%I
%S 3,5,7,17,19,89,107,521
%N Primes p such that 2^p  1 is a prime with primitive root 3.
%C If A096393(n) = 3, then A000043(n) belongs to this sequence.
%C 2^a(n)  1 is in A001123.
%C The first term that is unknown is a(9), which is either 2203 or 2281.
%H Discussion titled <a href="http://www.mersenneforum.org/showthread.php?t=5874">Conjecture about multiplicative order of 3 modulo a Mersenne prime</a>, (postings in mersenneforum.org), starting May 14 2006.
%H <a href="/index/Pri#primes_root">Index entries for primes by primitive root</a>
%t Select[Prime@Range[98], PrimitiveRoot[2^#  1] == 3 &]
%Y Subsequence of A000043.
%K hard,more,nonn
%O 1,1
%A _Arkadiusz Wesolowski_, Nov 20 2012
