refactor(Algebra/Star/*): Allow for star operation on non-associative algebras (#6562)

Typically a * operation on a mathematical structure `R` equipped with a multiplication is an involutive anti-automorphism i.e.
```
∀ r s : R, star (r * s) = star s * star r
```
Currently mathlib defines a class `StarSemigroup` to be a semigroup satisfying this property. However, the requirement for the multiplication to be associative is unnecessarily restrictive. There are important classes of star-algebra which are not associative (e.g. JB*-algebras).

This PR removes the requirement for a  `StarSemigroup` to be a semigroup, merely requiring it to have a multiplication.

I've changed the name from `StarSemigroup` to `StarMul` since it's no longer a semigroup.

[Zulip discussion](https://leanprover.zulipchat.com/#narrow/stream/116395-maths/topic/non-associative.20*-algebras)

Previously opened as a mathlib PR leanprover-community/mathlib3#17949



Co-authored-by: Christopher Hoskin <mans0954@users.noreply.github.com>
Co-authored-by: Eric Wieser <wieser.eric@gmail.com>

Author: mans0954

Mathlib/Algebra/Algebra/Unitization.lean

@@ -578,8 +578,9 @@ instance instStarModule [CommSemiring R] [StarRing R] [AddCommMonoid A] [StarAdd
     [Module R A] [StarModule R A] : StarModule R (Unitization R A) where
   star_smul r x := ext (by simp) (by simp)
 
-instance instStarRing [CommSemiring R] [StarRing R] [NonUnitalSemiring A] [StarRing A] [Module R A]
-    [IsScalarTower R A A] [SMulCommClass R A A] [StarModule R A] : StarRing (Unitization R A) :=
+instance instStarRing [CommSemiring R] [StarRing R] [NonUnitalNonAssocSemiring A] [StarRing A]
+    [Module R A] [StarModule R A] :
+    StarRing (Unitization R A) :=
   { Unitization.instStarAddMonoid with
     star_mul := fun x y =>
       ext (by simp [-star_mul']) (by simp [-star_mul', add_comm (star x.fst • star y.snd)]) }

Mathlib/Algebra/Star/Basic.lean

@@ -25,7 +25,7 @@ For now we simply do not introduce notations,
 as different users are expected to feel strongly about the relative merits of
 `r^*`, `r†`, `rᘁ`, and so on.
 
-Our star rings are actually star semirings, but of course we can prove
+Our star rings are actually star non-unital, non-associative, semirings, but of course we can prove
 `star_neg : star (-r) = - star r` when the underlying semiring is a ring.
 -/
 
@@ -121,21 +121,21 @@ export TrivialStar (star_trivial)
 
 attribute [simp] star_trivial
 
-/-- A `*`-semigroup is a semigroup `R` with an involutive operation `star`
+/-- A `*`-magma is a magma `R` with an involutive operation `star`
 such that `star (r * s) = star s * star r`.
 -/
-class StarSemigroup (R : Type u) [Semigroup R] extends InvolutiveStar R where
+class StarMul (R : Type u) [Mul R] extends InvolutiveStar R where
   /-- `star` skew-distributes over multiplication. -/
   star_mul : ∀ r s : R, star (r * s) = star s * star r
-#align star_semigroup StarSemigroup
+#align star_semigroup StarMul
 
-export StarSemigroup (star_mul)
+export StarMul (star_mul)
 
 attribute [simp 900] star_mul
 
-section StarSemigroup
+section StarMul
 
-variable [Semigroup R] [StarSemigroup R]
+variable [Mul R] [StarMul R]
 
 theorem star_star_mul (x y : R) : star (star x * y) = star y * x := by rw [star_mul, star_star]
 #align star_star_mul star_star_mul
@@ -164,17 +164,17 @@ theorem commute_star_comm {x y : R} : Commute (star x) y ↔ Commute x (star y)
   rw [← commute_star_star, star_star]
 #align commute_star_comm commute_star_comm
 
-end StarSemigroup
+end StarMul
 
 /-- In a commutative ring, make `simp` prefer leaving the order unchanged. -/
 @[simp]
-theorem star_mul' [CommSemigroup R] [StarSemigroup R] (x y : R) : star (x * y) = star x * star y :=
+theorem star_mul' [CommSemigroup R] [StarMul R] (x y : R) : star (x * y) = star x * star y :=
   (star_mul x y).trans (mul_comm _ _)
 #align star_mul' star_mul'
 
