# mathlibdocumentation

group_theory.perm.cycles

# Cyclic permutations #

## Main definitions #

In the following, f : equiv.perm β.

• equiv.perm.is_cycle: f.is_cycle when two nonfixed points of β are related by repeated application of f.
• equiv.perm.same_cycle: f.same_cycle x y when x and y are in the same cycle of f.

The following two definitions require that β is a fintype:

• equiv.perm.cycle_of: f.cycle_of x is the cycle of f that x belongs to.
• equiv.perm.cycle_factors: f.cycle_factors is a list of disjoint cyclic permutations that multiply to f.

## Main results #

• This file contains several closure results:
• closure_is_cycle : The symmetric group is generated by cycles
• closure_cycle_adjacent_swap : The symmetric group is generated by a cycle and an adjacent transposition
• closure_cycle_coprime_swap : The symmetric group is generated by a cycle and a coprime transposition
• closure_prime_cycle_swap : The symmetric group is generated by a prime cycle and a transposition

### is_cycle#

def equiv.perm.is_cycle {β : Type u_2} (f : equiv.perm β) :
Prop

A permutation is a cycle when any two nonfixed points of the permutation are related by repeated application of the permutation.

Equations
theorem equiv.perm.is_cycle.ne_one {β : Type u_2} {f : equiv.perm β} (h : f.is_cycle) :
f 1
theorem equiv.perm.is_cycle.two_le_card_support {α : Type u_1} [decidable_eq α] [fintype α] {f : equiv.perm α} (h : f.is_cycle) :
theorem equiv.perm.is_cycle_swap {α : Type u_1} [decidable_eq α] {x y : α} (hxy : x y) :
theorem equiv.perm.is_swap.is_cycle {α : Type u_1} [decidable_eq α] {f : equiv.perm α} (hf : f.is_swap) :
theorem equiv.perm.is_cycle.inv {β : Type u_2} {f : equiv.perm β} (hf : f.is_cycle) :
theorem equiv.perm.is_cycle.is_cycle_conj {β : Type u_2} {f g : equiv.perm β} (hf : f.is_cycle) :
((g * f) * g⁻¹).is_cycle
theorem equiv.perm.is_cycle.exists_gpow_eq {β : Type u_2} {f : equiv.perm β} (hf : f.is_cycle) {x y : β} (hx : f x x) (hy : f y y) :
∃ (i : ), (f ^ i) x = y
theorem equiv.perm.is_cycle.exists_pow_eq {β : Type u_2} [fintype β] {f : equiv.perm β} (hf : f.is_cycle) {x y : β} (hx : f x x) (hy : f y y) :
∃ (i : ), (f ^ i) x = y
def equiv.perm.is_cycle.gpowers_equiv_support {α : Type u_1} [decidable_eq α] [fintype α] {σ : equiv.perm α} (hσ : σ.is_cycle) :

The subgroup generated by a cycle is in bijection with its support

Equations
@[simp]
theorem equiv.perm.is_cycle.gpowers_equiv_support_apply {α : Type u_1} [decidable_eq α] [fintype α] {σ : equiv.perm α} (hσ : σ.is_cycle) {n : } :
(hσ.gpowers_equiv_support) σ ^ n, _⟩ = ^ n) (classical.some hσ), _⟩
@[simp]
theorem equiv.perm.is_cycle.gpowers_equiv_support_symm_apply {α : Type u_1} [decidable_eq α] [fintype α] {σ : equiv.perm α} (hσ : σ.is_cycle) (n : ) :
(hσ.gpowers_equiv_support.symm) ^ n) (classical.some hσ), _⟩ = σ ^ n, _⟩
theorem equiv.perm.order_of_is_cycle {α : Type u_1} [decidable_eq α] [fintype α] {σ : equiv.perm α} (hσ : σ.is_cycle) :
theorem equiv.perm.is_cycle_swap_mul_aux₁ {α : Type u_1} [decidable_eq α] (n : ) {b x : α} {f : equiv.perm α} (hb : ( (f x)) * f) b b) (h : (f ^ n) (f x) = b) :
∃ (i : ), (((equiv.swap x (f x)) * f) ^ i) (f x) = b
theorem equiv.perm.is_cycle_swap_mul_aux₂ {α : Type u_1} [decidable_eq α] (n : ) {b x : α} {f : equiv.perm α} (hb : ( (f x)) * f) b b) (h : (f ^ n) (f x) = b) :
∃ (i : ), (((equiv.swap x (f x)) * f) ^ i) (f x) = b
theorem equiv.perm.is_cycle.eq_swap_of_apply_apply_eq_self {α : Type u_1} [decidable_eq α] {f : equiv.perm α} (hf : f.is_cycle) {x : α} (hfx : f x x) (hffx : f (f x) = x) :
f = (f x)
theorem equiv.perm.is_cycle.swap_mul {α : Type u_1} [decidable_eq α] {f : equiv.perm α} (hf : f.is_cycle) {x : α} (hx : f x x) (hffx : f (f x) x) :
((equiv.swap x (f x)) * f).is_cycle
theorem equiv.perm.is_cycle.sign {α : Type u_1} [decidable_eq α] [fintype α] {f : equiv.perm α} (hf : f.is_cycle) :
= -(-1) ^ f.support.card
theorem equiv.perm.is_cycle_of_is_cycle_pow {α : Type u_1} [decidable_eq α] [fintype α] {σ : equiv.perm α} {n : } (h1 : ^ n).is_cycle) (h2 : σ.support ^ n).support) :
theorem equiv.perm.is_cycle.extend_domain {β : Type u_2} {α : Type u_1} {p : β → Prop} (f : α ) {g : equiv.perm α} (h : g.is_cycle) :

