proposition: Lemma center_class_formula G : #|G| = #|'Z(G)| + \sum_(xG in [set x ^: G | x in G :\: 'C(G)]) #|xG|.
proof: Proof. by rewrite acts_sum_card_orbit ?cardsID // astabsJ normsD ?norms_cent ?normG. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient action gproduct gfunctor) mathcomp(cyclic)