 /-- `star` as a `MulEquiv` from `R` to `Rᵐᵒᵖ` -/
 @[simps apply]
-def starMulEquiv [Semigroup R] [StarSemigroup R] : R ≃* Rᵐᵒᵖ :=
+def starMulEquiv [Mul R] [StarMul R] : R ≃* Rᵐᵒᵖ :=
   { (InvolutiveStar.star_involutive.toPerm star).trans opEquiv with
     toFun := fun x => MulOpposite.op (star x)
     map_mul' := fun x y => by simp only [star_mul, op_mul] }
@@ -183,7 +183,7 @@ def starMulEquiv [Semigroup R] [StarSemigroup R] : R ≃* Rᵐᵒᵖ :=
 
 /-- `star` as a `MulAut` for commutative `R`. -/
 @[simps apply]
-def starMulAut [CommSemigroup R] [StarSemigroup R] : MulAut R :=
+def starMulAut [CommSemigroup R] [StarMul R] : MulAut R :=
   { InvolutiveStar.star_involutive.toPerm star with
     toFun := star
     map_mul' := star_mul' }
@@ -193,32 +193,32 @@ def starMulAut [CommSemigroup R] [StarSemigroup R] : MulAut R :=
 variable (R)
 
 @[simp]
-theorem star_one [Monoid R] [StarSemigroup R] : star (1 : R) = 1 :=
+theorem star_one [MulOneClass R] [StarMul R] : star (1 : R) = 1 :=
   op_injective <| (starMulEquiv : R ≃* Rᵐᵒᵖ).map_one.trans (op_one _).symm
 #align star_one star_one
 
 variable {R}
 
 @[simp]
-theorem star_pow [Monoid R] [StarSemigroup R] (x : R) (n : ℕ) : star (x ^ n) = star x ^ n :=
+theorem star_pow [Monoid R] [StarMul R] (x : R) (n : ℕ) : star (x ^ n) = star x ^ n :=
   op_injective <|
     ((starMulEquiv : R ≃* Rᵐᵒᵖ).toMonoidHom.map_pow x n).trans (op_pow (star x) n).symm
 #align star_pow star_pow
 
 @[simp]
-theorem star_inv [Group R] [StarSemigroup R] (x : R) : star x⁻¹ = (star x)⁻¹ :=
+theorem star_inv [Group R] [StarMul R] (x : R) : star x⁻¹ = (star x)⁻¹ :=
   op_injective <| ((starMulEquiv : R ≃* Rᵐᵒᵖ).toMonoidHom.map_inv x).trans (op_inv (star x)).symm
 #align star_inv star_inv
 
 @[simp]
-theorem star_zpow [Group R] [StarSemigroup R] (x : R) (z : ℤ) : star (x ^ z) = star x ^ z :=
+theorem star_zpow [Group R] [StarMul R] (x : R) (z : ℤ) : star (x ^ z) = star x ^ z :=
   op_injective <|
     ((starMulEquiv : R ≃* Rᵐᵒᵖ).toMonoidHom.map_zpow x z).trans (op_zpow (star x) z).symm
 #align star_zpow star_zpow
 
 /-- When multiplication is commutative, `star` preserves division. -/
 @[simp]
-theorem star_div [CommGroup R] [StarSemigroup R] (x y : R) : star (x / y) = star x / star y :=
+theorem star_div [CommGroup R] [StarMul R] (x y : R) : star (x / y) = star x / star y :=
   map_div (starMulAut : R ≃* R) _ _
 #align star_div star_div
 
@@ -227,17 +227,17 @@ theorem star_div [CommGroup R] [StarSemigroup R] (x y : R) : star (x / y) = star
 See note [reducible non-instances].
 -/
 @[reducible]
-def starSemigroupOfComm {R : Type*} [CommMonoid R] : StarSemigroup R where
+def starMulOfComm {R : Type*} [CommMonoid R] : StarMul R where
   star := id
   star_involutive _ := rfl
   star_mul := mul_comm
-#align star_semigroup_of_comm starSemigroupOfComm
+#align star_semigroup_of_comm starMulOfComm
 
 section
 
-attribute [local instance] starSemigroupOfComm
+attribute [local instance] starMulOfComm
 
