mathlib documentation

algebra.opposites

Algebraic operations on αᵒᵖ #

This file records several basic facts about the opposite of an algebraic structure, e.g. the opposite of a ring is a ring (with multiplication x * y = yx). Use is made of the identity functions op : α → αᵒᵖ and unop : αᵒᵖ → α.

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@[instance]
def opposite.has_zero (α : Type u) [has_zero α] :
has_zero αᵒᵖ
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@[instance]
def opposite.has_one (α : Type u) [has_one α] :
has_one αᵒᵖ
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@[instance]
def opposite.has_add (α : Type u) [has_add α] :
has_add αᵒᵖ
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@[instance]
def opposite.has_sub (α : Type u) [has_sub α] :
has_sub αᵒᵖ
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@[instance]
def opposite.has_neg (α : Type u) [has_neg α] :
has_neg αᵒᵖ
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@[instance]
def opposite.has_mul (α : Type u) [has_mul α] :
has_mul αᵒᵖ
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@[instance]
def opposite.has_inv (α : Type u) [has_inv α] :
has_inv αᵒᵖ
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@[instance]
def opposite.has_scalar (α : Type u) (R : Type u_1) [has_scalar R α] :
has_scalar R αᵒᵖ
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@[simp]
theorem opposite.op_zero (α : Type u) [has_zero α] :
opposite.op 0 = 0
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@[simp]
theorem opposite.unop_zero (α : Type u) [has_zero α] :
opposite.unop 0 = 0
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@[simp]
theorem opposite.op_one (α : Type u) [has_one α] :
opposite.op 1 = 1
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@[simp]
theorem opposite.unop_one (α : Type u) [has_one α] :
opposite.unop 1 = 1
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@[simp]
theorem opposite.op_add {α : Type u} [has_add α] (x y : α) :
opposite.op (x + y) = opposite.op x + opposite.op y
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@[simp]
theorem opposite.unop_add {α : Type u} [has_add α] (x y : αᵒᵖ) :
opposite.unop (x + y) = opposite.unop x + opposite.unop y
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@[simp]
theorem opposite.op_neg {α : Type u} [has_neg α] (x : α) :
opposite.op (-x) = -opposite.op x
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@[simp]
theorem opposite.unop_neg {α : Type u} [has_neg α] (x : αᵒᵖ) :
opposite.unop (-x) = -opposite.unop x
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@[simp]
theorem opposite.op_mul {α : Type u} [has_mul α] (x y : α) :
opposite.op (x * y) = (opposite.op y) * opposite.op x
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@[simp]
theorem opposite.unop_mul {α : Type u} [has_mul α] (x y : αᵒᵖ) :
opposite.unop (x * y) = (opposite.unop y) * opposite.unop x
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@[simp]
theorem opposite.op_inv {α : Type u} [has_inv α] (x : α) :
opposite.op x⁻¹ = (opposite.op x)⁻¹
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@[simp]
theorem opposite.unop_inv {α : Type u} [has_inv α] (x : αᵒᵖ) :
opposite.unop x⁻¹ = (opposite.unop x)⁻¹
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@[simp]
theorem opposite.op_sub {α : Type u} [add_group α] (x y : α) :
opposite.op (x - y) = opposite.op x - opposite.op y
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@[simp]
theorem opposite.unop_sub {α : Type u} [add_group α] (x y : αᵒᵖ) :
opposite.unop (x - y) = opposite.unop x - opposite.unop y
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@[simp]
theorem opposite.op_smul {α : Type u} {R : Type u_1} [has_scalar R α] (c : R) (a : α) :
opposite.op (c • a) = c • opposite.op a
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@[simp]
theorem opposite.unop_smul {α : Type u} {R : Type u_1} [has_scalar R α] (c : R) (a : αᵒᵖ) :
opposite.unop (c • a) = c • opposite.unop a
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@[instance]
def opposite.add_semigroup (α : Type u) [add_semigroup α] :
add_semigroup αᵒᵖ
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@[instance]
def opposite.add_left_cancel_semigroup (α : Type u) [add_left_cancel_semigroup α] :
add_left_cancel_semigroup αᵒᵖ
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@[instance]
def opposite.add_right_cancel_semigroup (α : Type u) [add_right_cancel_semigroup α] :
add_right_cancel_semigroup αᵒᵖ
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@[instance]
def opposite.add_comm_semigroup (α : Type u) [add_comm_semigroup α] :
add_comm_semigroup αᵒᵖ
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@[instance]
def opposite.nontrivial (α : Type u) [nontrivial α] :
nontrivial αᵒᵖ
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@[simp]
theorem opposite.unop_eq_zero_iff {α : Type u_1} [has_zero α] (a : αᵒᵖ) :
opposite.unop a = 0 ↔ a = 0
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@[simp]
theorem opposite.op_eq_zero_iff {α : Type u_1} [has_zero α] (a : α) :
opposite.op a = 0 ↔ a = 0
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theorem opposite.unop_ne_zero_iff {α : Type u_1} [has_zero α] (a : αᵒᵖ) :
opposite.unop a ≠ 0 ↔ a ≠ 0
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theorem opposite.op_ne_zero_iff {α : Type u_1} [has_zero α] (a : α) :
opposite.op a ≠ 0 ↔ a ≠ 0
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@[instance]
def opposite.add_zero_class (α : Type u) [add_zero_class α] :
add_zero_class αᵒᵖ
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@[instance]
def opposite.add_monoid (α : Type u) [add_monoid α] :
add_monoid αᵒᵖ
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@[instance]
def opposite.add_comm_monoid (α : Type u) [add_comm_monoid α] :
add_comm_monoid αᵒᵖ
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@[instance]
def opposite.add_group (α : Type u) [add_group α] :
add_group αᵒᵖ
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@[instance]
def opposite.add_comm_group (α : Type u) [add_comm_group α] :
