Power operations on monoids and groups
The power operation on monoids and groups.
We separate this from group, because it depends on ℕ
,
which in turn depends on other parts of algebra.
Notation
The class has_pow α β
provides the notation a^b
for powers.
We define instances of has_pow M ℕ
, for monoids M
, and has_pow G ℤ
for groups G
.
We also define infix operators •ℕ
and •ℤ
for scalar multiplication by a natural and an integer
numbers, respectively.
Implementation details
We adopt the convention that 0^0 = 1
.
The power operation in a monoid. a^n = a*a*...*a
n times.
Equations
- monoid.pow a (n + 1) = a * monoid.pow a n
- monoid.pow a 0 = 1
Equations
- monoid.has_pow = {pow := monoid.pow (monoid.to_has_one M)}
Commutativity
First we prove some facts about semiconj_by
and commute
. They do not require any theory about
pow
and/or nsmul
and will be useful later in this file.
(Additive) monoid
Commutative (additive) monoid
Equations
- _ = _
Equations
- _ = _
Equations
- group.has_pow = {pow := gpow _inst_1}
Equations
Equations
Bernoulli's inequality for n : ℕ
, -2 ≤ a
.
Bernoulli's inequality reformulated to estimate a^n
.
Monoid homomorphisms from multiplicative ℕ
are defined by the image
of multiplicative.of_add 1
.
Equations
- powers_hom M = {to_fun := λ (x : M), {to_fun := λ (n : multiplicative ℕ), x ^ n.to_add, map_one' := _, map_mul' := _}, inv_fun := λ (f : multiplicative ℕ →* M), ⇑f (multiplicative.of_add 1), left_inv := _, right_inv := _}
Monoid homomorphisms from multiplicative ℤ
are defined by the image
of multiplicative.of_add 1
.
Equations
- gpowers_hom G = {to_fun := λ (x : G), {to_fun := λ (n : multiplicative ℤ), x ^ n.to_add, map_one' := _, map_mul' := _}, inv_fun := λ (f : multiplicative ℤ →* G), ⇑f (multiplicative.of_add 1), left_inv := _, right_inv := _}
Additive homomorphisms from ℕ
are defined by the image of 1
.
Additive homomorphisms from ℤ
are defined by the image of 1
.
Commutativity (again)
Facts about semiconj_by
and commute
that require gpow
or gsmul
, or the fact that integer
multiplication equals semiring multiplication.
Moving to the opposite monoid commutes with taking powers.