proposition: Lemma odd_not_extremal2 : odd #|G| -> ~~ extremal2 G.
proof: Proof. rewrite /extremal2 /extremal_class; case: logn => // n'. case: andP => [[n_gt1 isoG] | _]. by rewrite (card_isog isoG) card_2dihedral ?oddX. case: andP => [[n_gt2 isoG] | _]. by rewrite (card_isog isoG) card_quaternion ?oddX. case: andP => [[n_gt3 isoG] | _]. by rewrite (card_isog isoG) card_semidihedral ?oddX. by case: ifP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset prime binomial) mathcomp(fingroup morphism perm automorphism presentation) mathcomp(quotient action commutator gproduct gfunctor) mathcomp(ssralg countalg finalg zmodp cyclic pgroup center gseries) mathcomp(nilpotent sylow abelian finmodule matrix maximal)