A formal extension of the Hypergeometric Function to two variables, resulting in four kinds of functions (Appell 1925),
Appell defined the functions in 1880, and Picard showed in 1881 that they may all be expressed by
Integrals of the form
References
Appell, P. ``Sur les fonctions hypergéométriques de plusieurs variables.'' In Mémoir. Sci. Math.
Paris: Gauthier-Villars, 1925.
Bailey, W. N. Generalised Hypergeometric Series.
Cambridge, England: Cambridge University Press, p. 73, 1935.
Iyanaga, S. and Kawada, Y. (Eds.). Encyclopedic Dictionary of Mathematics. Cambridge, MA: MIT Press,
p. 1461, 1980.