# mathlib3documentation

algebra.regular.basic

# Regular elements #

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We introduce left-regular, right-regular and regular elements, along with their to_additive analogues add-left-regular, add-right-regular and add-regular elements.

By definition, a regular element in a commutative ring is a non-zero divisor. Lemma is_regular_of_ne_zero implies that every non-zero element of an integral domain is regular. Since it assumes that the ring is a cancel_monoid_with_zero it applies also, for instance, to ℕ.

The lemmas in Section mul_zero_class show that the 0 element is (left/right-)regular if and only if the mul_zero_class is trivial. This is useful when figuring out stopping conditions for regular sequences: if 0 is ever an element of a regular sequence, then we can extend the sequence by adding one further 0.

The final goal is to develop part of the API to prove, eventually, results about non-zero-divisors.

def is_left_regular {R : Type u_1} [has_mul R] (c : R) :
Prop

A left-regular element is an element c such that multiplication on the left by c is injective.

Equations
def is_add_left_regular {R : Type u_1} [has_add R] (c : R) :
Prop

An add-left-regular element is an element c such that addition on the left by c is injective. -/

Equations
def is_add_right_regular {R : Type u_1} [has_add R] (c : R) :
Prop

An add-right-regular element is an element c such that addition on the right by c is injective.

Equations
def is_right_regular {R : Type u_1} [has_mul R] (c : R) :
Prop

A right-regular element is an element c such that multiplication on the right by c is injective.

Equations
structure is_add_regular {R : Type u_2} [has_add R] (c : R) :
Prop
• left :
• right :

An add-regular element is an element c such that addition by c both on the left and on the right is injective.

structure is_regular {R : Type u_1} [has_mul R] (c : R) :
Prop
• left :
• right :

A regular element is an element c such that multiplication by c both on the left and on the right is injective.

@[protected]
@[protected]
theorem mul_le_cancellable.is_left_regular {R : Type u_1} [has_mul R] {a : R} (ha : mul_le_cancellable a) :
theorem is_left_regular.right_of_commute {R : Type u_1} [has_mul R] {a : R} (ca : (b : R), b) (h : is_left_regular a) :
theorem commute.is_regular_iff {R : Type u_1} [has_mul R] {a : R} (ca : (b : R), b) :
theorem is_left_regular.mul {R : Type u_1} [semigroup R] {a b : R} (lra : is_left_regular a) (lrb : is_left_regular b) :

In a semigroup, the product of left-regular elements is left-regular.

theorem is_right_regular.mul {R : Type u_1} [semigroup R] {a b : R} (rra : is_right_regular a) (rrb : is_right_regular b) :

In a semigroup, the product of right-regular elements is right-regular.

theorem is_left_regular.of_mul {R : Type u_1} [semigroup R] {a b : R} (ab : is_left_regular (a * b)) :

If an element b becomes left-regular after multiplying it on the left by a left-regular element, then b is left-regular.

theorem is_add_left_regular.of_add {R : Type u_1} {a b : R} (ab : is_add_left_regular (a + b)) :

If an element b becomes add-left-regular after adding to it on the left a add-left-regular element, then b is add-left-regular.

@[simp]
theorem mul_is_left_regular_iff {R : Type u_1} [semigroup R] {a : R} (b : R) (ha : is_left_regular a) :

An element is left-regular if and only if multiplying it on the left by a left-regular element is left-regular.

@[simp]
theorem add_is_add_left_regular_iff {R : Type u_1} {a : R} (b : R) (ha : is_add_left_regular a) :

theorem is_add_right_regular.of_add {R : Type u_1} {a b : R} (ab : is_add_right_regular (b + a)) :

If an element b becomes add-right-regular after adding to it on the right a add-right-regular element, then b is add-right-regular.

theorem is_right_regular.of_mul {R : Type u_1} [semigroup R] {a b : R} (ab : is_right_regular (b * a)) :

If an element b becomes right-regular after multiplying it on the right by a right-regular element, then b is right-regular.

@[simp]
theorem mul_is_right_regular_iff {R : Type u_1} [semigroup R] {a : R} (b : R) (ha : is_right_regular a) :

An element is right-regular if and only if multiplying it on the right with a right-regular element is right-regular.

