1,1

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,...}.

A122150[ a(n) ] = A122152[n].

Do[f=Numerator[Sum[(-1)^(k+1)*1/2^Prime[k], {k, 1, n}]]; If[PrimeQ[f], Print[{n, f}]], {n, 1, 1000}]

Cf. A122150, A122152, A122153.

Sequence in context: A084315 A062165 A001056 this_sequence A082732 A009286 A076663

Adjacent sequences: A122148 A122149 A122150 this_sequence A122152 A122153 A122154

nonn

Alexander Adamchuk (alex(AT)kolmogorov.com), Aug 22 2006