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References
Abramowitz, M. and Stegun, C. A. (Eds.).  ``Riemann's Differential Equation.''  §15.6 in
  Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, 9th printing.
  New York: Dover, pp. 564-565, 1972.
 
Morse, P. M. and Feshbach, H.  Methods of Theoretical Physics, Part I.  New York: McGraw-Hill, pp. 541-543, 1953.