%I #15 Dec 24 2014 23:30:38
%S 29,37,41,47,49,53,67,71,73,103,107,109,139,151,179,223,229,251,269,
%T 277,311,349,353,433,457,487,503,599,601,613,619,643,739,757,827,839,
%U 1031,1061,1117
%N Numbers n such that (2^n+1)/3 is a semiprime.
%C First unknown: 1123.
%C If (2^n+1)/3 is a semiprime, n must be prime or the square of a prime; the only known square of a prime in this sequence is 49.
%H Samuel Wagstaff, <a href="http://www.cerias.purdue.edu/homes/ssw/cun/index.html">The Cunningham Project</a>
%e a(1) = 29 so (2^29+1)/3 = 178956971 = 59 * 3033169 is a semiprime.
%Y Cf. A000978, A085724, A007583.
%K nonn
%O 1,1
%A _Eric Chen_, Dec 24 2014