In addition to the four Appell functions there are other sums of double
series that cannot be expressed as a product of two
functions, and which satisfy pairs of linear partial differential equations of
the second order. Two examples are provided by
16.14.5 |
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16.14.6 |
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(The region of convergence is not quite maximal.) See
ErdΓ©lyi et al. (1953a, §§5.7.1β5.7.2) for further information.