Smallest k such that sigma(2^(k*n)) is prime.

Terms

a(0) =1a(1) =1a(2) =2a(3) =1a(4) =6a(5) =1a(6) =18a(7) =2a(8) =2a(9) =3a(10) =8a(11) =1a(12) =40a(13) =9a(14) =2a(15) =1a(16) =177728a(17) =1a(18) =120a(19) =3a(20) =6a(21) =4a(22) =32906a(23) =95a(24) =868a(25) =20a(26) =1648a(27) =346a(28) =1996040a(29) =1