add_comm_group αᵒᵖ
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@[instance]
def opposite.semigroup (α : Type u) [semigroup α] :
semigroup αᵒᵖ
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@[instance]
def opposite.left_cancel_semigroup (α : Type u) [right_cancel_semigroup α] :
left_cancel_semigroup αᵒᵖ
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@[instance]
def opposite.right_cancel_semigroup (α : Type u) [left_cancel_semigroup α] :
right_cancel_semigroup αᵒᵖ
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@[instance]
def opposite.comm_semigroup (α : Type u) [comm_semigroup α] :
comm_semigroup αᵒᵖ
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@[simp]
theorem opposite.unop_eq_one_iff {α : Type u_1} [has_one α] (a : αᵒᵖ) :
opposite.unop a = 1 ↔ a = 1
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@[simp]
theorem opposite.op_eq_one_iff {α : Type u_1} [has_one α] (a : α) :
opposite.op a = 1 ↔ a = 1
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@[instance]
def opposite.mul_one_class (α : Type u) [mul_one_class α] :
mul_one_class αᵒᵖ
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@[instance]
def opposite.monoid (α : Type u) [monoid α] :
monoid αᵒᵖ
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@[instance]
def opposite.left_cancel_monoid (α : Type u) [right_cancel_monoid α] :
left_cancel_monoid αᵒᵖ
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@[instance]
def opposite.right_cancel_monoid (α : Type u) [left_cancel_monoid α] :
right_cancel_monoid αᵒᵖ
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@[instance]
def opposite.cancel_monoid (α : Type u) [cancel_monoid α] :
cancel_monoid αᵒᵖ
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@[instance]
def opposite.comm_monoid (α : Type u) [comm_monoid α] :
comm_monoid αᵒᵖ
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@[instance]
def opposite.cancel_comm_monoid (α : Type u) [cancel_comm_monoid α] :
cancel_comm_monoid αᵒᵖ
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@[instance]
def opposite.div_inv_monoid (α : Type u) [div_inv_monoid α] :
div_inv_monoid αᵒᵖ
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@[instance]
def opposite.group (α : Type u) [group α] :
group αᵒᵖ
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@[instance]
def opposite.comm_group (α : Type u) [comm_group α] :
comm_group αᵒᵖ
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@[instance]
def opposite.distrib (α : Type u) [distrib α] :
distrib αᵒᵖ
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@[instance]
def opposite.mul_zero_class (α : Type u) [mul_zero_class α] :
mul_zero_class αᵒᵖ
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@[instance]
def opposite.mul_zero_one_class (α : Type u) [mul_zero_one_class α] :
mul_zero_one_class αᵒᵖ
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@[instance]
def opposite.semigroup_with_zero (α : Type u) [semigroup_with_zero α] :
semigroup_with_zero αᵒᵖ
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@[instance]
def opposite.monoid_with_zero (α : Type u) [monoid_with_zero α] :
monoid_with_zero αᵒᵖ
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@[instance]
def opposite.non_unital_non_assoc_semiring (α : Type u) [non_unital_non_assoc_semiring α] :
non_unital_non_assoc_semiring αᵒᵖ
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@[instance]
def opposite.non_unital_semiring (α : Type u) [non_unital_semiring α] :
non_unital_semiring αᵒᵖ
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@[instance]
def opposite.non_assoc_semiring (α : Type u) [non_assoc_semiring α] :
non_assoc_semiring αᵒᵖ
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@[instance]
def opposite.semiring (α : Type u) [semiring α] :
semiring αᵒᵖ
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@[instance]
def opposite.comm_semiring (α : Type u) [comm_semiring α] :
comm_semiring αᵒᵖ
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@[instance]
def opposite.ring (α : Type u) [ring α] :
ring αᵒᵖ
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@[instance]
def opposite.comm_ring (α : Type u) [comm_ring α] :
comm_ring αᵒᵖ
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@[instance]
def opposite.no_zero_divisors (α : Type u) [has_zero α] [has_mul α] [no_zero_divisors α] :
no_zero_divisors αᵒᵖ
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@[instance]
def opposite.integral_domain (α : Type u) [integral_domain α] :
integral_domain αᵒᵖ
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@[instance]
def opposite.group_with_zero (α : Type u) [group_with_zero α] :
group_with_zero αᵒᵖ
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@[instance]
def opposite.division_ring (α : Type u) [division_ring α] :
division_ring αᵒᵖ
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@[instance]
def opposite.field (α : Type u) [field α] :
field αᵒᵖ
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@[instance]
def opposite.mul_action (α : Type u) (R : Type u_1) [monoid R] [mul_action R α] :
mul_action R αᵒᵖ
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@[instance]
def opposite.distrib_mul_action (α : Type u) (R : Type u_1) [monoid R] [add_monoid α] [distrib_mul_action R α] :
distrib_mul_action R αᵒᵖ
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@[instance]
def opposite.mul_distrib_mul_action (α : Type u) (R : Type u_1) [monoid R] [monoid α] [mul_distrib_mul_action R α] :
mul_distrib_mul_action R αᵒᵖ
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@[instance]
def opposite.is_scalar_tower (α : Type u) {M : Type u_1} {N : Type u_2} [has_scalar M N] [has_scalar M α] [has_scalar N α] [is_scalar_tower M N α] :
is_scalar_tower M N αᵒᵖ
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@[instance]
def opposite.smul_comm_class (α : Type u) {M : Type u_1} {N : Type u_2} [has_scalar M α] [has_scalar N α] [smul_comm_class M N α] :
smul_comm_class M N αᵒᵖ
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@[instance]
def has_mul.to_has_opposite_scalar (α : Type u) [has_mul α] :
has_scalar αᵒᵖ α

