A250291 - OEIS

%I #22 Feb 26 2025 02:10:32

%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 1031,1061,1117,1123,1217

%N Numbers k such that (2^k+1)/3 is a semiprime.

%C If (2^k+1)/3 is a semiprime, k must be prime or the square of a prime; the only known square of a prime in this sequence is 49.

%C a(42) >= 1259.

%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,more

%O 1,1

%A _Eric Chen_, Dec 24 2014

%E a(40)-a(41) from _Max Alekseyev_, Feb 25 2025