idempotent representation 3 16G20 2 16S34 2 16W10 2 20C05 2 20M 2 47L30 2 94B15 2 94B60 2 Cayley table 2 Hilbert space 2 anti-isomorphism 2 char-complete set 2 characteristic property 2 dihedral group 2 group algebra 2 group code 2 minimal code 2 projection 2 quaternion group 2 semigroup 2 unitarization 2 16G60; 15A21 1 18G31 1 18N50 1 46K10 1 46L05 1 Dold--Kan correspondence 1 extended Dynkin diagram 1