a(n) = binomial(n,21).

Terms

a(0) =1a(1) =22a(2) =253a(3) =2024a(4) =12650a(5) =65780a(6) =296010a(7) =1184040a(8) =4292145a(9) =14307150a(10) =44352165a(11) =129024480a(12) =354817320a(13) =927983760a(14) =2319959400a(15) =5567902560a(16) =12875774670a(17) =28781143380a(18) =62359143990a(19) =131282408400a(20) =269128937220a(21) =538257874440