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a(n) = smallest k such that k*n*2^n+1 is prime.
1

%I #13 Jun 15 2017 22:54:53

%S 1,2,3,3,4,2,3,6,4,4,9,3,3,5,9,7,3,3,3,5,3,7,19,5,5,2,3,7,7,9,5,15,3,

%T 10,10,7,14,6,8,6,25,6,50,45,13,4,18,31,27,2,4,33,18,2,41,18,10,9,6,7,

%U 3,32,4,39,18,17,11,30,17,18,7,51,38,11,15,13,9,10,24,2

%N a(n) = smallest k such that k*n*2^n+1 is prime.

%C If k = 1 then n*2^n+1 is a Cullen prime (A050920).

%H Pierre CAMI, <a href="/A288159/b288159.txt">Table of n, a(n) for n = 1..10000</a>

%t Table[k = 1; While[! PrimeQ[k n*2^n + 1], k++]; k, {n, 80}] (* _Michael De Vlieger_, Jun 09 2017 *)

%o (PARI) a(n) = my(k=1); while (! isprime(k*n*2^n+1), k++); k; \\ _Michel Marcus_, Jun 07 2017

%Y Cf. A050920, A288158.

%K nonn

%O 1,2

%A _Pierre CAMI_, Jun 06 2017