lean2/library/algebra/group_power.lean
Jeremy Avigad 4a36f843f7 refactor(library/algebra/group_power,library/*): change definition of pow
I changed the definition of pow so that a^(succ n) reduces to a * a^n rather than a^n * a.

This has the nice effect that on nat and int, where multiplication is defined by recursion on the right,
a^1 reduces to a, and a^2 reduces to a * a.

The change was a pain in the neck, and in retrospect maybe not worth it, but oh, well.
2015-08-14 18:49:57 -07:00

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/-
Copyright (c) 2015 Jeremy Avigad. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Author: Jeremy Avigad
The power operation on monoids and groups. We separate this from group, because it depends on
nat, which in turn depends on other parts of algebra.
We have "pow a n" for natural number powers, and "gpow a i" for integer powers. The notation
a^n is used for the first, but users can locally redefine it to gpow when needed.
Note: power adopts the convention that 0^0=1.
-/
import data.nat.basic data.int.basic
namespace algebra
variables {A : Type}
/- monoid -/
section monoid
open nat
variable [s : monoid A]
include s
definition pow (a : A) : → A
| 0 := 1
| (n+1) := a * pow n
infix [priority algebra.prio] `^` := pow
theorem pow_zero (a : A) : a^0 = 1 := rfl
theorem pow_succ (a : A) (n : ) : a^(succ n) = a * a^n := rfl
theorem pow_one (a : A) : a^1 = a := !mul_one
theorem pow_two (a : A) : a^2 = a * a :=
calc
a^2 = a * (a * 1) : rfl
... = a * a : mul_one
theorem pow_three (a : A) : a^3 = a * (a * a) :=
calc
a^3 = a * (a * (a * 1)) : rfl
... = a * (a * a) : mul_one
theorem pow_four (a : A) : a^4 = a * (a * (a * a)) :=
calc
a^4 = a * a^3 : rfl
... = a * (a * (a * a)) : pow_three
theorem pow_succ' (a : A) : ∀n, a^(succ n) = a^n * a
| 0 := by rewrite [pow_succ, *pow_zero, one_mul, mul_one]
| (succ n) := by rewrite [pow_succ, pow_succ' at {1}, pow_succ, mul.assoc]
theorem one_pow : ∀ n : , 1^n = (1:A)
| 0 := rfl
| (succ n) := by rewrite [pow_succ, one_mul, one_pow]
theorem pow_add (a : A) (m n : ) : a^(m + n) = a^m * a^n :=
begin
induction n with n ih,
{rewrite [nat.add_zero, pow_zero, mul_one]},
rewrite [add_succ, *pow_succ', ih, mul.assoc]
end
theorem pow_mul (a : A) (m : ) : ∀ n, a^(m * n) = (a^m)^n
| 0 := by rewrite [nat.mul_zero, pow_zero]
| (succ n) := by rewrite [nat.mul_succ, pow_add, pow_succ', pow_mul]
theorem pow_comm (a : A) (m n : ) : a^m * a^n = a^n * a^m :=
by rewrite [-*pow_add, nat.add.comm]
end monoid
/- commutative monoid -/
section comm_monoid
open nat
variable [s : comm_monoid A]
include s
theorem mul_pow (a b : A) : ∀ n, (a * b)^n = a^n * b^n
| 0 := by rewrite [*pow_zero, mul_one]
| (succ n) := by rewrite [*pow_succ', mul_pow, *mul.assoc, mul.left_comm a]
end comm_monoid
section group
variable [s : group A]
include s
section nat
open nat
theorem inv_pow (a : A) : ∀n, (a⁻¹)^n = (a^n)⁻¹
| 0 := by rewrite [*pow_zero, one_inv]
| (succ n) := by rewrite [pow_succ, pow_succ', inv_pow, mul_inv]
theorem pow_sub (a : A) {m n : } (H : m ≥ n) : a^(m - n) = a^m * (a^n)⁻¹ :=
assert H1 : m - n + n = m, from nat.sub_add_cancel H,
have H2 : a^(m - n) * a^n = a^m, by rewrite [-pow_add, H1],
eq_mul_inv_of_mul_eq H2
theorem pow_inv_comm (a : A) : ∀m n, (a⁻¹)^m * a^n = a^n * (a⁻¹)^m
| 0 n := by rewrite [*pow_zero, one_mul, mul_one]
| m 0 := by rewrite [*pow_zero, one_mul, mul_one]
| (succ m) (succ n) := by rewrite [pow_succ' at {1}, pow_succ at {1}, pow_succ', pow_succ,
*mul.assoc, inv_mul_cancel_left, mul_inv_cancel_left, pow_inv_comm]
end nat
open int
definition gpow (a : A) : → A
| (of_nat n) := a^n
| -[1+n] := (a^(nat.succ n))⁻¹
private lemma gpow_add_aux (a : A) (m n : nat) :
gpow a ((of_nat m) + -[1+n]) = gpow a (of_nat m) * gpow a (-[1+n]) :=
or.elim (nat.lt_or_ge m (nat.succ n))
(assume H : (#nat m < nat.succ n),
