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

Terms

a(0) =1a(1) =3a(2) =8a(3) =19a(4) =41a(5) =81a(6) =148a(7) =253a(8) =409a(9) =631a(10) =936a(11) =1343a(12) =1873a(13) =2549a(14) =3396a(15) =4441a(16) =5713a(17) =7243a(18) =9064a(19) =11211a(20) =13721a(21) =16633a(22) =19988a(23) =23829a(24) =28201a(25) =33151a(26) =38728a(27) =44983a(28) =51969a(29) =59741