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 A092561 #20 Jun 18 2022 04:04:19 %S A092561 15,511,2047,8388607,137438953471,2199023255551,562949953421311, %T A092561 576460752303423487,147573952589676412927,9671406556917033397649407, %U A092561 158456325028528675187087900671,2535301200456458802993406410751 %N A092561 "Mersenne" semiprimes, semiprimes of the form 2^k-1. %H A092561 Amiram Eldar, <a href="/A092561/b092561.txt">Table of n, a(n) for n = 1..42</a> %F A092561 a(n) = A000225(A085724(n)). - _Amiram Eldar_, Jun 18 2022 %e A092561 2047 is a member because 2047 = 2^11 - 1 = 23*89. %t A092561 a := Select[Range[2,120], ! PrimeQ[2^# - 1] && Length[Divisors[2^# - 1]] <= 4 &]; 2^a - 1 (* _Stefan Steinerberger_, Apr 12 2006 *) %t A092561 Select[2^Range[0,110]-1, PrimeOmega[#] == 2&] (* _Harvey P. Dale_, Feb 22 2013 *) %Y A092561 Corresponding k: A085724. %Y A092561 Cf. A000225, A092558, A092559, A092562. %K A092561 nonn %O A092561 1,1 %A A092561 _Zak Seidov_, Feb 27 2004 %E A092561 One more term from _Stefan Steinerberger_, Apr 12 2006