Euler, L. (1755). Institutiones calculi differentialis cum eius usu in analysi finitorum ac doctrina serierum. Impensis Academia Imperialis Scientiarum Petropolitan. Berolini ex officina Michlis, Reprint (1787), Ticini typographeo P. Galeatii [we refer to this reprint]. Also in (Euler, OO, Series I, 10). - References - Scientific Research Publishing