A113916 - OEIS

%I #3 Oct 31 2013 12:17:35

%S 3,7,19,31,307,421

%N Numbers n such that (p^n-1)/(p-1) is prime where p is the n-th prime.

%C Note that n must itself be a prime.

%t Do[ p = Prime[n]; If[PrimeQ[(p^n - 1)/(p - 1)], Print[n]], {n, 1800}] (* _Robert G. Wilson v_ *)

%K nonn

%O 1,1

%A Joao da Silva (zxawyh66(AT)yahoo.com), Jan 30 2006