Doron Zeilberger Partial difference equations in m<sub>1</sub>>=m<sub>2</sub>>= ... >=m<sub>n</sub>>=0 and their applications to combinatorics. 65-77 1980 31 Discrete Mathematics 1 http://dx.doi.org/10.1016/0012-365X(80)90173-9 db/journals/dm/dm31.html#Zeilberger80