a(n) = (n + 1)*(4*n^2 + 14*n + 9)/3.

Terms

a(0) =3a(1) =18a(2) =53a(3) =116a(4) =215a(5) =358a(6) =553a(7) =808a(8) =1131a(9) =1530a(10) =2013a(11) =2588a(12) =3263a(13) =4046a(14) =4945a(15) =5968a(16) =7123a(17) =8418a(18) =9861a(19) =11460a(20) =13223a(21) =15158a(22) =17273a(23) =19576a(24) =22075a(25) =24778a(26) =27693a(27) =30828a(28) =34191a(29) =37790