# mathlibdocumentation

algebra.invertible

# Invertible elements #

This file defines a typeclass invertible a for elements a with a two-sided multiplicative inverse.

The intent of the typeclass is to provide a way to write e.g. ⅟2 in a ring like ℤ[1/2] where some inverses exist but there is no general ⁻¹ operator; or to specify that a field has characteristic ≠ 2. It is the Type-valued analogue to the Prop-valued is_unit.

For constructions of the invertible element given a characteristic, see algebra/char_p/invertible and other lemmas in that file.

## Notation #

• ⅟a is invertible.inv_of a, the inverse of a

## Implementation notes #

The invertible class lives in Type, not Prop, to make computation easier. If multiplication is associative, invertible is a subsingleton anyway.

The simp normal form tries to normalize ⅟a to a ⁻¹. Otherwise, it pushes ⅟ inside the expression as much as possible.

Since invertible a is not a Prop (but it is a subsingleton), we have to be careful about coherence issues: we should avoid having multiple non-defeq instances for invertible a in the same context. This file plays it safe and uses def rather than instance for most definitions, users can choose which instances to use at the point of use.

For example, here's how you can use an invertible 1 instance:

variables {α : Type*} [monoid α]

def something_that_needs_inverses (x : α) [invertible x] := sorry

section
local attribute [instance] invertible_one
def something_one := something_that_needs_inverses 1
end


## Tags #

invertible, inverse element, inv_of, a half, one half, a third, one third, ½, ⅓

@[class]
structure invertible {α : Type u} [has_mul α] [has_one α] (a : α) :
Type u
• inv_of : α
• inv_of_mul_self : ( a) * a = 1
• mul_inv_of_self : a * a = 1

invertible a gives a two-sided multiplicative inverse of a.

Instances
@[simp]
theorem inv_of_mul_self {α : Type u} [has_mul α] [has_one α] (a : α) [invertible a] :
( a) * a = 1
@[simp]
theorem mul_inv_of_self {α : Type u} [has_mul α] [has_one α] (a : α) [invertible a] :
a * a = 1
@[simp]
theorem inv_of_mul_self_assoc {α : Type u} [monoid α] (a b : α) [invertible a] :
( a) * a * b = b
@[simp]
theorem mul_inv_of_self_assoc {α : Type u} [monoid α] (a b : α) [invertible a] :
a * ( a) * b = b
@[simp]
theorem mul_inv_of_mul_self_cancel {α : Type u} [monoid α] (a b : α) [invertible b] :
(a * b) * b = a
@[simp]
theorem mul_mul_inv_of_self_cancel {α : Type u} [monoid α] (a b : α) [invertible b] :
(a * b) * b = a
theorem inv_of_eq_right_inv {α : Type u} [monoid α] {a b : α} [invertible a] (hac : a * b = 1) :
a = b
theorem inv_of_eq_left_inv {α : Type u} [monoid α] {a b : α} [invertible a] (hac : b * a = 1) :
a = b
theorem invertible_unique {α : Type u} [monoid α] (a b : α) (h : a = b) [invertible a] [invertible b] :
a = b
@[instance]
def invertible.subsingleton {α : Type u} [monoid α] (a : α) :
def invertible.copy {α : Type u} [monoid α] {r : α} (hr : invertible r) (s : α) (hs : s = r) :

If r is invertible and s = r, then s is invertible.

Equations
@[simp]
theorem coe_unit_of_invertible {α : Type u} [monoid α] (a : α) [invertible a] :
= a
def unit_of_invertible {α : Type u} [monoid α] (a : α) [invertible a] :

An invertible element is a unit.

Equations
@[simp]
theorem coe_inv_unit_of_invertible {α : Type u} [monoid α] (a : α) [invertible a] :
= a
theorem is_unit_of_invertible {α : Type u} [monoid α] (a : α) [invertible a] :
def units.invertible {α : Type u} [monoid α] (u : units α) :

Units are invertible in their associated monoid.

