A.k.a. the Watson Quintuple Product Identity.
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(1) |
(2) |
(3) |
(4) |
See also Jacobi Triple Product, Ramanujan Theta Functions
References
Berndt, B. C. Ramanujan's Notebooks, Part III. New York: Springer-Verlag, 1985.
Borwein, J. M. and Borwein, P. B.
Pi & the AGM: A Study in Analytic Number Theory and Computational Complexity.
New York: Wiley, pp. 306-309, 1987.
Gasper, G. and Rahman, M. Basic Hypergeometric Series. Cambridge, England: Cambridge University Press, 1990.