-/-- Note that since `starSemigroupOfComm` is reducible, `simp` can already prove this. -/
+/-- Note that since `starMulOfComm` is reducible, `simp` can already prove this. -/
 theorem star_id_of_comm {R : Type*} [CommSemiring R] {x : R} : star x = x :=
   rfl
 #align star_id_of_comm star_id_of_comm
@@ -302,35 +302,35 @@ theorem star_zsmul [AddGroup R] [StarAddMonoid R] (x : R) (n : ℤ) : star (n 
   (starAddEquiv : R ≃+ R).toAddMonoidHom.map_zsmul _ _
 #align star_zsmul star_zsmul
 
-/-- A `*`-ring `R` is a (semi)ring with an involutive `star` operation which is additive
-which makes `R` with its multiplicative structure into a `*`-semigroup
+/-- A `*`-ring `R` is a non-unital, non-associative (semi)ring with an involutive `star` operation
+which is additive which makes `R` with its multiplicative structure into a `*`-multiplication
 (i.e. `star (r * s) = star s * star r`).  -/
-class StarRing (R : Type u) [NonUnitalSemiring R] extends StarSemigroup R where
+class StarRing (R : Type u) [NonUnitalNonAssocSemiring R] extends StarMul R where
   /-- `star` commutes with addition -/
   star_add : ∀ r s : R, star (r + s) = star r + star s
 #align star_ring StarRing
 
-instance (priority := 100) StarRing.toStarAddMonoid [NonUnitalSemiring R] [StarRing R] :
+instance (priority := 100) StarRing.toStarAddMonoid [NonUnitalNonAssocSemiring R] [StarRing R] :
     StarAddMonoid R where
   star_add := StarRing.star_add
 #align star_ring.to_star_add_monoid StarRing.toStarAddMonoid
 
 /-- `star` as a `RingEquiv` from `R` to `Rᵐᵒᵖ` -/
 @[simps apply]
-def starRingEquiv [NonUnitalSemiring R] [StarRing R] : R ≃+* Rᵐᵒᵖ :=
+def starRingEquiv [NonUnitalNonAssocSemiring R] [StarRing R] : R ≃+* Rᵐᵒᵖ :=
   { starAddEquiv.trans (MulOpposite.opAddEquiv : R ≃+ Rᵐᵒᵖ), starMulEquiv with
     toFun := fun x => MulOpposite.op (star x) }
 #align star_ring_equiv starRingEquiv
 #align star_ring_equiv_apply starRingEquiv_apply
 
 @[simp, norm_cast]
-theorem star_natCast [Semiring R] [StarRing R] (n : ℕ) : star (n : R) = n :=
+theorem star_natCast [NonAssocSemiring R] [StarRing R] (n : ℕ) : star (n : R) = n :=
   (congr_arg unop (map_natCast (starRingEquiv : R ≃+* Rᵐᵒᵖ) n)).trans (unop_natCast _)
 #align star_nat_cast star_natCast
 
 --Porting note: new theorem
 @[simp]
-theorem star_ofNat [Semiring R] [StarRing R] (n : ℕ) [n.AtLeastTwo] :
+theorem star_ofNat [NonAssocSemiring R] [StarRing R] (n : ℕ) [n.AtLeastTwo] :
     star (no_index (OfNat.ofNat n) : R) = OfNat.ofNat n :=
   star_natCast _
 
@@ -459,7 +459,7 @@ See note [reducible non-instances].
 -/
 @[reducible]
 def starRingOfComm {R : Type*} [CommSemiring R] : StarRing R :=
-  { starSemigroupOfComm with
+  { starMulOfComm with
     star := id
     star_add := fun _ _ => rfl }
 #align star_ring_of_comm starRingOfComm
@@ -486,7 +486,7 @@ export StarModule (star_smul)
 attribute [simp] star_smul
 
 /-- A commutative star monoid is a star module over itself via `Monoid.toMulAction`. -/
-instance StarSemigroup.to_starModule [CommMonoid R] [StarSemigroup R] : StarModule R R :=
+instance StarSemigroup.to_starModule [CommMonoid R] [StarMul R] : StarModule R R :=
   ⟨star_mul'⟩
 #align star_semigroup.to_star_module StarSemigroup.to_starModule
 
