a(n) = sigma_3(3n+2).

Terms

a(0) =9a(1) =126a(2) =585a(3) =1332a(4) =3096a(5) =4914a(6) =9198a(7) =12168a(8) =19782a(9) =24390a(10) =37449a(11) =43344a(12) =61740a(13) =68922a(14) =97236a(15) =103824a(16) =141759a(17) =148878a(18) =201240a(19) =205380a(20) =268128a(21) =276948a(22) =358722a(23) =357912a(24) =455886a(25) =458208a(26) =589806a(27) =571788a(28) =715572a(29) =704970