a(n) = (3 + 2*n - 3*n^2 + 4*n^3 - 3*((-1 + n) mod 2))/6.

Terms

a(0) =1a(1) =4a(2) =15a(3) =36a(4) =73a(5) =128a(6) =207a(7) =312a(8) =449a(9) =620a(10) =831a(11) =1084a(12) =1385a(13) =1736a(14) =2143a(15) =2608a(16) =3137a(17) =3732a(18) =4399a(19) =5140a(20) =5961a(21) =6864a(22) =7855a(23) =8936a(24) =10113a(25) =11388a(26) =12767a(27) =14252a(28) =15849a(29) =17560