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

Terms

a(0) =1a(1) =3a(2) =9a(3) =27a(4) =65a(5) =131a(6) =233a(7) =379a(8) =577a(9) =835a(10) =1161a(11) =1563a(12) =2049a(13) =2627a(14) =3305a(15) =4091a(16) =4993a(17) =6019a(18) =7177a(19) =8475a(20) =9921a(21) =11523a(22) =13289a(23) =15227a(24) =17345a(25) =19651a(26) =22153a(27) =24859a(28) =27777a(29) =30915