a(n) = 27*n^2/2 + 45*n/2 - 12 (n>=1).

Terms

a(0) =24a(1) =87a(2) =177a(3) =294a(4) =438a(5) =609a(6) =807a(7) =1032a(8) =1284a(9) =1563a(10) =1869a(11) =2202a(12) =2562a(13) =2949a(14) =3363a(15) =3804a(16) =4272a(17) =4767a(18) =5289a(19) =5838a(20) =6414a(21) =7017a(22) =7647a(23) =8304a(24) =8988a(25) =9699a(26) =10437a(27) =11202a(28) =11994a(29) =12813