a(n) = ceiling(n^3/3).

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =22a(5) =42a(6) =72a(7) =115a(8) =171a(9) =243a(10) =334a(11) =444a(12) =576a(13) =733a(14) =915a(15) =1125a(16) =1366a(17) =1638a(18) =1944a(19) =2287a(20) =2667a(21) =3087a(22) =3550a(23) =4056a(24) =4608a(25) =5209a(26) =5859a(27) =6561a(28) =7318a(29) =8130