%I #8 Mar 07 2013 03:37:47

%S 1,1,2,1,6,3,5,12,3,5,8,6,5,3,18,8,41,5,11,14,66,8,2,9,18,8,21,12,30,

%T 16,6,1,6,8,33,2,53,23,2,5,21,15,6,12,20,5,5,28,53,80,11,53,9,5,56,2,

%U 120,5,23,16,9,8,5,9,24,25,62,131,41,51,12,3,99,56,2,37,155,3,23,55,29,9,2,33

%N Least a(n) such that a(n)*2^n*(2^n-1)+1 is prime.

%C Numbers n such that a(n)=2 are found in A006598. - _Michel Marcus_, Mar 07 2013

%e 1*2^5*(2^5-1)+1=993=3*331

%e 2*2^5*(2^5-1)+1=1985=5*397

%e 3*2^5*(2^5-1)+1=2977=13*229

%e 4*2^5*(2^5-1)+1=3969=3*1323

%e 5*2^5*(2^5-1)+1=4961=11*451

%e 6*2^5*(2^5-1)+1=5953 prime so a(5)=6

%o (PARI) a(n) = {k = 1; while (! isprime(k*2^n*(2^n-1) + 1), k++); return (k);} \\ _Michel Marcus_, Mar 07 2013

%K easy,nonn

%O 1,3

%A _Pierre CAMI_, Nov 18 2004