# A181964 (b-file synthesized from sequence entry) 1 1 2 1 3 6 4 36 5 240 6 2160 7 20160 8 241920 9 2903040 10 39916800 11 578793600 12 9580032000 13 161902540800 14 3007651046400 15 58845346560000 16 1234444603392000 17 26854400821248000 18 624231436308480000 19 15083992450695168000 20 385614968295997440000