a(n) = (n+1)*(n^4-4*n^3+11*n^2-8*n+12)/12.

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =65a(5) =186a(6) =462a(7) =1016a(8) =2025a(9) =3730a(10) =6446a(11) =10572a(12) =16601a(13) =25130a(14) =36870a(15) =52656a(16) =73457a(17) =100386a(18) =134710a(19) =177860a(20) =231441a(21) =297242a(22) =377246a(23) =473640a(24) =588825a(25) =725426a(26) =886302a(27) =1074556a(28) =1293545a(29) =1546890