@[simp]
theorem add_is_add_right_regular_iff {R : Type u_1} {a : R} (b : R) (ha : is_add_right_regular a) :

Two elements a and b are add-regular if and only if both sums a + b and b + a are add-regular.

theorem is_regular_mul_and_mul_iff {R : Type u_1} [semigroup R] {a b : R} :

Two elements a and b are regular if and only if both products a * b and b * a are regular.

The "most used" implication of add_and_add_iff, with split hypotheses, instead of ∧.

theorem is_regular.and_of_mul_of_mul {R : Type u_1} [semigroup R] {a b : R} (ab : is_regular (a * b)) (ba : is_regular (b * a)) :

The "most used" implication of mul_and_mul_iff, with split hypotheses, instead of ∧.

theorem is_left_regular.subsingleton {R : Type u_1} (h : is_left_regular 0) :

The element 0 is left-regular if and only if R is trivial.

theorem is_right_regular.subsingleton {R : Type u_1} (h : is_right_regular 0) :

The element 0 is right-regular if and only if R is trivial.

theorem is_regular.subsingleton {R : Type u_1} (h : is_regular 0) :

The element 0 is regular if and only if R is trivial.

The element 0 is left-regular if and only if R is trivial.

theorem not_is_left_regular_zero_iff {R : Type u_1}  :

In a non-trivial mul_zero_class, the 0 element is not left-regular.

The element 0 is right-regular if and only if R is trivial.

theorem not_is_right_regular_zero_iff {R : Type u_1}  :

In a non-trivial mul_zero_class, the 0 element is not right-regular.

theorem is_regular_iff_subsingleton {R : Type u_1}  :

The element 0 is regular if and only if R is trivial.

theorem is_left_regular.ne_zero {R : Type u_1} {a : R} [nontrivial R] (la : is_left_regular a) :
a 0

A left-regular element of a nontrivial mul_zero_class is non-zero.

theorem is_right_regular.ne_zero {R : Type u_1} {a : R} [nontrivial R] (ra : is_right_regular a) :
a 0

A right-regular element of a nontrivial mul_zero_class is non-zero.

theorem is_regular.ne_zero {R : Type u_1} {a : R} [nontrivial R] (la : is_regular a) :
a 0

A regular element of a nontrivial mul_zero_class is non-zero.

theorem not_is_left_regular_zero {R : Type u_1} [nR : nontrivial R] :

In a non-trivial ring, the element 0 is not left-regular -- with typeclasses.

theorem not_is_right_regular_zero {R : Type u_1} [nR : nontrivial R] :

In a non-trivial ring, the element 0 is not right-regular -- with typeclasses.

theorem not_is_regular_zero {R : Type u_1} [nontrivial R] :

In a non-trivial ring, the element 0 is not regular -- with typeclasses.

theorem is_regular_one {R : Type u_1}  :

If multiplying by 1 on either side is the identity, 1 is regular.

theorem is_add_regular_zero {R : Type u_1}  :

If adding 0 on either side is the identity, 0 is regular.

theorem is_regular_mul_iff {R : Type u_1} {a b : R} :

A product is regular if and only if the factors are.

A sum is add-regular if and only if the summands are.

theorem is_left_regular_of_mul_eq_one {R : Type u_1} [monoid R] {a b : R} (h : b * a = 1) :

An element admitting a left inverse is left-regular.

theorem is_add_left_regular_of_add_eq_zero {R : Type u_1} [add_monoid R] {a b : R} (h : b + a = 0) :

theorem is_add_right_regular_of_add_eq_zero {R : Type u_1} [add_monoid R] {a b : R} (h : a + b = 0) :

theorem is_right_regular_of_mul_eq_one {R : Type u_1} [monoid R] {a b : R} (h : a * b = 1) :

An element admitting a right inverse is right-regular.

If R is an additive monoid, an element in add_units R is add-regular.

theorem units.is_regular {R : Type u_1} [monoid R] (a : Rˣ) :

If R is a monoid, an element in Rˣ is regular.

theorem is_unit.is_regular {R : Type u_1} [monoid R] {a : R} (ua : is_unit a) :

A unit in a monoid is regular.

theorem is_left_regular_of_left_cancel_semigroup {R : Type u_1} (g : R) :

Elements of a left cancel semigroup are left regular.

theorem is_right_regular_of_right_cancel_semigroup {R : Type u_1} (g : R) :

Elements of a right cancel semigroup are right regular.

theorem is_regular_of_cancel_monoid {R : Type u_1} (g : R) :

Elements of a cancel monoid are regular. Cancel semigroups do not appear to exist.