# A002303 (b-file synthesized from sequence entry) 4 16 5 272 6 3968 7 56320 8 814080 9 12207360 10 191431680 11 3149752320 12 54428774400 13 987559372800 14 18797300121600 15 374883257548800 16 7822865085235200 17 170560590520320000 18 3879770715684864000 19 91945674412720128000