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

Terms

a(0) =1a(1) =2a(2) =6a(3) =15a(4) =31a(5) =56a(6) =92a(7) =141a(8) =205a(9) =286a(10) =386a(11) =507a(12) =651a(13) =820a(14) =1016a(15) =1241a(16) =1497a(17) =1786a(18) =2110a(19) =2471a(20) =2871a(21) =3312a(22) =3796a(23) =4325a(24) =4901a(25) =5526a(26) =6202a(27) =6931a(28) =7715a(29) =8556