# A024420 (b-file synthesized from sequence entry) 0 1 1 1 2 1 3 4 4 22 5 108 6 612 7 4416 8 36576 9 331200 10 3319200 11 36806400 12 445046400 13 5813579520 14 81716947200 15 1230656716800 16 19761225523200 17 336973967769600 18 6082189179494400 19 115851849523200000 20 2322322137354240000 21 48869666136023040000