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

Terms

a(0) =1a(1) =6a(2) =17a(3) =37a(4) =69a(5) =116a(6) =181a(7) =267a(8) =377a(9) =514a(10) =681a(11) =881a(12) =1117a(13) =1392a(14) =1709a(15) =2071a(16) =2481a(17) =2942a(18) =3457a(19) =4029a(20) =4661a(21) =5356a(22) =6117a(23) =6947a(24) =7849a(25) =8826a(26) =9881a(27) =11017a(28) =12237a(29) =13544