a(n) = binomial(n,20).

Terms

a(0) =1a(1) =21a(2) =231a(3) =1771a(4) =10626a(5) =53130a(6) =230230a(7) =888030a(8) =3108105a(9) =10015005a(10) =30045015a(11) =84672315a(12) =225792840a(13) =573166440a(14) =1391975640a(15) =3247943160a(16) =7307872110a(17) =15905368710a(18) =33578000610a(19) =68923264410a(20) =137846528820a(21) =269128937220a(22) =513791607420a(23) =960566918220