characterization of the submonoid map in the case of an algebra. S is made explicit to help with type inference

Instances For
theorem Algebra.mem_algebraMapSubmonoid_of_mem {R : Type u} [] {S : Type u_2} [] [Algebra R S] {M : } (x : { x // x M }) :
↑() x
theorem Algebra.mul_sub_algebraMap_commutes {R : Type u} {A : Type w} [] [Ring A] [Algebra R A] (x : A) (r : R) :
x * (x - ↑() r) = (x - ↑() r) * x
theorem Algebra.mul_sub_algebraMap_pow_commutes {R : Type u} {A : Type w} [] [Ring A] [Algebra R A] (x : A) (r : R) (n : ) :
x * (x - ↑() r) ^ n = (x - ↑() r) ^ n * x
@[reducible]
def Algebra.semiringToRing (R : Type u) {A : Type w} [] [] [Algebra R A] :

A Semiring that is an Algebra over a commutative ring carries a natural Ring structure. See note [reducible non-instances].

Instances For
instance Module.End.instAlgebra (R : Type u) (S : Type v) (M : Type w) [] [] [] [Module R M] [Module S M] [] [SMul R S] [] :
Algebra R ()
theorem Module.algebraMap_end_eq_smul_id (R : Type u) (S : Type v) (M : Type w) [] [] [] [Module R M] [Module S M] [] [SMul R S] [] (a : R) :
↑(algebraMap R ()) a = a LinearMap.id
@[simp]
theorem Module.algebraMap_end_apply (R : Type u) (S : Type v) (M : Type w) [] [] [] [Module R M] [Module S M] [] [SMul R S] [] (a : R) (m : M) :
↑(↑(algebraMap R ()) a) m = a m
@[simp]
theorem Module.ker_algebraMap_end (K : Type u) (V : Type v) [] [] [Module K V] (a : K) (ha : a 0) :
theorem Module.End_algebraMap_isUnit_inv_apply_eq_iff {R : Type u} (S : Type v) {M : Type w} [] [] [] [Module R M] [Module S M] [] [SMul R S] [] {x : R} (h : IsUnit (↑(algebraMap R ()) x)) (m : M) (m' : M) :
()⁻¹ m = m' m = x m'
theorem Module.End_algebraMap_isUnit_inv_apply_eq_iff' {R : Type u} (S : Type v) {M : Type w} [] [] [] [Module R M] [Module S M] [] [SMul R S] [] {x : R} (h : IsUnit (↑(algebraMap R ()) x)) (m : M) (m' : M) :
m' = ()⁻¹ m m = x m'
theorem LinearMap.map_algebraMap_mul {R : Type u_1} {A : Type u_2} {B : Type u_3} [] [] [] [Algebra R A] [Algebra R B] (f : A →ₗ[R] B) (a : A) (r : R) :
f (↑() r * a) = ↑() r * f a

An alternate statement of LinearMap.map_smul for when algebraMap is more convenient to work with than •.

theorem LinearMap.map_mul_algebraMap {R : Type u_1} {A : Type u_2} {B : Type u_3} [] [] [] [Algebra R A] [Algebra R B] (f : A →ₗ[R] B) (a : A) (r : R) :
f (a * ↑() r) = f a * ↑() r
instance algebraNat {R : Type u_1} [] :

Semiring ⥤ ℕ-Alg

@[simp]
theorem RingHom.map_rat_algebraMap {R : Type u_1} {S : Type u_2} [] [] [] [] (f : R →+* S) (r : ) :
f (↑() r) = ↑() r
instance algebraRat {α : Type u_1} [] [] :
@[simp]
instance algebraInt (R : Type u_1) [Ring R] :

Ring ⥤ ℤ-Alg

@[simp]
theorem algebraMap_int_eq (R : Type u_1) [Ring R] :

A special case of eq_intCast' that happens to be true definitionally

theorem NoZeroSMulDivisors.of_algebraMap_injective {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] [] (h : Function.Injective ↑()) :

If algebraMap R A is injective and A has no zero divisors, R-multiples in A are zero only if one of the factors is zero.

Cannot be an instance because there is no Injective (algebraMap R A) typeclass.

theorem NoZeroSMulDivisors.algebraMap_injective (R : Type u_1) (A : Type u_2) [] [Ring A] [] [Algebra R A] [] :
theorem NeZero.of_noZeroSMulDivisors (R : Type u_1) (A : Type u_2) (n : ) [] [NeZero n] [Ring A] [] [Algebra R A] [] :
NeZero n
instance NoZeroSMulDivisors.Algebra.noZeroSMulDivisors {R : Type u_1} {A : Type u_2} [] [] [Algebra R A] [] [] :
theorem algebra_compatible_smul {R : Type u_1} [] (A : Type u_2) [] [Algebra R A] {M : Type u_3} [] [Module A M] [Module R M] [] (r : R) (m : M) :
r m = ↑() r m
@[simp]
theorem algebraMap_smul {R : Type u_1} [] (A : Type u_2) [] [Algebra R A] {M : Type u_3} [] [Module A M] [Module R M] [] (r : R) (m : M) :
↑() r m = r m
theorem intCast_smul {k : Type u_5} {V : Type u_6} [] [] [Module k V] (r : ) (x : V) :
r x = r x
theorem NoZeroSMulDivisors.trans (R : Type u_5) (A : Type u_6) (M : Type u_7) [] [Ring A] [] [Algebra R A] [] [Module R M] [Module A M] [] [] [] :
instance IsScalarTower.to_smulCommClass {R : Type u_1} [] {A : Type u_2} [] [Algebra R A] {M : Type u_3} [] [Module A M] [Module R M] [] :
instance IsScalarTower.to_smulCommClass' {R : Type u_1} [] {A : Type u_2} [] [Algebra R A] {M : Type u_3} [] [Module A M] [Module R M] [] :
instance Algebra.to_smulCommClass {R : Type u_5} {A : Type u_6} [] [] [Algebra R A] :
theorem smul_algebra_smul_comm {R : Type u_1} [] {A : Type u_2} [] [Algebra R A] {M : Type u_3} [] [Module A M] [Module R M] [] (r : R) (a : A) (m : M) :
a r m = r a m
def LinearMap.ltoFun (R : Type u) (M : Type v) (A : Type w) [] [] [Module R M] [] [Algebra R A] :
(M →ₗ[R] A) →ₗ[A] MA

A-linearly coerce an R-linear map from M to A to a function, given an algebra A over a commutative semiring R and M a module over R.

Instances For

TODO: The following lemmas no longer involve Algebra at all, and could be moved closer to Algebra/Module/Submodule.lean. Currently this is tricky because ker, range, ⊤, and ⊥ are all defined in LinearAlgebra/Basic.lean.

@[simp]
theorem LinearMap.ker_restrictScalars (R : Type u_1) {S : Type u_2} {M : Type u_3} {N : Type u_4} [] [] [SMul R S] [] [Module R M] [Module S M] [] [] [Module R N] [Module S N] [] (f : M →ₗ[S] N) :
LinearMap.ker (R f) =