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 A176896 #19 Feb 01 2017 09:12:37 %S A176896 9,21,51,69,77,93,111,115,155,183,187,213,237,247,259,267,287,291,319, %T A176896 323,339,355,371,403,407,411,413,427,437,453,471,519,535,579,583,591, %U A176896 597,611,629,655,669,671,679,717,723,731,755,763,789,791,807,815,831 %N A176896 Semiprimes s such that (s-1)/2 is also semiprime. %C A176896 Semiprime analog of safe primes (A005385). - _Zak Seidov_, Feb 01 2017 %H A176896 Zak Seidov, <a href="/A176896/b176896.txt">Table of n, a(n) for n = 1..1000</a> %F A176896 a(n) = 2*A111153(n) + 1. - _Zak Seidov_, Sep 10 2012 %e A176896 a(1)=9 because (3*3 - 1)/2 = 4 = 2*2; a(2)=21 because (3*7 - 1)/2 = 10 = 2*5. %t A176896 Select[Range[1000],PrimeOmega[#]==PrimeOmega[(#-1)/2]==2&] (* _Harvey P. Dale_, Jan 28 2016 *) %Y A176896 Cf. A001358, A005385, A111153. %K A176896 nonn %O A176896 1,1 %A A176896 _Juri-Stepan Gerasimov_, Apr 28 2010 %E A176896 Corrected (497 and 745 removed) by _R. J. Mathar_, May 01 2010