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

Terms

a(0) =2a(1) =2a(2) =4a(3) =2a(4) =25a(5) =2a(6) =59049a(7) =4a(8) =4a(9) =5a(10) =256a(11) =2a(12) =282475249a(13) =243a(14) =4a(15) =2a(16) =729a(17) =2a(19) =8a(20) =64a(21) =16a(22) =25a(23) =1331a(24) =594823321a(25) =16807a(26) =38950081a(27) =151a(28) =361a(29) =2a(31) =19902511