In addition to the four Appell functions there are 24 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


(The region of convergence
is not quite maximal.) See
Erdélyi et al. (1953a, §§5.7.1–5.7.2) for further information.