Like has_mul.to_has_scalar, but multiplies on the right.

See also monoid.to_opposite_mul_action and monoid_with_zero.to_opposite_mul_action.

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@[simp]
theorem opposite.op_smul_eq_mul (α : Type u) [has_mul α] {a a' : α} :
opposite.op a • a' = a' * a
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@[instance]
def semigroup.opposite_smul_comm_class (α : Type u) [semigroup α] :
smul_comm_class αᵒᵖ α α
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@[instance]
def semigroup.opposite_smul_comm_class' (α : Type u) [semigroup α] :
smul_comm_class α αᵒᵖ α
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@[instance]
def monoid.to_opposite_mul_action (α : Type u) [monoid α] :
mul_action αᵒᵖ α

Like monoid.to_mul_action, but multiplies on the right.

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@[instance]
def is_scalar_tower.opposite_mid {M : Type u_1} {N : Type u_2} [monoid N] [has_scalar M N] [smul_comm_class M N N] :
is_scalar_tower M Nᵒᵖ N
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@[instance]
def smul_comm_class.opposite_mid {M : Type u_1} {N : Type u_2} [monoid N] [has_scalar M N] [is_scalar_tower M N N] :
smul_comm_class M Nᵒᵖ N
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@[instance]
def left_cancel_monoid.to_has_faithful_opposite_scalar (α : Type u) [left_cancel_monoid α] :
has_faithful_scalar αᵒᵖ α

monoid.to_opposite_mul_action is faithful on cancellative monoids.

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@[instance]
def cancel_monoid_with_zero.to_has_faithful_opposite_scalar (α : Type u) [cancel_monoid_with_zero α] [nontrivial α] :
has_faithful_scalar αᵒᵖ α

monoid.to_opposite_mul_action is faithful on nontrivial cancellative monoids with zero.

