In: Advanced Math
Let S be a non-empty set (finite or otherwise) and Σ the group of permutations on S. Suppose ∼ is an equivalence relation on S. Prove (a) {ρ ∈ Σ : x ∼ ρ(x) (∀x ∈ S)} is a subgroup of Σ. (b) The elements ρ ∈ Σ for which, for every x and y in S, ρ(x) ∼ ρ(y) if and only if x ∼ y is a subgroup of Σ.