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Least prime p such that 3*p*2^n-1 is prime.
3

%I #8 Sep 23 2019 10:20:59

%S 2,2,2,2,5,2,2,3,11,17,2,23,29,3,5,3,43,2,7,17,19,3,11,23,13,19,19,7,

%T 43,23,71,5,19,2,59,11,59,2,47,31,89,5,2,3,19,107,11,71,59,23,29,7,61,

%U 89,2,173,11,17,37,37,31,23,19,2,199,103,59,37,139,17,67,41,43

%N Least prime p such that 3*p*2^n-1 is prime.

%H Pierre CAMI, <a href="/A130325/b130325.txt">Table of n, a(n) for n = 0..500</a>

%e 3*2*2^0-1=5 is prime so for n=0 p=2.

%e 3*2*2^1-1=11 is prime so for n=1 p=2.

%o (PARI) a(n) = my(p=2); while (!isprime(3*p*2^n-1), p=nextprime(p+1)); p; \\ _Michel Marcus_, Sep 23 2019

%Y Cf. A130326, A130327.

%K nonn

%O 0,1

%A _Pierre CAMI_, May 24 2007