Semirings and rings

This file defines semirings, rings and domains. This is analogous to Algebra.Group.Defs and Algebra.Group.Basic, the difference being that the former is about + and * separately, while the present file is about their interaction.

Main definitions

• Distrib: Typeclass for distributivity of multiplication over addition.
• HasDistribNeg: Typeclass for commutativity of negation and multiplication. This is useful when dealing with multiplicative submonoids which are closed under negation without being closed under addition, for example Units.
• (NonUnital)(NonAssoc)(Semi)Ring: Typeclasses for possibly non-unital or non-associative rings and semirings. Some combinations are not defined yet because they haven't found use. For Lie Rings, there is a type synonym CommutatorRing defined in Mathlib/Algebra/Algebra/NonUnitalHom.lean turning the bracket into a multiplication so that the instance instNonUnitalNonAssocSemiringCommutatorRing can be defined.

Tags

Semiring, CommSemiring, Ring, CommRing, domain, IsDomain, nonzero, units

Previously an import dependency on Mathlib.Algebra.Group.Basic had crept in. In general, the .Defs files in the basic algebraic hierarchy should only depend on earlier .Defs files, without importing .Basic theory development.

These assert_not_exists statements guard against this returning.

Distrib class

class Distrib (R : Type u_1) extends Mul , Add : Type u_1

A typeclass stating that multiplication is left and right distributive over addition.

• mul : R → R → R
• add : R → R → R
• left_distrib : ∀ (a b c : R), a * (b + c) = a * b + a * c

  Multiplication is left distributive over addition

• right_distrib : ∀ (a b c : R), (a + b) * c = a * c + b * c

  Multiplication is right distributive over addition

theorem Distrib.left_distrib {R : Type u_1} [self : Distrib R] (a : R) (b : R) (c : R) : a * (b + c) = a * b + a * c

Multiplication is left distributive over addition

theorem Distrib.right_distrib {R : Type u_1} [self : Distrib R] (a : R) (b : R) (c : R) : (a + b) * c = a * c + b * c

Multiplication is right distributive over addition

class LeftDistribClass (R : Type u_1) [Mul R] [Add R] : Prop

A typeclass stating that multiplication is left distributive over addition.

• left_distrib : ∀ (a b c : R), a * (b + c) = a * b + a * c

  Multiplication is left distributive over addition

theorem LeftDistribClass.left_distrib {R : Type u_1} : ∀ {inst : Mul R} {inst_1 : Add R} [self : LeftDistribClass R] (a b c : R), a * (b + c) = a * b + a * c

Multiplication is left distributive over addition

class RightDistribClass (R : Type u_1) [Mul R] [Add R] : Prop

A typeclass stating that multiplication is right distributive over addition.

• right_distrib : ∀ (a b c : R), (a + b) * c = a * c + b * c

  Multiplication is right distributive over addition

theorem RightDistribClass.right_distrib {R : Type u_1} : ∀ {inst : Mul R} {inst_1 : Add R} [self : RightDistribClass R] (a b c : R), (a + b) * c = a * c + b * c

Multiplication is right distributive over addition

@[instance 100]

instance Distrib.leftDistribClass (R : Type u_1) [Distrib R] : LeftDistribClass R

Equations

• ⋯ = ⋯

@[instance 100]

instance Distrib.rightDistribClass (R : Type u_1) [Distrib R] : RightDistribClass R

Equations

• ⋯ = ⋯

theorem left_distrib {R : Type v} [Mul R] [Add R] [LeftDistribClass R] (a : R) (b : R) (c : R) : a * (b + c) = a * b + a * c

theorem mul_add {R : Type v} [Mul R] [Add R] [LeftDistribClass R] (a : R) (b : R) (c : R) : a * (b + c) = a * b + a * c

Alias of left_distrib.

theorem right_distrib {R : Type v} [Mul R] [Add R] [RightDistribClass R] (a : R) (b : R) (c : R) : (a + b) * c = a * c + b * c

theorem add_mul {R : Type v} [Mul R] [Add R] [RightDistribClass R] (a : R) (b : R) (c : R) : (a + b) * c = a * c + b * c

Alias of right_distrib.

theorem distrib_three_right {R : Type v} [Mul R] [Add R] [RightDistribClass R] (a : R) (b : R) (c : R) (d : R) : (a + b + c) * d = a * d + b * d + c * d

Classes of semirings and rings

We make sure that the canonical path from NonAssocSemiring to Ring passes through Semiring, as this is a path which is followed all the time in linear algebra where the defining semilinear map σ : R →+* S depends on the NonAssocSemiring structure of R and S while the module definition depends on the Semiring structure.

