formal/lean/mathlib/logic/function/iterate.lean

/-
Copyright (c) 2020 Yury Kudryashov. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Yury Kudryashov
-/
import logic.function.conjugate

/-!
# Iterations of a function

In this file we prove simple properties of `nat.iterate f n` a.k.a. `f^[n]`:

* `iterate_zero`, `iterate_succ`, `iterate_succ'`, `iterate_add`, `iterate_mul`:
  formulas for `f^[0]`, `f^[n+1]` (two versions), `f^[n+m]`, and `f^[n*m]`;

* `iterate_id` : `id^[n]=id`;

* `injective.iterate`, `surjective.iterate`, `bijective.iterate` :
  iterates of an injective/surjective/bijective function belong to the same class;

* `left_inverse.iterate`, `right_inverse.iterate`, `commute.iterate_left`, `commute.iterate_right`,
  `commute.iterate_iterate`:
  some properties of pairs of functions survive under iterations

* `iterate_fixed`, `semiconj.iterate_*`, `semiconj₂.iterate`:
  if `f` fixes a point (resp., semiconjugates unary/binary operarations), then so does `f^[n]`.

-/

universes u v

variables {α : Type u} {β : Type v}
namespace function

variable (f : α → α)

@[simp] theorem iterate_zero : f^[0] = id := rfl

theorem iterate_zero_apply (x : α) : f^[0] x = x := rfl

@[simp] theorem iterate_succ (n : ℕ) : f^[n.succ] = (f^[n]) ∘ f := rfl

theorem iterate_succ_apply (n : ℕ) (x : α) : f^[n.succ] x = (f^[n]) (f x) := rfl

@[simp] theorem iterate_id (n : ℕ) : (id : α → α)^[n] = id :=
nat.rec_on n rfl $ λ n ihn, by rw [iterate_succ, ihn, comp.left_id]

theorem iterate_add : ∀ (m n : ℕ), f^[m + n] = (f^[m]) ∘ (f^[n])
| m 0 := rfl
| m (nat.succ n) := by rw [nat.add_succ, iterate_succ, iterate_succ, iterate_add]

theorem iterate_add_apply (m n : ℕ) (x : α) : f^[m + n] x = (f^[m] (f^[n] x)) :=
by rw iterate_add

@[simp] theorem iterate_one : f^[1] = f := funext $ λ a, rfl

lemma iterate_mul (m : ℕ) : ∀ n, f^[m * n] = (f^[m]^[n])
| 0 := by simp only [nat.mul_zero, iterate_zero]
| (n + 1) := by simp only [nat.mul_succ, nat.mul_one, iterate_one, iterate_add, iterate_mul n]

variable {f}

theorem iterate_fixed {x} (h : f x = x) (n : ℕ) : f^[n] x = x :=
nat.rec_on n rfl $ λ n ihn, by rw [iterate_succ_apply, h, ihn]

theorem injective.iterate (Hinj : injective f) (n : ℕ) : injective (f^[n]) :=
nat.rec_on n injective_id $ λ n ihn, ihn.comp Hinj

theorem surjective.iterate (Hsurj : surjective f) (n : ℕ) : surjective (f^[n]) :=
nat.rec_on n surjective_id $ λ n ihn, ihn.comp Hsurj

theorem bijective.iterate (Hbij : bijective f) (n : ℕ) : bijective (f^[n]) :=
⟨Hbij.1.iterate n, Hbij.2.iterate n⟩

namespace semiconj

lemma iterate_right {f : α → β} {ga : α → α} {gb : β → β} (h : semiconj f ga gb) (n : ℕ) :
  semiconj f (ga^[n]) (gb^[n]) :=
nat.rec_on n id_right $ λ n ihn, ihn.comp_right h

lemma iterate_left {g : ℕ → α → α} (H : ∀ n, semiconj f (g n) (g $ n + 1)) (n k : ℕ) :
  semiconj (f^[n]) (g k) (g $ n + k) :=
begin
  induction n with n ihn generalizing k,
  { rw [nat.zero_add], exact id_left },
  { rw [nat.succ_eq_add_one, nat.add_right_comm, nat.add_assoc],
    exact (H k).comp_left (ihn (k + 1)) }
end

end semiconj

namespace commute

variable {g : α → α}

lemma iterate_right (h : commute f g) (n : ℕ) : commute f (g^[n]) := h.iterate_right n

