# A013174 (b-file synthesized from sequence entry) 0 1 1 2 2 4 3 8 4 16 5 80 6 640 7 4160 8 21760 9 177920 10 2291200 11 25548800 12 223436800 13 2560563200 14 44438118400 15 702660608000 16 8725110784000 17 129295622144000 18 2796896567296000 19 57197188186112000 20 926039081943040000