Reducible Linear Difference Operators. (Short Communications). Charles H. Franke — 1972 Aequationes mathematicae
A Characterization of Linear Difference Equations Which are Solvable by Elementary Operations (Short Communication) CHARLES H. FRANKE — 1973 Aequationes mathematicae
Linearly Reducible Difference Operators (Short Communication) CHARLES H. FRANKE — 1971 Aequationes mathematicae
A Characterization of Linear Difference Equations Which are Solvable by Elementary Operations. Charles H. Franke — 1974 Aequationes mathematicae