It is not currently possible to adjust priorities by hand (see lean4#2115). Instead, the last declared instance is used, so we make sure that Semiring is declared after NonAssocRing, so that Semiring -> NonAssocSemiring is tried before NonAssocRing -> NonAssocSemiring. TODO: clean this once lean4#2115 is fixed

class NonUnitalNonAssocSemiring (α : Type u) extends AddCommMonoid , Distrib , MulZeroClass , AddMonoid , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

A not-necessarily-unital, not-necessarily-associative semiring. See CommutatorRing and the documentation thereof in case you need a NonUnitalNonAssocSemiring instance on a Lie ring or a Lie algebra.

class NonUnitalSemiring (α : Type u) extends NonUnitalNonAssocSemiring , SemigroupWithZero , AddCommMonoid , Distrib , Semigroup , MulZeroClass , AddMonoid , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

An associative but not-necessarily unital semiring.

class NonAssocSemiring (α : Type u) extends NonUnitalNonAssocSemiring , MulZeroOneClass , AddCommMonoidWithOne , MulOneClass , AddMonoidWithOne , AddCommMonoid , Distrib , MulZeroClass , NatCast , AddMonoid , One , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

A unital but not-necessarily-associative semiring.

class NonUnitalNonAssocRing (α : Type u) extends AddCommGroup , NonUnitalNonAssocSemiring , AddGroup , AddCommMonoid , SubNegMonoid , Distrib , MulZeroClass , AddMonoid , Neg , Sub , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

A not-necessarily-unital, not-necessarily-associative ring.

class NonUnitalRing (α : Type u_1) extends NonUnitalNonAssocRing , NonUnitalSemiring , AddCommGroup , NonUnitalNonAssocSemiring , AddGroup , SemigroupWithZero , AddCommMonoid , SubNegMonoid , Distrib , Semigroup , MulZeroClass , AddMonoid , Neg , Sub , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u_1

An associative but not-necessarily unital ring.

class NonAssocRing (α : Type u_1) extends NonUnitalNonAssocRing , NonAssocSemiring , AddCommGroupWithOne , AddCommGroup , NonUnitalNonAssocSemiring , MulZeroOneClass , AddGroupWithOne , AddCommMonoidWithOne , MulOneClass , IntCast , AddMonoidWithOne , AddGroup , AddCommMonoid , SubNegMonoid , Distrib , MulZeroClass , NatCast , AddMonoid , One , Neg , Sub , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u_1

A unital but not-necessarily-associative ring.

class Semiring (α : Type u) extends NonUnitalSemiring , NonAssocSemiring , MonoidWithZero , NonUnitalNonAssocSemiring , Monoid , MulZeroOneClass , SemigroupWithZero , AddCommMonoidWithOne , MulOneClass , AddMonoidWithOne , AddCommMonoid , Distrib , Semigroup , MulZeroClass , NatCast , AddMonoid , One , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

A Semiring is a type with addition, multiplication, a 0 and a 1 where addition is commutative and associative, multiplication is associative and left and right distributive over addition, and 0 and 1 are additive and multiplicative identities.

class Ring (R : Type u) extends Semiring , AddCommGroup , AddGroupWithOne , NonUnitalSemiring , NonAssocSemiring , MonoidWithZero , NonUnitalNonAssocSemiring , Monoid , MulZeroOneClass , SemigroupWithZero , AddCommMonoidWithOne , MulOneClass , IntCast , AddMonoidWithOne , AddGroup , AddCommMonoid , Distrib , Semigroup , MulZeroClass , NatCast , SubNegMonoid , AddMonoid , One , Neg , Sub , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

A Ring is a Semiring with negation making it an additive group.

