* PDF copy of Appell's article at Goettingen University* PDF copy of a second article on Appell's equations and Gauss's principle
=== Публикации П. Аппеля по данному вопросу ===* PDF copy of Appell’s article at Goettingen University* PDF copy of a second article on Appell’s equations and Gauss’s principle* "Берёзкин Е.