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

Terms

a(0) =1a(1) =5a(2) =21a(3) =57a(4) =121a(5) =221a(6) =365a(7) =561a(8) =817a(9) =1141a(10) =1541a(11) =2025a(12) =2601a(13) =3277a(14) =4061a(15) =4961a(16) =5985a(17) =7141a(18) =8437a(19) =9881a(20) =11481a(21) =13245a(22) =15181a(23) =17297a(24) =19601a(25) =22101a(26) =24805a(27) =27721a(28) =30857a(29) =34221