a(n) = (9 + 14*n + 12*n^2 + 4*n^3)/3.

Terms

a(0) =3a(1) =13a(2) =39a(3) =89a(4) =171a(5) =293a(6) =463a(7) =689a(8) =979a(9) =1341a(10) =1783a(11) =2313a(12) =2939a(13) =3669a(14) =4511a(15) =5473a(16) =6563a(17) =7789a(18) =9159a(19) =10681a(20) =12363a(21) =14213a(22) =16239a(23) =18449a(24) =20851a(25) =23453a(26) =26263a(27) =29289a(28) =32539a(29) =36021