a(n) = n^3/6 + 3*n^2/4 + 7*n/3 + 7/8 + (-1)^n/8.

Terms

a(0) =1a(1) =4a(2) =10a(3) =19a(4) =33a(5) =52a(6) =78a(7) =111a(8) =153a(9) =204a(10) =266a(11) =339a(12) =425a(13) =524a(14) =638a(15) =767a(16) =913a(17) =1076a(18) =1258a(19) =1459a(20) =1681a(21) =1924a(22) =2190a(23) =2479a(24) =2793a(25) =3132a(26) =3498a(27) =3891a(28) =4313a(29) =4764