Numbers n such that n and sigma(n) are prime powers (of the form p^k, p prime, k>=1).

Terms

a(0) =2a(1) =3a(2) =4a(3) =7a(4) =9a(5) =16a(6) =25a(7) =31a(8) =64a(9) =81a(10) =127a(11) =289a(12) =729a(13) =1681a(14) =2401a(15) =3481a(16) =4096a(17) =5041a(18) =7921a(19) =8191a(20) =10201a(21) =15625a(22) =17161a(23) =27889a(24) =28561a(25) =29929a(26) =65536a(27) =83521a(28) =85849a(29) =131071