a(n) is the smallest number k such that sigma(k) and sigma(k^n) are both primes.

Terms

a(0) =2a(1) =2a(2) =4a(3) =2a(4) =25a(5) =2a(6) =262144a(7) =4a(8) =4a(9) =64a(10) =734449a(11) =2a(13) =9066121a(14) =4a(15) =2a(16) =729a(17) =2a(19) =5041a(20) =64a(21) =16a(22) =25a(24) =16610312161a(25) =2607021481a(26) =38950081a(28) =5331481a(29) =2