%N Numbers n such that the n-th prime is of the form 3k-1.
%C A003627 indexed by A000040.
%F a(n)=k such that A000040(k) = A003627(n).
%t PrimePi/@Select[3Range[0,250]-1,PrimeQ] (* From Harvey P. Dale, Apr 26 2011 *)
%Y A003627 gives primes arising in A024893, A087370, A088879.
%Y A133677 is another version.
%A _Ray Chandler_, Dec 26 2003