Equations
@[simp]
theorem inv_of_units {α : Type u} [monoid α] (u : units α) [invertible u] :
theorem is_unit.nonempty_invertible {α : Type u} [monoid α] {a : α} (h : is_unit a) :
def is_unit.invertible {α : Type u} [monoid α] {a : α} (h : is_unit a) :

Convert is_unit to invertible using classical.choice.

Prefer casesI h.nonempty_invertible over letI := h.invertible if you want to avoid choice.

Equations
@[simp]
theorem nonempty_invertible_iff_is_unit {α : Type u} [monoid α] (a : α) :
def invertible_of_group {α : Type u} [group α] (a : α) :

Each element of a group is invertible.

Equations
@[simp]
theorem inv_of_eq_group_inv {α : Type u} [group α] (a : α) [invertible a] :
def invertible_one {α : Type u} [monoid α] :

1 is the inverse of itself

Equations
@[simp]
theorem inv_of_one {α : Type u} [monoid α] [invertible 1] :
1 = 1
def invertible_neg {α : Type u} [ring α] (a : α) [invertible a] :

-⅟a is the inverse of -a

Equations
@[simp]
theorem inv_of_neg {α : Type u} [ring α] (a : α) [invertible a] [invertible (-a)] :
(-a) = - a
@[simp]
theorem one_sub_inv_of_two {α : Type u} [ring α] [invertible 2] :
1 - 2 = 2
@[instance]
def invertible_inv_of {α : Type u} [has_one α] [has_mul α] {a : α} [invertible a] :

a is the inverse of ⅟a.

Equations
@[simp]
theorem inv_of_inv_of {α : Type u} [monoid α] {a : α} [invertible a] [invertible ( a)] :
( a) = a
def invertible_mul {α : Type u} [monoid α] (a b : α) [invertible a] [invertible b] :

⅟b * ⅟a is the inverse of a * b

Equations
@[simp]
theorem inv_of_mul {α : Type u} [monoid α] (a b : α) [invertible a] [invertible b] [invertible (a * b)] :
(a * b) = ( b) * a
theorem commute.inv_of_right {α : Type u} [monoid α] {a b : α} [invertible b] (h : b) :
( b)
theorem commute.inv_of_left {α : Type u} [monoid α] {a b : α} [invertible b] (h : a) :
commute ( b) a
theorem commute_inv_of {M : Type u_1} [has_one M] [has_mul M] (m : M) [invertible m] :
( m)
theorem nonzero_of_invertible {α : Type u} (a : α) [invertible a] :
a 0
def invertible_of_nonzero {α : Type u} {a : α} (h : a 0) :

a⁻¹ is an inverse of a if a ≠ 0

Equations
@[simp]
theorem inv_of_eq_inv {α : Type u} (a : α) [invertible a] :
@[simp]
theorem inv_mul_cancel_of_invertible {α : Type u} (a : α) [invertible a] :
a⁻¹ * a = 1
@[simp]
theorem mul_inv_cancel_of_invertible {α : Type u} (a : α) [invertible a] :
a * a⁻¹ = 1
@[simp]
theorem div_mul_cancel_of_invertible {α : Type u} (a b : α) [invertible b] :
(a / b) * b = a
@[simp]
theorem mul_div_cancel_of_invertible {α : Type u} (a b : α) [invertible b] :
a * b / b = a
@[simp]
theorem div_self_of_invertible {α : Type u} (a : α) [invertible a] :
a / a = 1
def invertible_div {α : Type u} (a b : α) [invertible a] [invertible b] :

b / a is the inverse of a / b

Equations
@[simp]
theorem inv_of_div {α : Type u} (a b : α) [invertible a] [invertible b] [invertible (a / b)] :
(a / b) = b / a
def invertible_inv {α : Type u} {a : α} [invertible a] :

a is the inverse of a⁻¹

Equations
def invertible.map {R : Type u_1} {S : Type u_2} [monoid R] [monoid S] (f : R →* S) (r : R) [invertible r] :

Monoid homs preserve invertibility.

Equations