# A006157 (b-file synthesized from sequence entry) 2 1 3 5 4 28 5 180 6 1320 7 10920 8 100800 9 1028160 10 11491200 11 139708800 12 1836172800 13 25945920000 14 392302310400 15 6320426112000 16 108101081088000 17 1956280854528000 18 37347179950080000 19 750144785854464000 20 15813863053148160000 21 349121438173347840000