# A144883 (b-file synthesized from sequence entry) 0 1 1 3 2 21 3 96 4 684 5 4320 6 35640 7 293760 8 2881440 9 29695680 10 345643200 11 4296499200 12 58518028800 13 851751936000 14 13338801100800 15 222145265664000 16 3934764739584000 17 73675580977152000 18 1455693852146688000 19 30238671742304256000 20 659012259039805440000