a(n) = 1 + (6 + (11 + (6 + n)*n)*n)*n/24.

Terms

a(0) =1a(1) =2a(2) =6a(3) =16a(4) =36a(5) =71a(6) =127a(7) =211a(8) =331a(9) =496a(10) =716a(11) =1002a(12) =1366a(13) =1821a(14) =2381a(15) =3061a(16) =3877a(17) =4846a(18) =5986a(19) =7316a(20) =8856a(21) =10627a(22) =12651a(23) =14951a(24) =17551a(25) =20476a(26) =23752a(27) =27406a(28) =31466a(29) =35961