Semirings

theorem add_one_mul {α : Type u} [Add α] [MulOneClass α] [RightDistribClass α] (a : α) (b : α) : (a + 1) * b = a * b + b

theorem mul_add_one {α : Type u} [Add α] [MulOneClass α] [LeftDistribClass α] (a : α) (b : α) : a * (b + 1) = a * b + a

theorem one_add_mul {α : Type u} [Add α] [MulOneClass α] [RightDistribClass α] (a : α) (b : α) : (1 + a) * b = b + a * b

theorem mul_one_add {α : Type u} [Add α] [MulOneClass α] [LeftDistribClass α] (a : α) (b : α) : a * (1 + b) = a + a * b

theorem two_mul {α : Type u} [NonAssocSemiring α] (n : α) : 2 * n = n + n

theorem mul_two {α : Type u} [NonAssocSemiring α] (n : α) : n * 2 = n + n

theorem ite_zero_mul {α : Type u} [MulZeroClass α] (P : Prop) [Decidable P] (a : α) (b : α) : (if P then a else 0) * b = if P then a * b else 0

theorem mul_ite_zero {α : Type u} [MulZeroClass α] (P : Prop) [Decidable P] (a : α) (b : α) : (a * if P then b else 0) = if P then a * b else 0

theorem ite_zero_mul_ite_zero {α : Type u} [MulZeroClass α] (P : Prop) (Q : Prop) [Decidable P] [Decidable Q] (a : α) (b : α) : ((if P then a else 0) * if Q then b else 0) = if P ∧ Q then a * b else 0

theorem mul_boole {α : Type u_1} [MulZeroOneClass α] (P : Prop) [Decidable P] (a : α) : (a * if P then 1 else 0) = if P then a else 0

theorem boole_mul {α : Type u_1} [MulZeroOneClass α] (P : Prop) [Decidable P] (a : α) : (if P then 1 else 0) * a = if P then a else 0

class NonUnitalNonAssocCommSemiring (α : Type u) extends NonUnitalNonAssocSemiring , CommMagma , AddCommMonoid , Distrib , MulZeroClass , AddMonoid , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

A not-necessarily-unital, not-necessarily-associative, but commutative semiring.

class NonUnitalCommSemiring (α : Type u) extends NonUnitalSemiring , CommSemigroup , NonUnitalNonAssocSemiring , SemigroupWithZero , AddCommMonoid , Distrib , Semigroup , MulZeroClass , AddMonoid , AddCommSemigroup , AddSemigroup , AddZeroClass , CommMagma , Mul , Zero , AddCommMagma , Add : Type u

A non-unital commutative semiring is a NonUnitalSemiring with commutative multiplication. In other words, it is a type with the following structures: additive commutative monoid (AddCommMonoid), commutative semigroup (CommSemigroup), distributive laws (Distrib), and multiplication by zero law (MulZeroClass).

class CommSemiring (R : Type u) extends Semiring , CommMonoid , NonUnitalSemiring , NonAssocSemiring , MonoidWithZero , NonUnitalNonAssocSemiring , Monoid , MulZeroOneClass , SemigroupWithZero , AddCommMonoidWithOne , CommSemigroup , MulOneClass , AddMonoidWithOne , AddCommMonoid , Distrib , Semigroup , MulZeroClass , NatCast , AddMonoid , One , AddCommSemigroup , AddSemigroup , AddZeroClass , CommMagma , Mul , Zero , AddCommMagma , Add : Type u

A commutative semiring is a semiring with commutative multiplication.

@[instance 100]

instance CommSemiring.toNonUnitalCommSemiring {α : Type u} [CommSemiring α] : NonUnitalCommSemiring α

Equations

• CommSemiring.toNonUnitalCommSemiring = NonUnitalCommSemiring.mk ⋯

@[instance 100]

instance CommSemiring.toCommMonoidWithZero {α : Type u} [CommSemiring α] : CommMonoidWithZero α

Equations

• CommSemiring.toCommMonoidWithZero = CommMonoidWithZero.mk ⋯ ⋯

theorem add_mul_self_eq {α : Type u} [CommSemiring α] (a : α) (b : α) : (a + b) * (a + b) = a * a + 2 * a * b + b * b

theorem add_sq {α : Type u} [CommSemiring α] (a : α) (b : α) : (a + b) ^ 2 = a ^ 2 + 2 * a * b + b ^ 2

theorem add_sq' {α : Type u} [CommSemiring α] (a : α) (b : α) : (a + b) ^ 2 = a ^ 2 + b ^ 2 + 2 * a * b

theorem add_pow_two {α : Type u} [CommSemiring α] (a : α) (b : α) : (a + b) ^ 2 = a ^ 2 + 2 * a * b + b ^ 2

Alias of add_sq.

class HasDistribNeg (α : Type u_1) [Mul α] extends InvolutiveNeg , Neg : Type u_1

Typeclass for a negation operator that distributes across multiplication.

