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%I #2 Mar 31 2012 13:20:28
%S 5,19,327235662833,
%T 578175370366880553282134492422436321419543414585625120508329411643068012549226892303,
%U 39731908913255031966162449696446781074231732174358868548789339497630379824042353480418749055951
%N Primes in A122150[n] = Numerator[ Sum[ (-1)^(k+1) * 1/2^Prime[k], {k,1,n} ] ].
%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,...}. Indices of primes in A122150[n] are listed in A122151[n] = {3,4,13,60,66,75,175,...}.
%F a(n) = A122150[ A122151[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, A122151, A122153.
%K nonn
%O 1,1
%A _Alexander Adamchuk_, Aug 22 2006
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