(n^3 - n + 15)/3.

Terms

a(0) =5a(1) =7a(2) =13a(3) =25a(4) =45a(5) =75a(6) =117a(7) =173a(8) =245a(9) =335a(10) =445a(11) =577a(12) =733a(13) =915a(14) =1125a(15) =1365a(16) =1637a(17) =1943a(18) =2285a(19) =2665a(20) =3085a(21) =3547a(22) =4053a(23) =4605a(24) =5205a(25) =5855a(26) =6557a(27) =7313a(28) =8125a(29) =8995