This is useful for dealing with submonoids of a ring that contain -1 without having to duplicate lemmas.

• neg : α → α
• neg_neg : ∀ (x : α), - -x = x
• neg_mul : ∀ (x y : α), -x * y = -(x * y)

  Negation is left distributive over multiplication

• mul_neg : ∀ (x y : α), x * -y = -(x * y)

  Negation is right distributive over multiplication

theorem HasDistribNeg.neg_mul {α : Type u_1} : ∀ {inst : Mul α} [self : HasDistribNeg α] (x y : α), -x * y = -(x * y)

Negation is left distributive over multiplication

theorem HasDistribNeg.mul_neg {α : Type u_1} : ∀ {inst : Mul α} [self : HasDistribNeg α] (x y : α), x * -y = -(x * y)

Negation is right distributive over multiplication

@[simp]

theorem neg_mul {α : Type u} [Mul α] [HasDistribNeg α] (a : α) (b : α) : -a * b = -(a * b)

@[simp]

theorem mul_neg {α : Type u} [Mul α] [HasDistribNeg α] (a : α) (b : α) : a * -b = -(a * b)

theorem neg_mul_neg {α : Type u} [Mul α] [HasDistribNeg α] (a : α) (b : α) : -a * -b = a * b

theorem neg_mul_eq_neg_mul {α : Type u} [Mul α] [HasDistribNeg α] (a : α) (b : α) : -(a * b) = -a * b

theorem neg_mul_eq_mul_neg {α : Type u} [Mul α] [HasDistribNeg α] (a : α) (b : α) : -(a * b) = a * -b

theorem neg_mul_comm {α : Type u} [Mul α] [HasDistribNeg α] (a : α) (b : α) : -a * b = a * -b

theorem neg_eq_neg_one_mul {α : Type u} [MulOneClass α] [HasDistribNeg α] (a : α) : -a = -1 * a

theorem mul_neg_one {α : Type u} [MulOneClass α] [HasDistribNeg α] (a : α) : a * -1 = -a

An element of a ring multiplied by the additive inverse of one is the element's additive inverse.

theorem neg_one_mul {α : Type u} [MulOneClass α] [HasDistribNeg α] (a : α) : -1 * a = -a

The additive inverse of one multiplied by an element of a ring is the element's additive inverse.

@[instance 100]

instance MulZeroClass.negZeroClass {α : Type u} [MulZeroClass α] [HasDistribNeg α] : NegZeroClass α

Equations

• MulZeroClass.negZeroClass = NegZeroClass.mk ⋯

Rings

@[instance 100]

instance NonUnitalNonAssocRing.toHasDistribNeg {α : Type u} [NonUnitalNonAssocRing α] : HasDistribNeg α

Equations

• NonUnitalNonAssocRing.toHasDistribNeg = HasDistribNeg.mk ⋯ ⋯

theorem mul_sub_left_distrib {α : Type u} [NonUnitalNonAssocRing α] (a : α) (b : α) (c : α) : a * (b - c) = a * b - a * c

theorem mul_sub {α : Type u} [NonUnitalNonAssocRing α] (a : α) (b : α) (c : α) : a * (b - c) = a * b - a * c

Alias of mul_sub_left_distrib.

theorem mul_sub_right_distrib {α : Type u} [NonUnitalNonAssocRing α] (a : α) (b : α) (c : α) : (a - b) * c = a * c - b * c

theorem sub_mul {α : Type u} [NonUnitalNonAssocRing α] (a : α) (b : α) (c : α) : (a - b) * c = a * c - b * c

Alias of mul_sub_right_distrib.

theorem sub_one_mul {α : Type u} [NonAssocRing α] (a : α) (b : α) : (a - 1) * b = a * b - b

theorem mul_sub_one {α : Type u} [NonAssocRing α] (a : α) (b : α) : a * (b - 1) = a * b - a

theorem one_sub_mul {α : Type u} [NonAssocRing α] (a : α) (b : α) : (1 - a) * b = b - a * b

theorem mul_one_sub {α : Type u} [NonAssocRing α] (a : α) (b : α) : a * (1 - b) = a - a * b

