This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A250291 #22 Feb 26 2025 02:10:32 %S A250291 29,37,41,47,49,53,67,71,73,103,107,109,139,151,179,223,229,251,269, %T A250291 277,311,349,353,433,457,487,503,599,601,613,619,643,739,757,827,839, %U A250291 1031,1061,1117,1123,1217 %N A250291 Numbers k such that (2^k+1)/3 is a semiprime. %C A250291 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 A250291 a(42) >= 1259. %H A250291 Samuel Wagstaff, <a href="http://www.cerias.purdue.edu/homes/ssw/cun/index.html">The Cunningham Project</a>. %e A250291 a(1) = 29 so (2^29+1)/3 = 178956971 = 59 * 3033169 is a semiprime. %Y A250291 Cf. A000978, A085724, A007583. %K A250291 nonn,more %O A250291 1,1 %A A250291 _Eric Chen_, Dec 24 2014 %E A250291 a(40)-a(41) from _Max Alekseyev_, Feb 25 2025