[Merged by Bors] - feat: Create a partition from an element of Sym

Mathlib/Combinatorics/Enumerative/Partition.lean

@@ -99,6 +99,34 @@ def ofSums (n : ℕ) (l : Multiset ℕ) (hl : l.sum = n) : Partition n where
 def ofMultiset (l : Multiset ℕ) : Partition l.sum := ofSums _ l rfl
 #align nat.partition.of_multiset Nat.Partition.ofMultiset
 
+/-- An element `s` of `Sym σ n` induces a partition given by its multiplicities. -/
+def ofSym {n : ℕ} {σ : Type*} (s : Sym σ n) [DecidableEq σ] : n.Partition where
+  parts := s.1.dedup.map s.1.count
+  parts_pos := by simp [Multiset.count_pos]
+  parts_sum := by
+    show ∑ a ∈ s.1.toFinset, count a s.1 = n
+    rw [toFinset_sum_count_eq]
+    exact s.2
+
+variable {n : ℕ} {σ τ : Type*} [DecidableEq σ] [DecidableEq τ]
+
+@[simp] lemma ofSym_map (e : σ ≃ τ) (s : Sym σ n) :
+    ofSym (s.map e) = ofSym s := by
+  simp only [ofSym, Sym.val_eq_coe, Sym.coe_map, toFinset_val, mk.injEq]
+  rw [Multiset.dedup_map_of_injective e.injective]
+  simp only [map_map, Function.comp_apply]
+  congr; funext i
+  rw [← Multiset.count_map_eq_count' e _ e.injective]
+
+/-- An equivalence between `σ` and `τ` induces an equivalence between the subtypes of `Sym σ n` and
+`Sym τ n` corresponding to a given partition. -/
+def ofSymShapeEquiv (μ : Partition n) (e : σ ≃ τ) :
+    {x : Sym σ n // ofSym x = μ} ≃ {x : Sym τ n // ofSym x = μ} where
+  toFun := fun x => ⟨Sym.equivCongr e x, by simp [ofSym_map, x.2]⟩
+  invFun := fun x => ⟨Sym.equivCongr e.symm x, by simp [ofSym_map, x.2]⟩
+  left_inv := by intro x; simp
+  right_inv := by intro x; simp
+
 /-- The partition of exactly one part. -/
 def indiscrete (n : ℕ) : Partition n := ofSums n {n} rfl
 #align nat.partition.indiscrete_partition Nat.Partition.indiscrete
@@ -123,6 +151,9 @@ instance UniquePartitionZero : Unique (Partition 0) where
 instance UniquePartitionOne : Unique (Partition 1) where
   uniq _ := Partition.ext _ _ <| by simp
 
+@[simp] lemma ofSym_one (s : Sym σ 1) : ofSym s = indiscrete 1 := by
+  ext; simp
+
 /-- The number of times a positive integer `i` appears in the partition `ofSums n l hl` is the same
 as the number of times it appears in the multiset `l`.
 (For `i = 0`, `Partition.non_zero` combined with `Multiset.count_eq_zero_of_not_mem` gives that

Mathlib/Data/Sym/Basic.lean

@@ -534,6 +534,19 @@ theorem mem_append_iff {s' : Sym α m} : a ∈ s.append s' ↔ a ∈ s ∨ a ∈
   Multiset.mem_add
 #align sym.mem_append_iff Sym.mem_append_iff
 
+/-- `a ↦ {a}` as an equivalence between `α` and `Sym α 1`. -/
+@[simps apply]
+def oneEquiv : α ≃ Sym α 1 where
+  toFun a := ⟨{a}, by simp⟩
+  invFun s := (Equiv.subtypeQuotientEquivQuotientSubtype
+      (·.length = 1) _ (fun l ↦ Iff.rfl) (fun l l' ↦ by rfl) s).liftOn
+    (fun l ↦ l.1.head <| List.length_pos.mp <| by simp)
+    fun ⟨_, _⟩ ⟨_, h⟩ ↦ fun perm ↦ by
+      obtain ⟨a, rfl⟩ := List.length_eq_one.mp h
+      exact List.eq_of_mem_singleton (perm.mem_iff.mp <| List.head_mem _)
+  left_inv a := by rfl
+  right_inv := by rintro ⟨⟨l⟩, h⟩; obtain ⟨a, rfl⟩ := List.length_eq_one.mp h; rfl
+
 /-- Fill a term `m : Sym α (n - i)` with `i` copies of `a` to obtain a term of `Sym α n`.
 This is a convenience wrapper for `m.append (replicate i a)` that adjusts the term using
 `Sym.cast`. -/