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.