a(n) = (8 - 2*n + 11*n^2 - 6*n^3 + n^4)/4.

Terms

a(0) =3a(1) =4a(2) =5a(3) =12a(4) =37a(5) =98a(6) =219a(7) =430a(8) =767a(9) =1272a(10) =1993a(11) =2984a(12) =4305a(13) =6022a(14) =8207a(15) =10938a(16) =14299a(17) =18380a(18) =23277a(19) =29092a(20) =35933a(21) =43914a(22) =53155a(23) =63782a(24) =75927a(25) =89728a(26) =105329a(27) =122880a(28) =142537a(29) =164462