a(0)=1, a(n) = sigma_3(3n).

Terms

a(0) =1a(1) =28a(2) =252a(3) =757a(4) =2044a(5) =3528a(6) =6813a(7) =9632a(8) =16380a(9) =20440a(10) =31752a(11) =37296a(12) =55261a(13) =61544a(14) =86688a(15) =95382a(16) =131068a(17) =137592a(18) =183960a(19) =192080a(20) =257544a(21) =260408a(22) =335664a(23) =340704a(24) =442845a(25) =441028a(26) =553896a(27) =551881a(28) =703136a(29) =682920