Numbers n such that sigma(n) is a prime power (A025475).

Terms

a(0) =3a(1) =7a(2) =21a(3) =31a(4) =81a(5) =93a(6) =127a(7) =217a(8) =381a(9) =400a(10) =651a(11) =889a(12) =2667a(13) =3937a(14) =8191a(15) =11811a(16) =24573a(17) =27559a(18) =57337a(19) =82677a(20) =131071a(21) =172011a(22) =253921a(23) =393213a(24) =524287a(25) =761763a(26) =917497a(27) =1040257a(28) =1572861a(29) =1777447