@[instance 100]

instance Ring.toNonUnitalRing {α : Type u} [Ring α] : NonUnitalRing α

Equations

• Ring.toNonUnitalRing = NonUnitalRing.mk ⋯

@[instance 100]

instance Ring.toNonAssocRing {α : Type u} [Ring α] : NonAssocRing α

Equations

• Ring.toNonAssocRing = NonAssocRing.mk ⋯ ⋯ ⋯ ⋯ ⋯ ⋯

@[instance 200]

instance instSemiring {α : Type u} [Ring α] : Semiring α

The instance from Ring to Semiring happens often in linear algebra, for which all the basic definitions are given in terms of semirings, but many applications use rings or fields. We increase a little bit its priority above 100 to try it quickly, but remaining below the default 1000 so that more specific instances are tried first.

Equations

• instSemiring = Semiring.mk ⋯ ⋯ ⋯ ⋯ Semiring.npow ⋯ ⋯

class NonUnitalNonAssocCommRing (α : Type u) extends NonUnitalNonAssocRing , NonUnitalNonAssocCommSemiring , AddCommGroup , NonUnitalNonAssocSemiring , AddGroup , CommMagma , AddCommMonoid , SubNegMonoid , Distrib , MulZeroClass , AddMonoid , Neg , Sub , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

A non-unital non-associative commutative ring is a NonUnitalNonAssocRing with commutative multiplication.

class NonUnitalCommRing (α : Type u) extends NonUnitalRing , NonUnitalNonAssocCommRing , NonUnitalNonAssocRing , NonUnitalSemiring , NonUnitalNonAssocCommSemiring , AddCommGroup , NonUnitalNonAssocSemiring , AddGroup , SemigroupWithZero , CommMagma , AddCommMonoid , SubNegMonoid , Distrib , Semigroup , MulZeroClass , AddMonoid , Neg , Sub , AddCommSemigroup , AddSemigroup , AddZeroClass , Mul , Zero , AddCommMagma , Add : Type u

A non-unital commutative ring is a NonUnitalRing with commutative multiplication.

@[instance 100]

instance NonUnitalCommRing.toNonUnitalCommSemiring {α : Type u} [s : NonUnitalCommRing α] : NonUnitalCommSemiring α

Equations

• NonUnitalCommRing.toNonUnitalCommSemiring = NonUnitalCommSemiring.mk ⋯

class CommRing (α : Type u) extends Ring , CommMonoid , Semiring , AddCommGroup , AddGroupWithOne , NonUnitalSemiring , NonAssocSemiring , MonoidWithZero , NonUnitalNonAssocSemiring , Monoid , MulZeroOneClass , SemigroupWithZero , AddCommMonoidWithOne , CommSemigroup , MulOneClass , IntCast , AddMonoidWithOne , AddGroup , AddCommMonoid , Distrib , Semigroup , MulZeroClass , NatCast , SubNegMonoid , AddMonoid , One , Neg , Sub , AddCommSemigroup , AddSemigroup , AddZeroClass , CommMagma , Mul , Zero , AddCommMagma , Add : Type u

A commutative ring is a ring with commutative multiplication.

@[instance 100]

instance CommRing.toCommSemiring {α : Type u} [s : CommRing α] : CommSemiring α

Equations

• CommRing.toCommSemiring = CommSemiring.mk ⋯

@[instance 100]

instance CommRing.toNonUnitalCommRing {α : Type u} [s : CommRing α] : NonUnitalCommRing α

Equations

• CommRing.toNonUnitalCommRing = NonUnitalCommRing.mk ⋯

@[instance 100]

instance CommRing.toAddCommGroupWithOne {α : Type u} [s : CommRing α] : AddCommGroupWithOne α

Equations

• CommRing.toAddCommGroupWithOne = AddCommGroupWithOne.mk ⋯ ⋯ ⋯ ⋯

class IsDomain (α : Type u) [Semiring α] extends IsCancelMulZero , Nontrivial , IsLeftCancelMulZero , IsRightCancelMulZero : Prop

A domain is a nontrivial semiring such that multiplication by a non zero element is cancellative on both sides. In other words, a nontrivial semiring R satisfying ∀ {a b c : R}, a ≠ 0 → a * b = a * c → b = c and ∀ {a b c : R}, b ≠ 0 → a * b = c * b → a = c.

This is implemented as a mixin for Semiring α. To obtain an integral domain use [CommRing α] [IsDomain α].