a(n) = floor(n^3/9).

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =7a(5) =13a(6) =24a(7) =38a(8) =56a(9) =81a(10) =111a(11) =147a(12) =192a(13) =244a(14) =304a(15) =375a(16) =455a(17) =545a(18) =648a(19) =762a(20) =888a(21) =1029a(22) =1183a(23) =1351a(24) =1536a(25) =1736a(26) =1952a(27) =2187a(28) =2439a(29) =2709