a(n) = (4*n^3 - 9*n^2 + 11*n + 3)/3.

Terms

a(0) =1a(1) =3a(2) =7a(3) =21a(4) =53a(5) =111a(6) =203a(7) =337a(8) =521a(9) =763a(10) =1071a(11) =1453a(12) =1917a(13) =2471a(14) =3123a(15) =3881a(16) =4753a(17) =5747a(18) =6871a(19) =8133a(20) =9541a(21) =11103a(22) =12827a(23) =14721a(24) =16793a(25) =19051a(26) =21503a(27) =24157a(28) =27021a(29) =30103