|
%I #31 Mar 31 2012 13:22:06
%S 1,1,3,1,3,5,7,0,8,7,6,5,12,11,15,1,15,13,18,17,12,7,18,9,0,21,25,21,
%T 18,25,30,0,28,27,29,33,36,23,30,0,39,41,0,17,42,43,35,0,30,29,41,21,
%U 47,45,51,0,55,0,47,41,51,21,23,0,47,63,66,65,58
%N Greatest k <= n such that 2^n+2^k+1 is prime, or 0 if no such k exists.
%C Up to n=6601, a(n) = 0 for 916 values of n and a(n) >= n/2 for 4574 values of n.
%H Pierre CAMI, <a href="/A179760/b179760.txt">Table of n, a(n) for n = 1..10000</a>
%t Table[k=n; While[k>0 && ! PrimeQ[2^n+2^k+1], k--]; k, {n, 100}]
%o (PARI) A179760(n)={forstep(i=n,1,-1, if(isprime(2^n+2^i+1),return(i)));return(0)}
%Y Cf. A106821, A178993, A179291, A179013.
%K nonn
%O 1,3
%A _Pierre CAMI_, Jan 12 2011
|
