Documentation

Mathlib.Algebra.Ring.Action.Basic

Group action on rings #

This file defines the typeclass of monoid acting on semirings MulSemiringAction M R, and the corresponding typeclass of invariant subrings.

Note that Algebra does not satisfy the axioms of MulSemiringAction.

Implementation notes #

There is no separate typeclass for group acting on rings, group acting on fields, etc. They are all grouped under MulSemiringAction.

Tags #

group action, invariant subring

class MulSemiringAction (M : Type u) (R : Type v) [Monoid M] [Semiring R] extends DistribMulAction :
Type (max u v)

Typeclass for multiplicative actions by monoids on semirings.

This combines DistribMulAction with MulDistribMulAction.

  • smul : MRR
  • one_smul : ∀ (b : R), 1 b = b
  • mul_smul : ∀ (x y : M) (b : R), (x * y) b = x y b
  • smul_zero : ∀ (a : M), a 0 = 0
  • smul_add : ∀ (a : M) (x y : R), a (x + y) = a x + a y
  • smul_one : ∀ (g : M), g 1 = 1

    Multipliying 1 by a scalar gives 1

  • smul_mul : ∀ (g : M) (x y : R), g (x * y) = g x * g y

    Scalar multiplication distributes across multiplication

Instances
    theorem MulSemiringAction.smul_one {M : Type u} {R : Type v} [Monoid M] [Semiring R] [self : MulSemiringAction M R] (g : M) :
    g 1 = 1

    Multipliying 1 by a scalar gives 1

    theorem MulSemiringAction.smul_mul {M : Type u} {R : Type v} [Monoid M] [Semiring R] [self : MulSemiringAction M R] (g : M) (x : R) (y : R) :
    g (x * y) = g x * g y

    Scalar multiplication distributes across multiplication

    @[simp]
    theorem MulSemiringAction.toRingHom_apply (M : Type u_1) [Monoid M] (R : Type v) [Semiring R] [MulSemiringAction M R] (x : M) :
    ∀ (x_1 : R), (MulSemiringAction.toRingHom M R x) x_1 = x x_1
    def MulSemiringAction.toRingHom (M : Type u_1) [Monoid M] (R : Type v) [Semiring R] [MulSemiringAction M R] (x : M) :
    R →+* R

    Each element of the monoid defines a semiring homomorphism.

    Equations
    Instances For

      The tautological action by R →+* R on R.

      This generalizes Function.End.applyMulAction.

      Equations
      @[simp]
      theorem RingHom.smul_def (R : Type v) [Semiring R] (f : R →+* R) (a : R) :
      f a = f a

      RingHom.applyMulSemiringAction is faithful.

      Equations
      • =
      @[reducible, inline]
      abbrev MulSemiringAction.compHom {M : Type u_1} {N : Type u_2} [Monoid M] [Monoid N] (R : Type v) [Semiring R] (f : N →* M) [MulSemiringAction M R] :

      Compose a MulSemiringAction with a MonoidHom, with action f r' • m. See note [reducible non-instances].

      Equations
      Instances For