Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the 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