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A176896 Semiprimes s such that (s-1)/2 is also semiprime. 3

%I

%S 9,21,51,69,77,93,111,115,155,183,187,213,237,247,259,267,287,291,319,

%T 323,339,355,371,403,407,411,413,427,437,453,471,519,535,579,583,591,

%U 597,611,629,655,669,671,679,717,723,731,755,763,789,791,807,815,831

%N Semiprimes s such that (s-1)/2 is also semiprime.

%C Semiprime analog of safe primes (A005385). - _Zak Seidov_, Feb 01 2017

%H Zak Seidov, <a href="/A176896/b176896.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) = 2*A111153(n) + 1. - _Zak Seidov_, Sep 10 2012

%e a(1)=9 because (3*3 - 1)/2 = 4 = 2*2; a(2)=21 because (3*7 - 1)/2 = 10 = 2*5.

%t Select[Range[1000],PrimeOmega[#]==PrimeOmega[(#-1)/2]==2&] (* _Harvey P. Dale_, Jan 28 2016 *)

%Y Cf. A001358, A005385, A111153.

%K nonn

%O 1,1

%A _Juri-Stepan Gerasimov_, Apr 28 2010

%E Corrected (497 and 745 removed) by _R. J. Mathar_, May 01 2010

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Last modified May 16 11:54 EDT 2021. Contains 343942 sequences. (Running on oeis4.)