lemma iterate_left (h : commute f g) (n : ℕ) : commute (f^[n]) g := (h.symm.iterate_right n).symm

lemma iterate_iterate (h : commute f g) (m n : ℕ) : commute (f^[m]) (g^[n]) :=
(h.iterate_left m).iterate_right n

lemma iterate_eq_of_map_eq (h : commute f g) (n : ℕ) {x} (hx : f x = g x) : f^[n] x = (g^[n]) x :=
nat.rec_on n rfl $ λ n ihn,
by simp only [iterate_succ_apply, hx, (h.iterate_left n).eq, ihn, ((refl g).iterate_right n).eq]

lemma comp_iterate (h : commute f g) (n : ℕ) : (f ∘ g)^[n] = (f^[n]) ∘ (g^[n]) :=
begin
  induction n with n ihn, { refl },
  funext x,
  simp only [ihn, (h.iterate_right n).eq, iterate_succ, comp_app]
end

variable (f)

lemma iterate_self (n : ℕ) : commute (f^[n]) f := (refl f).iterate_left n

lemma self_iterate (n : ℕ) : commute f (f^[n]) := (refl f).iterate_right n

lemma iterate_iterate_self (m n : ℕ) : commute (f^[m]) (f^[n]) := (refl f).iterate_iterate m n

end commute

lemma semiconj₂.iterate {f : α → α} {op : α → α → α} (hf : semiconj₂ f op op) (n : ℕ) :
  semiconj₂ (f^[n]) op op :=
nat.rec_on n (semiconj₂.id_left op) (λ n ihn, ihn.comp hf)

variable (f)

theorem iterate_succ' (n : ℕ) : f^[n.succ] = f ∘ (f^[n]) :=
by rw [iterate_succ, (commute.self_iterate f n).comp_eq]

theorem iterate_succ_apply' (n : ℕ) (x : α) : f^[n.succ] x = f (f^[n] x) :=
by rw [iterate_succ']

theorem iterate_pred_comp_of_pos {n : ℕ} (hn : 0 < n) : f^[n.pred] ∘ f = (f^[n]) :=
by rw [← iterate_succ, nat.succ_pred_eq_of_pos hn]

theorem comp_iterate_pred_of_pos {n : ℕ} (hn : 0 < n) : f ∘ (f^[n.pred]) = (f^[n]) :=
by rw [← iterate_succ', nat.succ_pred_eq_of_pos hn]

/-- A recursor for the iterate of a function. -/
def iterate.rec (p : α → Sort*) {f : α → α} (h : ∀ a, p a → p (f a)) {a : α} (ha : p a) (n : ℕ) :
  p (f^[n] a) :=
nat.rec ha (λ m, by { rw iterate_succ', exact h _ }) n

lemma iterate.rec_zero (p : α → Sort*) {f : α → α} (h : ∀ a, p a → p (f a)) {a : α} (ha : p a) :
  iterate.rec p h ha 0 = ha :=
rfl

variable {f}

theorem left_inverse.iterate {g : α → α} (hg : left_inverse g f) (n : ℕ) :
  left_inverse (g^[n]) (f^[n]) :=
nat.rec_on n (λ _, rfl) $ λ n ihn, by { rw [iterate_succ', iterate_succ], exact ihn.comp hg }

theorem right_inverse.iterate {g : α → α} (hg : right_inverse g f) (n : ℕ) :
  right_inverse (g^[n]) (f^[n]) :=
hg.iterate n

lemma iterate_comm (f : α → α) (m n : ℕ) : f^[n]^[m] = (f^[m]^[n]) :=
(iterate_mul _ _ _).symm.trans (eq.trans (by rw nat.mul_comm) (iterate_mul _ _ _))

lemma iterate_commute (m n : ℕ) : commute (λ f : α → α, f^[m]) (λ f, f^[n]) :=
λ f, iterate_comm f m n

end function

namespace list
open function

theorem foldl_const (f : α → α) (a : α) (l : list β) : l.foldl (λ b _, f b) a = (f^[l.length]) a :=
begin
  induction l with b l H generalizing a,
  { refl },
  { rw [length_cons, foldl, iterate_succ_apply, H] }
end

theorem foldr_const (f : β → β) (b : β) : Π l : list α, l.foldr (λ _, f) b = (f^[l.length]) b
| []     := rfl
| (a::l) := by rw [length_cons, foldr, foldr_const l, iterate_succ_apply']

end list