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%I #17 Oct 28 2023 09:24:31
%S 9,15,21,33,35,39,49,51,55,57,69,77,87,93,111,115,123,129,141,159,177,
%T 183,201,203,213,219,237,249,267,291,295,303,309,321,327,329,339,361,
%U 377,381,393,411,417,437,447,451,453,471,473,489,501,515,519
%N Semiprimes s such that 2^(s-1) mod s is also a semiprime.
%C Semiprimes s such that A062173(s) is also a semiprime.
%H François Huppé, <a href="/A175626/b175626.txt">Table of n, a(n) for n = 1..10000</a>
%e 9 is in the sequence because 9 and (2^(9-1) mod 9)=4 are both semiprime.
%t semiPrimeQ[x_] := Plus @@ Last /@ FactorInteger@ x == 2; fQ[n_] := semiPrimeQ@ n && semiPrimeQ@ PowerMod[2, n - 1, n]; Select[ Range@ 525, fQ]
%Y Cf. A001358, A062173, A175168.
%K nonn
%O 1,1
%A _Juri-Stepan Gerasimov_, Dec 04 2010
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