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%I #10 Mar 31 2012 13:22:09
%S 1,1,1,1,1,1,3,3,1,1,5,5,3,9,7,1,5,1,3,19,9,11,7,1,11,1,27,25,9,11,3,
%T 1,21,19,1,19,3,1,7,9,9,3,27,5,1,5,5,7,3,1,41,1,9,17,1,11,1,27,3,105,
%U 33,15,23,29,19,43,37,15,3,11,27,9,57,5,3,3,35
%N Least odd number k such that (k*2^n-1)*k*2^n-1 or (k*2^n-1)*k*2^n+1 or (k*2^n+1)*k*2^n-1 or (k*2^n+1)*k*2^n+1 is prime.
%C As N increases, it appears that (sum_{k=1..N} a(k)) / (sum_{k=1..N} k) tends to 0.24
%H Pierre CAMI, <a href="/A187371/b187371.txt">Table of n, a(n) for n = 1..4000</a>
%F a(n) = least of A187367(n), A187368(n), A187369(n), A187370(n).
%t Table[k = 1; While[! PrimeQ[(k*2^n - 1)*k*2^n - 1] && ! PrimeQ[(k*2^n - 1)*k*2^n + 1] && ! PrimeQ[(k*2^n + 1)*k*2^n - 1] && ! PrimeQ[(k*2^n + 1)*k*2^n + 1], k = k + 2]; k, {n, 100}]
%Y Cf. A187367, A187368, A187369, A187370.
%K nonn
%O 1,7
%A _Pierre CAMI_, Mar 09 2011
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