### same_cycle#

def equiv.perm.same_cycle {β : Type u_2} (f : equiv.perm β) (x y : β) :
Prop

The equivalence relation indicating that two points are in the same cycle of a permutation.

Equations
theorem equiv.perm.same_cycle.refl {β : Type u_2} (f : equiv.perm β) (x : β) :
theorem equiv.perm.same_cycle.symm {β : Type u_2} (f : equiv.perm β) {x y : β} :
f.same_cycle x yf.same_cycle y x
theorem equiv.perm.same_cycle.trans {β : Type u_2} (f : equiv.perm β) {x y z : β} :
f.same_cycle x yf.same_cycle y zf.same_cycle x z
theorem equiv.perm.same_cycle.apply_eq_self_iff {β : Type u_2} {f : equiv.perm β} {x y : β} :
f.same_cycle x y(f x = x f y = y)
theorem equiv.perm.is_cycle.same_cycle {β : Type u_2} {f : equiv.perm β} (hf : f.is_cycle) {x y : β} (hx : f x x) (hy : f y y) :
@[protected, instance]
def equiv.perm.same_cycle.decidable_rel {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) :
Equations
theorem equiv.perm.same_cycle_apply {β : Type u_2} {f : equiv.perm β} {x y : β} :
f.same_cycle x (f y) f.same_cycle x y
theorem equiv.perm.same_cycle_cycle {β : Type u_2} {f : equiv.perm β} {x : β} (hx : f x x) :
f.is_cycle ∀ {y : β}, f.same_cycle x y f y y
theorem equiv.perm.same_cycle_inv {β : Type u_2} (f : equiv.perm β) {x y : β} :
theorem equiv.perm.same_cycle_inv_apply {β : Type u_2} {f : equiv.perm β} {x y : β} :

### cycle_of#

def equiv.perm.cycle_of {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) (x : α) :

f.cycle_of x is the cycle of the permutation f to which x belongs.

Equations
theorem equiv.perm.cycle_of_apply {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) (x y : α) :
(f.cycle_of x) y = ite (f.same_cycle x y) (f y) y
theorem equiv.perm.cycle_of_inv {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) (x : α) :
@[simp]
theorem equiv.perm.cycle_of_pow_apply_self {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) (x : α) (n : ) :
(f.cycle_of x ^ n) x = (f ^ n) x
@[simp]
theorem equiv.perm.cycle_of_gpow_apply_self {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) (x : α) (n : ) :
(f.cycle_of x ^ n) x = (f ^ n) x
theorem equiv.perm.same_cycle.cycle_of_apply {α : Type u_1} [decidable_eq α] [fintype α] {f : equiv.perm α} {x y : α} (h : f.same_cycle x y) :
(f.cycle_of x) y = f y
theorem equiv.perm.cycle_of_apply_of_not_same_cycle {α : Type u_1} [decidable_eq α] [fintype α] {f : equiv.perm α} {x y : α} (h : ¬f.same_cycle x y) :
(f.cycle_of x) y = y
@[simp]
theorem equiv.perm.cycle_of_apply_self {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) (x : α) :
(f.cycle_of x) x = f x
theorem equiv.perm.is_cycle.cycle_of_eq {α : Type u_1} [decidable_eq α] [fintype α] {f : equiv.perm α} (hf : f.is_cycle) {x : α} (hx : f x x) :
f.cycle_of x = f
@[simp]
theorem equiv.perm.cycle_of_eq_one_iff {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) {x : α} :
f.cycle_of x = 1 f x = x
theorem equiv.perm.is_cycle.cycle_of {α : Type u_1} [decidable_eq α] [fintype α] {f : equiv.perm α} (hf : f.is_cycle) {x : α} :
f.cycle_of x = ite (f x = x) 1 f
theorem equiv.perm.cycle_of_one {α : Type u_1} [decidable_eq α] [fintype α] (x : α) :
1.cycle_of x = 1
theorem equiv.perm.is_cycle_cycle_of {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) {x : α} (hx : f x x) :