@@ -517,9 +517,9 @@ end
 
 namespace Units
 
-variable [Monoid R] [StarSemigroup R]
+variable [Monoid R] [StarMul R]
 
-instance : StarSemigroup Rˣ where
+instance : StarMul Rˣ where
   star u :=
     { val := star u
       inv := star ↑u⁻¹
@@ -543,12 +543,12 @@ instance {A : Type*} [Star A] [SMul R A] [StarModule R A] : StarModule Rˣ A :=
 
 end Units
 
-theorem IsUnit.star [Monoid R] [StarSemigroup R] {a : R} : IsUnit a → IsUnit (star a)
+theorem IsUnit.star [Monoid R] [StarMul R] {a : R} : IsUnit a → IsUnit (star a)
   | ⟨u, hu⟩ => ⟨Star.star u, hu ▸ rfl⟩
 #align is_unit.star IsUnit.star
 
 @[simp]
-theorem isUnit_star [Monoid R] [StarSemigroup R] {a : R} : IsUnit (star a) ↔ IsUnit a :=
+theorem isUnit_star [Monoid R] [StarMul R] {a : R} : IsUnit (star a) ↔ IsUnit a :=
   ⟨fun h => star_star a ▸ h.star, IsUnit.star⟩
 #align is_unit_star isUnit_star
 
@@ -560,14 +560,14 @@ theorem Ring.inverse_star [Semiring R] [StarRing R] (a : R) :
   rw [Ring.inverse_non_unit _ ha, Ring.inverse_non_unit _ (mt isUnit_star.mp ha), star_zero]
 #align ring.inverse_star Ring.inverse_star
 
-instance Invertible.star {R : Type*} [Monoid R] [StarSemigroup R] (r : R) [Invertible r] :
+instance Invertible.star {R : Type*} [MulOneClass R] [StarMul R] (r : R) [Invertible r] :
     Invertible (star r) where
   invOf := Star.star (⅟ r)
   invOf_mul_self := by rw [← star_mul, mul_invOf_self, star_one]
   mul_invOf_self := by rw [← star_mul, invOf_mul_self, star_one]
 #align invertible.star Invertible.star
 
-theorem star_invOf {R : Type*} [Monoid R] [StarSemigroup R] (r : R) [Invertible r]
+theorem star_invOf {R : Type*} [Monoid R] [StarMul R] (r : R) [Invertible r]
     [Invertible (star r)] : star (⅟ r) = ⅟ (star r) := by
   have : star (⅟ r) = star (⅟ r) * ((star r) * ⅟ (star r)) := by
     simp only [mul_invOf_self, mul_one]
@@ -594,7 +594,7 @@ theorem op_star [Star R] (r : R) : op (star r) = star (op r) :=
 instance [InvolutiveStar R] : InvolutiveStar Rᵐᵒᵖ where
   star_involutive r := unop_injective (star_star r.unop)
 
-instance [Monoid R] [StarSemigroup R] : StarSemigroup Rᵐᵒᵖ where
+instance [Mul R] [StarMul R] : StarMul Rᵐᵒᵖ where
   star_mul x y := unop_injective (star_mul y.unop x.unop)
 
 instance [AddMonoid R] [StarAddMonoid R] : StarAddMonoid Rᵐᵒᵖ where
@@ -607,7 +607,7 @@ end MulOpposite
 
 /-- A commutative star monoid is a star module over its opposite via
 `Monoid.toOppositeMulAction`. -/
-instance StarSemigroup.toOpposite_starModule [CommMonoid R] [StarSemigroup R] :
+instance StarSemigroup.toOpposite_starModule [CommMonoid R] [StarMul R] :
     StarModule Rᵐᵒᵖ R :=
   ⟨fun r s => star_mul' s r.unop⟩
 #align star_semigroup.to_opposite_star_module StarSemigroup.toOpposite_starModule

