-- Mathlib/GroupTheory/GroupAction/FixedPoints.lean
theorem set_mem_fixedBy_of_movedBy_subset {s : Set α} {g : G} (s_subset : (fixedBy α g)ᶜ ⊆ s) :
    s ∈ fixedBy (Set α) g

Proof state:
case mpr
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
a✝ : a ∈ s
⊢ g • a ∈ s

Tactic:
apply s_subset

Result:
case mpr.a
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
a✝ : a ∈ s
⊢ g • a ∈ (fixedBy α g)ᶜ

Proof state:
case h
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
⊢ a ∈ g⁻¹ • s ↔ a ∈ s

Tactic:
rw [Set.mem_inv_smul_set_iff]

Result:
case h
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
⊢ g • a ∈ s ↔ a ∈ s

Proof state:
case mpr.a
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
a✝ : a ∈ s
⊢ g • a ∈ (fixedBy α g)ᶜ

Tactic:
rwa [Set.mem_compl_iff, smul_mem_fixedBy_iff_mem_fixedBy]

Result:
no goals

Proof state:
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
⊢ s ∈ fixedBy (Set α) g

Tactic:
rw [← fixedBy_inv]

Result:
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
⊢ s ∈ fixedBy (Set α) g⁻¹

Proof state:
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
⊢ g • a ∈ s ↔ a ∈ s

Tactic:
constructor <;> (intro; apply s_subset)

Result:
case mp.a
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
a✝ : g • a ∈ s
⊢ a ∈ (fixedBy α g)ᶜ

case mpr.a
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
a✝ : a ∈ s
⊢ g • a ∈ (fixedBy α g)ᶜ

Proof state:
case mp.a
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
a✝ : g • a ∈ s
⊢ a ∈ (fixedBy α g)ᶜ

Tactic:
exact a_moved

Result:
no goals

Proof state:
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_fixed : a ∈ fixedBy α g
⊢ g • a ∈ s ↔ a ∈ s

Tactic:
rw [a_fixed]

Result:
no goals

Proof state:
case pos
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
h✝ : a ∈ fixedBy α g
⊢ g • a ∈ s ↔ a ∈ s

case neg
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
h✝ : a ∉ fixedBy α g
⊢ g • a ∈ s ↔ a ∈ s

Tactic:
case pos a_fixed =>
  rw [a_fixed]

Result:
case neg
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
h✝ : a ∉ fixedBy α g
⊢ g • a ∈ s ↔ a ∈ s

Proof state:
case mpr
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
⊢ a ∈ s → g • a ∈ s

Tactic:
intro

Result:
case mpr
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
a_moved : a ∉ fixedBy α g
a✝ : a ∈ s
⊢ g • a ∈ s

Proof state:
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
⊢ s ∈ fixedBy (Set α) g⁻¹

Tactic:
ext a

Result:
case h
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
⊢ a ∈ g⁻¹ • s ↔ a ∈ s

Proof state:
case h
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
⊢ g • a ∈ s ↔ a ∈ s

Tactic:
by_cases a ∈ fixedBy α g

Result:
case pos
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
h✝ : a ∈ fixedBy α g
⊢ g • a ∈ s ↔ a ∈ s

case neg
α : Type u_1
G : Type u_2
inst✝¹ : Group G
inst✝ : MulAction G α
s : Set α
g : G
s_subset : (fixedBy α g)ᶜ ⊆ s
a : α
h✝ : a ∉ fixedBy α g
⊢ g • a ∈ s ↔ a ∈ s