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theorem opposite.semiconj_by.op {α : Type u} [has_mul α] {a x y : α} (h : semiconj_by a x y) :
semiconj_by (opposite.op a) (opposite.op y) (opposite.op x)
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theorem opposite.semiconj_by.unop {α : Type u} [has_mul α] {a x y : αᵒᵖ} (h : semiconj_by a x y) :
semiconj_by (opposite.unop a) (opposite.unop y) (opposite.unop x)
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@[simp]
theorem opposite.semiconj_by_op {α : Type u} [has_mul α] {a x y : α} :
semiconj_by (opposite.op a) (opposite.op y) (opposite.op x) ↔ semiconj_by a x y
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@[simp]
theorem opposite.semiconj_by_unop {α : Type u} [has_mul α] {a x y : αᵒᵖ} :
semiconj_by (opposite.unop a) (opposite.unop y) (opposite.unop x) ↔ semiconj_by a x y
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theorem opposite.commute.op {α : Type u} [has_mul α] {x y : α} (h : commute x y) :
commute (opposite.op x) (opposite.op y)
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theorem opposite.commute.unop {α : Type u} [has_mul α] {x y : αᵒᵖ} (h : commute x y) :
commute (opposite.unop x) (opposite.unop y)
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@[simp]
theorem opposite.commute_op {α : Type u} [has_mul α] {x y : α} :
commute (opposite.op x) (opposite.op y) ↔ commute x y
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@[simp]
theorem opposite.commute_unop {α : Type u} [has_mul α] {x y : αᵒᵖ} :
commute (opposite.unop x) (opposite.unop y) ↔ commute x y
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def opposite.op_add_equiv {α : Type u} [has_add α] :
α ≃+ αᵒᵖ

The function op is an additive equivalence.

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@[simp]
theorem opposite.coe_op_add_equiv {α : Type u} [has_add α] :
⇑opposite.op_add_equiv = opposite.op
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@[simp]
theorem opposite.coe_op_add_equiv_symm {α : Type u} [has_add α] :
⇑(opposite.op_add_equiv.symm) = opposite.unop
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@[simp]
theorem opposite.op_add_equiv_to_equiv {α : Type u} [has_add α] :
opposite.op_add_equiv.to_equiv = opposite.equiv_to_opposite
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@[simp]
theorem mul_equiv.inv'_symm_apply (G : Type u_1) [group G] :
⇑((mul_equiv.inv' G).symm) = has_inv.inv ∘ opposite.unop
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@[simp]
theorem mul_equiv.inv'_apply (G : Type u_1) [group G] :
⇑(mul_equiv.inv' G) = opposite.op ∘ has_inv.inv
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def mul_equiv.inv' (G : Type u_1) [group G] :
G ≃* Gᵒᵖ

Inversion on a group is a mul_equiv to the opposite group. When G is commutative, there is mul_equiv.inv.

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@[simp]
theorem monoid_hom.to_opposite_apply {R : Type u_1} {S : Type u_2} [mul_one_class R] [mul_one_class S] (f : R →* S) (hf : ∀ (x y : R), commute (⇑f x) (⇑f y)) :
⇑(f.to_opposite hf) = opposite.op ∘ ⇑f
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def monoid_hom.to_opposite {R : Type u_1} {S : Type u_2} [mul_one_class R] [mul_one_class S] (f : R →* S) (hf : ∀ (x y : R), commute (⇑f x) (⇑f y)) :
R →* Sᵒᵖ

A monoid homomorphism f : R →* S such that f x commutes with f y for all x, y defines a monoid homomorphism to Sᵒᵖ.

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def ring_hom.to_opposite {R : Type u_1} {S : Type u_2} [semiring R] [semiring S] (f : R →+* S) (hf : ∀ (x y : R), commute (⇑f x) (⇑f y)) :
R →+* Sᵒᵖ

A ring homomorphism f : R →+* S such that f x commutes with f y for all x, y defines a ring homomorphism to Sᵒᵖ.

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@[simp]
theorem ring_hom.to_opposite_apply {R : Type u_1} {S : Type u_2} [semiring R] [semiring S] (f : R →+* S) (hf : ∀ (x y : R), commute (⇑f x) (⇑f y)) :
⇑(f.to_opposite hf) = opposite.op ∘ ⇑f
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def units.op_equiv {R : Type u_1} [monoid R] :
units Rᵒᵖ ≃* (units R)ᵒᵖ

The units of the opposites are equivalent to the opposites of the units.

