Finch, S. R. Mathematical Constants. Cambridge, England: Cambridge University Press, p. 106, 2003.Heath-Brown,
D. R. and Moroz, B. Z. "The Density of Rational Points on the Cubic
Surface ."
Math. Proc. Cambridge Philos. Soc.125, 385-395, 1999.Niklasch,
G. "Some Number-Theoretical Constants." http://www.gn-50uma.de/alula/essays/Moree/Moree.en.shtml.Sloane,
N. J. A. Sequence A118228 in "The
On-Line Encyclopedia of Integer Sequences."