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%I #19 Sep 08 2022 08:45:52
%S 3,3,3,15,45,3,99,45,51,141,153,177,411,45,45,267,237,75,75,207,111,
%T 111,123,159,57,375,1419,45,291,321,489,585,525,1623,579,45,27,1293,
%U 1059,255,2265,33,465,165,405,315,315,117,411,1725
%N a(n) is the least odd multiple of 3, k, such that k*2^n-1 is a Sophie Germain prime.
%H Pierre CAMI, <a href="/A176248/b176248.txt">Table of n, a(n) for n = 1..1073</a>
%e 3*2^6 - 1 = 3*64 - 1 = 191 that is the 15th Sophie Germain prime, so a(6) = 3. - _Bernard Schott_, Sep 16 2019
%o (PARI) a(n) = my(k=1, p); while(!(isprime(p=3*k*2^n-1) && isprime(2*p+1)), k+=2); 3*k; \\ _Michel Marcus_, Sep 16 2019
%o (Magma) sol:=[]; for n in [1..50] do k:=3; while not IsPrime(k*2^n-1) or not IsPrime(2*(k*2^n-1)+1) do k:=k+6; end while; Append(~sol,k); end for; sol; // _Marius A. Burtea_, Sep 16 2019
%Y Cf. A005384 (Sophie Germain primes).
%K nonn
%O 1,1
%A _Pierre CAMI_, Apr 13 2010
%E Name, data and b-file corrected by _Michel Marcus_, Sep 16 2019
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