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@[simp]
theorem units.coe_unop_op_equiv {R : Type u_1} [monoid R] (u : units Rᵒᵖ) :
↑(opposite.unop (⇑units.op_equiv u)) = opposite.unop ↑u
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@[simp]
theorem units.coe_op_equiv_symm {R : Type u_1} [monoid R] (u : (units R)ᵒᵖ) :
↑(⇑(units.op_equiv.symm) u) = opposite.op ↑(opposite.unop u)
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def monoid_hom.op {α : Type u_1} {β : Type u_2} [mul_one_class α] [mul_one_class β] :
(α →* β) ≃ (αᵒᵖ →* βᵒᵖ)

A hom α →* β can equivalently be viewed as a hom αᵒᵖ →* βᵒᵖ. This is the action of the (fully faithful) ᵒᵖ-functor on morphisms.

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@[simp]
theorem monoid_hom.op_apply_apply {α : Type u_1} {β : Type u_2} [mul_one_class α] [mul_one_class β] (f : α →* β) (ᾰ : αᵒᵖ) :
⇑(⇑monoid_hom.op f) ᾰ = (opposite.op ∘ ⇑f ∘ opposite.unop) ᾰ
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@[simp]
theorem monoid_hom.op_symm_apply_apply {α : Type u_1} {β : Type u_2} [mul_one_class α] [mul_one_class β] (f : αᵒᵖ →* βᵒᵖ) (ᾰ : α) :
⇑(⇑(monoid_hom.op.symm) f) ᾰ = (opposite.unop ∘ ⇑f ∘ opposite.op) ᾰ
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@[simp]
def monoid_hom.unop {α : Type u_1} {β : Type u_2} [mul_one_class α] [mul_one_class β] :
(αᵒᵖ →* βᵒᵖ) ≃ (α →* β)

The 'unopposite' of a monoid hom αᵒᵖ →* βᵒᵖ. Inverse to monoid_hom.op.

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def add_monoid_hom.op {α : Type u_1} {β : Type u_2} [add_zero_class α] [add_zero_class β] :
(α →+ β) ≃ (αᵒᵖ →+ βᵒᵖ)

A hom α →+ β can equivalently be viewed as a hom αᵒᵖ →+ βᵒᵖ. This is the action of the (fully faithful) ᵒᵖ-functor on morphisms.

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@[simp]
theorem add_monoid_hom.op_apply_apply {α : Type u_1} {β : Type u_2} [add_zero_class α] [add_zero_class β] (f : α →+ β) (ᾰ : αᵒᵖ) :
⇑(⇑add_monoid_hom.op f) ᾰ = (opposite.op ∘ ⇑f ∘ opposite.unop) ᾰ
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@[simp]
theorem add_monoid_hom.op_symm_apply_apply {α : Type u_1} {β : Type u_2} [add_zero_class α] [add_zero_class β] (f : αᵒᵖ →+ βᵒᵖ) (ᾰ : α) :
⇑(⇑(add_monoid_hom.op.symm) f) ᾰ = (opposite.unop ∘ ⇑f ∘ opposite.op) ᾰ
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@[simp]
def add_monoid_hom.unop {α : Type u_1} {β : Type u_2} [add_zero_class α] [add_zero_class β] :
(αᵒᵖ →+ βᵒᵖ) ≃ (α →+ β)

The 'unopposite' of an additive monoid hom αᵒᵖ →+ βᵒᵖ. Inverse to add_monoid_hom.op.

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@[simp]
theorem ring_hom.op_apply_apply {α : Type u_1} {β : Type u_2} [non_assoc_semiring α] [non_assoc_semiring β] (f : α →+* β) (ᾰ : αᵒᵖ) :
⇑(⇑ring_hom.op f) ᾰ = (⇑add_monoid_hom.op f.to_add_monoid_hom).to_fun ᾰ
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def ring_hom.op {α : Type u_1} {β : Type u_2} [non_assoc_semiring α] [non_assoc_semiring β] :
(α →+* β) ≃ (αᵒᵖ →+* βᵒᵖ)

A ring hom α →+* β can equivalently be viewed as a ring hom αᵒᵖ →+* βᵒᵖ. This is the action of the (fully faithful) ᵒᵖ-functor on morphisms.

