a(n) = 1 + n + ((n-1)*n^2)/2.

Terms

a(0) =1a(1) =2a(2) =5a(3) =13a(4) =29a(5) =56a(6) =97a(7) =155a(8) =233a(9) =334a(10) =461a(11) =617a(12) =805a(13) =1028a(14) =1289a(15) =1591a(16) =1937a(17) =2330a(18) =2773a(19) =3269a(20) =3821a(21) =4432a(22) =5105a(23) =5843a(24) =6649a(25) =7526a(26) =8477a(27) =9505a(28) =10613a(29) =11804