a(n) = binomial(n,19).

Terms

a(0) =1a(1) =20a(2) =210a(3) =1540a(4) =8855a(5) =42504a(6) =177100a(7) =657800a(8) =2220075a(9) =6906900a(10) =20030010a(11) =54627300a(12) =141120525a(13) =347373600a(14) =818809200a(15) =1855967520a(16) =4059928950a(17) =8597496600a(18) =17672631900a(19) =35345263800a(20) =68923264410a(21) =131282408400a(22) =244662670200a(23) =446775310800a(24) =800472431850