a(n) = (2*n^3 + 5*n^2 - 13*n)/2.

Terms

a(0) =-3a(1) =5a(2) =30a(3) =78a(4) =155a(5) =267a(6) =420a(7) =620a(8) =873a(9) =1185a(10) =1562a(11) =2010a(12) =2535a(13) =3143a(14) =3840a(15) =4632a(16) =5525a(17) =6525a(18) =7638a(19) =8870a(20) =10227a(21) =11715a(22) =13340a(23) =15108a(24) =17025a(25) =19097a(26) =21330a(27) =23730a(28) =26303a(29) =29055