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@[simp]
theorem ring_hom.op_symm_apply_apply {α : Type u_1} {β : Type u_2} [non_assoc_semiring α] [non_assoc_semiring β] (f : αᵒᵖ →+* βᵒᵖ) (ᾰ : α) :
⇑(⇑(ring_hom.op.symm) f) ᾰ = (⇑add_monoid_hom.unop f.to_add_monoid_hom).to_fun ᾰ
source
@[simp]
def ring_hom.unop {α : Type u_1} {β : Type u_2} [non_assoc_semiring α] [non_assoc_semiring β] :
(αᵒᵖ →+* βᵒᵖ) ≃ (α →+* β)

The 'unopposite' of a ring hom αᵒᵖ →+* βᵒᵖ. Inverse to ring_hom.op.

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def add_equiv.op {α : Type u_1} {β : Type u_2} [has_add α] [has_add β] :
α ≃+ β ≃ (αᵒᵖ ≃+ βᵒᵖ)

A iso α ≃+ β can equivalently be viewed as an iso αᵒᵖ ≃+ βᵒᵖ.

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@[simp]
theorem add_equiv.op_symm_apply {α : Type u_1} {β : Type u_2} [has_add α] [has_add β] (f : αᵒᵖ ≃+ βᵒᵖ) :
⇑(add_equiv.op.symm) f = opposite.op_add_equiv.trans (f.trans opposite.op_add_equiv.symm)
source
@[simp]
theorem add_equiv.op_apply {α : Type u_1} {β : Type u_2} [has_add α] [has_add β] (f : α ≃+ β) :
⇑add_equiv.op f = opposite.op_add_equiv.symm.trans (f.trans opposite.op_add_equiv)
source
@[simp]
def add_equiv.unop {α : Type u_1} {β : Type u_2} [has_add α] [has_add β] :
αᵒᵖ ≃+ βᵒᵖ ≃ (α ≃+ β)

The 'unopposite' of an iso αᵒᵖ ≃+ βᵒᵖ. Inverse to add_equiv.op.

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@[simp]
theorem mul_equiv.op_symm_apply_apply {α : Type u_1} {β : Type u_2} [has_mul α] [has_mul β] (f : αᵒᵖ ≃* βᵒᵖ) (ᾰ : α) :
⇑(⇑(mul_equiv.op.symm) f) ᾰ = (opposite.unop ∘ ⇑f ∘ opposite.op) ᾰ
source
@[simp]
theorem mul_equiv.op_symm_apply_symm_apply {α : Type u_1} {β : Type u_2} [has_mul α] [has_mul β] (f : αᵒᵖ ≃* βᵒᵖ) (ᾰ : β) :
⇑((⇑(mul_equiv.op.symm) f).symm) ᾰ = (opposite.unop ∘ ⇑(f.symm) ∘ opposite.op) ᾰ
source
def mul_equiv.op {α : Type u_1} {β : Type u_2} [has_mul α] [has_mul β] :
α ≃* β ≃ (αᵒᵖ ≃* βᵒᵖ)

A iso α ≃* β can equivalently be viewed as an iso αᵒᵖ ≃+ βᵒᵖ.

Equations
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@[simp]
theorem mul_equiv.op_apply_symm_apply {α : Type u_1} {β : Type u_2} [has_mul α] [has_mul β] (f : α ≃* β) (ᾰ : βᵒᵖ) :
⇑((⇑mul_equiv.op f).symm) ᾰ = (opposite.op ∘ ⇑(f.symm) ∘ opposite.unop) ᾰ
source
@[simp]
theorem mul_equiv.op_apply_apply {α : Type u_1} {β : Type u_2} [has_mul α] [has_mul β] (f : α ≃* β) (ᾰ : αᵒᵖ) :
⇑(⇑mul_equiv.op f) ᾰ = (opposite.op ∘ ⇑f ∘ opposite.unop) ᾰ
source
@[simp]
def mul_equiv.unop {α : Type u_1} {β : Type u_2} [has_mul α] [has_mul β] :
αᵒᵖ ≃* βᵒᵖ ≃ (α ≃* β)

The 'unopposite' of an iso αᵒᵖ ≃* βᵒᵖ. Inverse to mul_equiv.op.

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@[ext]
theorem add_monoid_hom.op_ext {α : Type u_1} {β : Type u_2} [add_zero_class α] [add_zero_class β] (f g : αᵒᵖ →+ β) (h : f.comp opposite.op_add_equiv.to_add_monoid_hom = g.comp opposite.op_add_equiv.to_add_monoid_hom) :
f = g

This ext lemma change equalities on αᵒᵖ →+ β to equalities on α →+ β. This is useful because there are often ext lemmas for specific αs that will apply to an equality of α →+ β such as finsupp.add_hom_ext'.