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%I #2 Mar 31 2012 13:20:28
%S 3,4,13,60,66,75,175
%N Indices n such that A122150[n] is a prime.
%C A122150[n] = Numerator[ Sum[ (-1)^(k+1) * 1/2^Prime[k], {k,1,n} ] ] begins {1,1,5,19,305,1219,19505,78019,1248305,79891519,319566077,20452228927, 327235662833,...}. Primes in A122150[n] are listed in A122152[n] = {5,19,327235662833,...}.
%F A122150[ a(n) ] = A122152[n].
%t Do[f=Numerator[Sum[(-1)^(k+1)*1/2^Prime[k],{k,1,n}]];If[PrimeQ[f],Print[{n,f}]],{n,1,1000}]
%Y Cf. A122150, A122152, A122153.
%K nonn
%O 1,1
%A _Alexander Adamchuk_, Aug 22 2006
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