assert H1 : (#nat nat.succ n - m > nat.zero), from nat.sub_pos_of_lt H,
calc
gpow a ((of_nat m) + -[1+n]) = gpow a (sub_nat_nat m (nat.succ n)) : rfl
... = gpow a (-[1+ nat.pred (nat.sub (nat.succ n) m)]) : {sub_nat_nat_of_lt H}
... = (pow a (nat.succ (nat.pred (nat.sub (nat.succ n) m))))⁻¹ : rfl
... = (pow a (nat.succ n) * (pow a m)⁻¹)⁻¹ :
by rewrite [nat.succ_pred_of_pos H1, pow_sub a (nat.le_of_lt H)]
... = pow a m * (pow a (nat.succ n))⁻¹ :
by rewrite [mul_inv, inv_inv]
... = gpow a (of_nat m) * gpow a (-[1+n]) : rfl)
(assume H : (#nat m ≥ nat.succ n),
calc
gpow a ((of_nat m) + -[1+n]) = gpow a (sub_nat_nat m (nat.succ n)) : rfl
... = gpow a (#nat m - nat.succ n) : {sub_nat_nat_of_ge H}
... = pow a m * (pow a (nat.succ n))⁻¹ : pow_sub a H
... = gpow a (of_nat m) * gpow a (-[1+n]) : rfl)
theorem gpow_add (a : A) : ∀i j : int, gpow a (i + j) = gpow a i * gpow a j
| (of_nat m) (of_nat n) := !pow_add
| (of_nat m) -[1+n] := !gpow_add_aux
| -[1+m] (of_nat n) := by rewrite [int.add.comm, gpow_add_aux, ↑gpow, -*inv_pow, pow_inv_comm]
| -[1+m] -[1+n] :=
calc
gpow a (-[1+m] + -[1+n]) = (a^(#nat nat.succ m + nat.succ n))⁻¹ : rfl
... = (a^(nat.succ m))⁻¹ * (a^(nat.succ n))⁻¹ : by rewrite [pow_add, pow_comm, mul_inv]
... = gpow a (-[1+m]) * gpow a (-[1+n]) : rfl
theorem gpow_comm (a : A) (i j : ) : gpow a i * gpow a j = gpow a j * gpow a i :=
by rewrite [-*gpow_add, int.add.comm]
end group
section ordered_ring
open nat
variable [s : linear_ordered_ring A]
include s
theorem pow_pos {a : A} (H : a > 0) (n : ) : pow a n > 0 :=
begin
induction n,
rewrite pow_zero,
apply zero_lt_one,
rewrite pow_succ',
apply mul_pos,
apply v_0, apply H
end
theorem pow_ge_one_of_ge_one {a : A} (H : a ≥ 1) (n : ) : pow a n ≥ 1 :=
begin
induction n,
rewrite pow_zero,
apply le.refl,
rewrite [pow_succ', -{1}mul_one],
apply mul_le_mul v_0 H zero_le_one,
apply le_of_lt,
apply pow_pos,
apply gt_of_ge_of_gt H zero_lt_one
end
local notation 2 := (1 : A) + 1
theorem pow_two_add (n : ) : pow 2 n + pow 2 n = pow 2 (succ n) :=
by rewrite [pow_succ', left_distrib, *mul_one]
end ordered_ring
/- additive monoid -/
section add_monoid
variable [s : add_monoid A]
include s
local attribute add_monoid.to_monoid [trans-instance]
open nat
definition nmul : → A → A := λ n a, pow a n
infix [priority algebra.prio] `⬝` := nmul
theorem zero_nmul (a : A) : (0:) ⬝ a = 0 := pow_zero a
theorem succ_nmul (n : ) (a : A) : nmul (succ n) a = a + (nmul n a) := pow_succ a n
theorem succ_nmul' (n : ) (a : A) : succ n ⬝ a = nmul n a + a := pow_succ' a n
theorem nmul_zero (n : ) : n ⬝ 0 = (0:A) := one_pow n
theorem one_nmul (a : A) : 1 ⬝ a = a := pow_one a
theorem add_nmul (m n : ) (a : A) : (m + n) ⬝ a = (m ⬝ a) + (n ⬝ a) := pow_add a m n
theorem mul_nmul (m n : ) (a : A) : (m * n) ⬝ a = m ⬝ (n ⬝ a) := eq.subst (nat.mul.comm n m) (pow_mul a n m)
theorem nmul_comm (m n : ) (a : A) : (m ⬝ a) + (n ⬝ a) = (n ⬝ a) + (m ⬝ a) := pow_comm a m n
end add_monoid
/- additive commutative monoid -/
section add_comm_monoid
open nat
variable [s : add_comm_monoid A]
include s
local attribute add_comm_monoid.to_comm_monoid [trans-instance]
theorem nmul_add (n : ) (a b : A) : n ⬝ (a + b) = (n ⬝ a) + (n ⬝ b) := mul_pow a b n
end add_comm_monoid
section add_group
variable [s : add_group A]
include s
local attribute add_group.to_group [trans-instance]
section nat
open nat
theorem nmul_neg (n : ) (a : A) : n ⬝ (-a) = -(n ⬝ a) := inv_pow a n
theorem sub_nmul {m n : } (a : A) (H : m ≥ n) : (m - n) ⬝ a = (m ⬝ a) + -(n ⬝ a) := pow_sub a H
theorem nmul_neg_comm (m n : ) (a : A) : (m ⬝ (-a)) + (n ⬝ a) = (n ⬝ a) + (m ⬝ (-a)) := pow_inv_comm a m n
end nat
open int
definition imul : → A → A := λ i a, gpow a i
theorem add_imul (i j : ) (a : A) : imul (i + j) a = imul i a + imul j a :=
gpow_add a i j
theorem imul_comm (i j : ) (a : A) : imul i a + imul j a = imul j a + imul i a := gpow_comm a i j
end add_group
end algebra