Mathlib/Algebra/Star/BigOperators.lean

@@ -19,7 +19,7 @@ variable {R : Type*}
 open BigOperators
 
 @[simp]
-theorem star_prod [CommMonoid R] [StarSemigroup R] {α : Type*} (s : Finset α) (f : α → R) :
+theorem star_prod [CommMonoid R] [StarMul R] {α : Type*} (s : Finset α) (f : α → R) :
     star (∏ x in s, f x) = ∏ x in s, star (f x) := map_prod (starMulAut : R ≃* R) _ _
 #align star_prod star_prod
 

Mathlib/Algebra/Star/CHSH.lean

@@ -82,7 +82,7 @@ The physical interpretation is that `A₀` and `A₁` are a pair of boolean obse
 are spacelike separated from another pair `B₀` and `B₁` of boolean observables.
 -/
 --@[nolint has_nonempty_instance] Porting note: linter does not exist
-structure IsCHSHTuple {R} [Monoid R] [StarSemigroup R] (A₀ A₁ B₀ B₁ : R) : Prop where
+structure IsCHSHTuple {R} [Monoid R] [StarMul R] (A₀ A₁ B₀ B₁ : R) : Prop where
   A₀_inv : A₀ ^ 2 = 1
   A₁_inv : A₁ ^ 2 = 1
   B₀_inv : B₀ ^ 2 = 1

Mathlib/Algebra/Star/Center.lean

@@ -10,7 +10,7 @@ import Mathlib.Algebra.Star.Pointwise
 
 /-! # `Set.center`, `Set.centralizer` and the `star` operation -/
 
-variable {R : Type*} [Semigroup R] [StarSemigroup R] {a : R} {s : Set R}
+variable {R : Type*} [Mul R] [StarMul R] {a : R} {s : Set R}
 
 theorem Set.star_mem_center (ha : a ∈ Set.center R) : star a ∈ Set.center R := by
   simpa only [star_mul, star_star] using fun g =>

Mathlib/Algebra/Star/Free.lean

@@ -23,7 +23,7 @@ namespace FreeMonoid
 
 variable {α : Type*}
 
-instance : StarSemigroup (FreeMonoid α) where
+instance : StarMul (FreeMonoid α) where
   star := List.reverse
   star_involutive := List.reverse_reverse
   star_mul := List.reverse_append

Mathlib/Algebra/Star/Module.lean

@@ -82,7 +82,7 @@ def starLinearEquiv (R : Type*) {A : Type*} [CommSemiring R] [StarRing R] [AddCo
     map_smul' := star_smul }
 #align star_linear_equiv starLinearEquiv
 
-variable (R : Type*) (A : Type*) [Semiring R] [StarSemigroup R] [TrivialStar R] [AddCommGroup A]
+variable (R : Type*) (A : Type*) [Semiring R] [StarMul R] [TrivialStar R] [AddCommGroup A]
   [Module R A] [StarAddMonoid A] [StarModule R A]
 
 /-- The self-adjoint elements of a star module, as a submodule. -/
@@ -184,7 +184,7 @@ def StarModule.decomposeProdAdjoint : A ≃ₗ[R] selfAdjoint A × skewAdjoint A
 
 @[simp]
 theorem algebraMap_star_comm {R A : Type*} [CommSemiring R] [StarRing R] [Semiring A]
-    [StarSemigroup A] [Algebra R A] [StarModule R A] (r : R) :
+    [StarMul A] [Algebra R A] [StarModule R A] (r : R) :
     algebraMap R A (star r) = star (algebraMap R A r) := by
   simp only [Algebra.algebraMap_eq_smul_one, star_smul, star_one]
 #align algebra_map_star_comm algebraMap_star_comm

Mathlib/Algebra/Star/NonUnitalSubalgebra.lean

@@ -26,10 +26,11 @@ instance instInvolutiveStar {S R : Type*} [InvolutiveStar R] [SetLike S R] [Star
     (s : S) : InvolutiveStar s where
   star_involutive r := Subtype.ext <| star_star (r : R)
 
