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Let 
See also Lemniscate
References
Abramowitz, M. and Stegun, C. A. (Eds.).
  Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, 9th printing.
  New York: Dover, 1972.
 
Borwein, J. M. and Borwein, P. B.  Pi & the AGM: A Study in Analytic Number Theory and Computational Complexity.
  New York: Wiley, 1987.
 
Finch, S.  ``Favorite Mathematical Constants.''  http://www.mathsoft.com/asolve/constant/gauss/gauss.html
 
Le Lionnais, F.  Les nombres remarquables.  Paris: Hermann, p. 37, 1983.
 
Todd, J.  ``The Lemniscate Constant.''  Comm. ACM 18, 14-19 and 462, 1975.