a(n) = Sum_{d|3} phi(d)*n^(3/d).

Terms

a(0) =0a(1) =3a(2) =12a(3) =33a(4) =72a(5) =135a(6) =228a(7) =357a(8) =528a(9) =747a(10) =1020a(11) =1353a(12) =1752a(13) =2223a(14) =2772a(15) =3405a(16) =4128a(17) =4947a(18) =5868a(19) =6897a(20) =8040a(21) =9303a(22) =10692a(23) =12213a(24) =13872a(25) =15675a(26) =17628a(27) =19737a(28) =22008a(29) =24447