-/-- In a star semigroup (i.e., a semigroup with an antimultiplicative involutive star operation),
-any star-closed subset which is also closed under multiplication is itself a star semigroup. -/
-instance instStarSemigroup {S R : Type*} [Semigroup R] [StarSemigroup R] [SetLike S R]
-    [MulMemClass S R] [StarMemClass S R] (s : S) : StarSemigroup s where
+/-- In a star magma (i.e., a multiplication with an antimultiplicative involutive star
+operation), any star-closed subset which is also closed under multiplication is itself a star
+magma. -/
+instance instStarMul {S R : Type*} [Mul R] [StarMul R] [SetLike S R]
+    [MulMemClass S R] [StarMemClass S R] (s : S) : StarMul s where
   star_mul _ _ := Subtype.ext <| star_mul _ _
 
 /-- In a `StarAddMonoid` (i.e., an additive monoid with an additive involutive star operation), any
@@ -39,11 +40,12 @@ instance instStarAddMonoid {S R : Type*} [AddMonoid R] [StarAddMonoid R] [SetLik
     [AddSubmonoidClass S R] [StarMemClass S R] (s : S) : StarAddMonoid s where
   star_add _ _ := Subtype.ext <| star_add _ _
 
-/-- In a star ring (i.e., a non-unital semiring with an additive, antimultiplicative, involutive
-star operation), an star-closed non-unital subsemiring is itself a star ring. -/
-instance instStarRing {S R : Type*} [NonUnitalSemiring R] [StarRing R] [SetLike S R]
+/-- In a star ring (i.e., a non-unital, non-associative, semiring with an additive,
+antimultiplicative, involutive star operation), a star-closed non-unital subsemiring is itself a
+star ring. -/
+instance instStarRing {S R : Type*} [NonUnitalNonAssocSemiring R] [StarRing R] [SetLike S R]
     [NonUnitalSubsemiringClass S R] [StarMemClass S R] (s : S) : StarRing s :=
-  { StarMemClass.instStarSemigroup s, StarMemClass.instStarAddMonoid s with }
+  { StarMemClass.instStarMul s, StarMemClass.instStarAddMonoid s with }
 
 /-- In a star `R`-module (i.e., `star (r • m) = (star r) • m`) any star-closed subset which is also
 closed under the scalar action by `R` is itself a star `R`-module. -/

Mathlib/Algebra/Star/Pi.lean

@@ -43,7 +43,7 @@ instance [∀ i, Star (f i)] [∀ i, TrivialStar (f i)] : TrivialStar (∀ i, f
 instance [∀ i, InvolutiveStar (f i)] : InvolutiveStar (∀ i, f i)
     where star_involutive _ := funext fun _ => star_star _
 
-instance [∀ i, Semigroup (f i)] [∀ i, StarSemigroup (f i)] : StarSemigroup (∀ i, f i)
+instance [∀ i, Mul (f i)] [∀ i, StarMul (f i)] : StarMul (∀ i, f i)
     where star_mul _ _ := funext fun _ => star_mul _ _
 
 instance [∀ i, AddMonoid (f i)] [∀ i, StarAddMonoid (f i)] : StarAddMonoid (∀ i, f i)

Mathlib/Algebra/Star/Pointwise.lean

@@ -117,7 +117,7 @@ theorem star_singleton {β : Type*} [InvolutiveStar β] (x : β) : ({x} : Set β
   rw [mem_star, mem_singleton_iff, mem_singleton_iff, star_eq_iff_star_eq, eq_comm]
 #align set.star_singleton Set.star_singleton
 
-protected theorem star_mul [Monoid α] [StarSemigroup α] (s t : Set α) : (s * t)⋆ = t⋆ * s⋆ := by
+protected theorem star_mul [Mul α] [StarMul α] (s t : Set α) : (s * t)⋆ = t⋆ * s⋆ := by
  simp_rw [← image_star, ← image2_mul, image_image2, image2_image_left, image2_image_right,
    star_mul, image2_swap _ s t]
 #align set.star_mul Set.star_mul
@@ -134,7 +134,7 @@ instance [Star α] [TrivialStar α] : TrivialStar (Set α) where
     ext1
     simp [star_trivial]
 
-protected theorem star_inv [Group α] [StarSemigroup α] (s : Set α) : s⁻¹⋆ = s⋆⁻¹ := by
+protected theorem star_inv [Group α] [StarMul α] (s : Set α) : s⁻¹⋆ = s⋆⁻¹ := by
   ext
   simp only [mem_star, mem_inv, star_inv]
 #align set.star_inv Set.star_inv