Numbers n such that both sigma(n) and sigma(sigma(n)) are odd numbers.

Terms

a(0) =1a(1) =81a(2) =400a(3) =32400a(4) =195938a(5) =224450a(6) =1705636a(7) =3648100a(8) =13645088a(9) =15870978a(10) =18180450a(11) =29184800a(12) =138156516a(13) =295496100a(14) =1055340196a(15) =1105252128a(16) =1476326929a(17) =2263475776a(18) =2323432804a(19) =2363968800a(20) =2592846400a(21) =2661528100a(22) =2686005218a(23) =2917410498a(24) =3564550178