Finch, S. R. Mathematical Constants. Cambridge, England: Cambridge University Press, p. 119, 2003.Finch,
S. R. "Errata and Addenda to Mathematical Constants." Mar. 6,
2015. https://oeis.org/A106533/a106533.pdf.Nesterenko,
Yu. V. "Modular Functions and Transcendence Questions." Sb. Math.187,
1319-1348, 1996. https://doi.org/10.1070/SM1996v187n09ABEH000158.Sloane,
N. J. A. Sequence A141848 in "The
On-Line Encyclopedia of Integer Sequences."Stevenhagen, P. "The
Number of Real Quadratic Fields Having Units of Negative Norm." Experiment.
Math.2, 121-136, 1993.