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,...}.

Table of n, a(n) for n=1..7.

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: A243764 A201821 A001056 * A294384 A216868 A082732

Adjacent sequences: A122148 A122149 A122150 * A122152 A122153 A122154

nonn

Alexander Adamchuk, Aug 22 2006

approved