### cycle_factors#

def equiv.perm.cycle_factors_aux {α : Type u_1} [decidable_eq α] [fintype α] (l : list α) (f : equiv.perm α) :
(∀ {x : α}, f x xx l){l // l.prod = f (∀ (g : , g l → g.is_cycle)

Given a list l : list α and a permutation f : perm α whose nonfixed points are all in l, recursively factors f into cycles.

Equations
def equiv.perm.cycle_factors {α : Type u_1} [decidable_eq α] [fintype α] [linear_order α] (f : equiv.perm α) :
{l // l.prod = f (∀ (g : , g l → g.is_cycle)

Factors a permutation f into a list of disjoint cyclic permutations that multiply to f.

Equations
def equiv.perm.trunc_cycle_factors {α : Type u_1} [decidable_eq α] [fintype α] (f : equiv.perm α) :
trunc {l // l.prod = f (∀ (g : , g l → g.is_cycle)

Factors a permutation f into a list of disjoint cyclic permutations that multiply to f, without a linear order.

Equations
theorem equiv.perm.cycle_induction_on {β : Type u_2} [fintype β] (P : → Prop) (σ : equiv.perm β) (base_one : P 1) (base_cycles : ∀ (σ : , σ.is_cycleP σ) (induction_disjoint : ∀ (σ τ : , σ.disjoint τσ.is_cycleP σP τP * τ)) :
P σ
theorem equiv.perm.closure_is_cycle {β : Type u_2} [fintype β] :
theorem equiv.perm.closure_cycle_adjacent_swap {α : Type u_1} [decidable_eq α] [fintype α] {σ : equiv.perm α} (h1 : σ.is_cycle) (h2 : σ.support = ) (x : α) :
theorem equiv.perm.closure_cycle_coprime_swap {α : Type u_1} [decidable_eq α] [fintype α] {n : } {σ : equiv.perm α} (h0 : n.coprime (fintype.card α)) (h1 : σ.is_cycle) (h2 : σ.support = finset.univ) (x : α) :
subgroup.closure {σ, (^ n) x)} =
theorem equiv.perm.closure_prime_cycle_swap {α : Type u_1} [decidable_eq α] [fintype α] {σ τ : equiv.perm α} (h0 : nat.prime (fintype.card α)) (h1 : σ.is_cycle) (h2 : σ.support = finset.univ) (h3 : τ.is_swap) :
theorem equiv.perm.is_conj_of_support_equiv {α : Type u_1} [decidable_eq α] [fintype α] {σ τ : equiv.perm α} (f : {x // x (σ.support)} {x // x (τ.support)}) (hf : ∀ (x : α) (hx : x (σ.support)), (f σ x, _⟩) = τ (f x, hx⟩)) :
τ
theorem equiv.perm.is_cycle.is_conj {α : Type u_1} [decidable_eq α] [fintype α] {σ τ : equiv.perm α} (hσ : σ.is_cycle) (hτ : τ.is_cycle) (h : σ.support.card = τ.support.card) :
τ
theorem equiv.perm.is_cycle.is_conj_iff {α : Type u_1} [decidable_eq α] [fintype α] {σ τ : equiv.perm α} (hσ : σ.is_cycle) (hτ : τ.is_cycle) :
@[simp]
theorem equiv.perm.support_conj {α : Type u_1} [decidable_eq α] [fintype α] {σ τ : equiv.perm α} :
((σ * τ) * σ⁻¹).support =
theorem equiv.perm.card_support_conj {α : Type u_1} [decidable_eq α] [fintype α] {σ τ : equiv.perm α} :
((σ * τ) * σ⁻¹).support.card = τ.support.card
theorem equiv.perm.disjoint.is_conj_mul {α : Type u_1} [fintype α] {σ τ π ρ : equiv.perm α} (hc1 : «π») (hc2 : ρ) (hd1 : σ.disjoint τ) (hd2 : «π».disjoint ρ) :
is_conj * τ) («π» * ρ)

### Fixed points #

theorem equiv.perm.fixed_point_card_lt_of_ne_one {α : Type u_1} [decidable_eq α] [fintype α] {σ : equiv.perm α} (h : σ 1) :
(finset.filter (λ (x : α), σ x = x) finset.univ).card <