1. General theory of linear difference equations;Birkhoff, George D.;Trans. Amer. Math. Soc.,1911
2. Formal theory of irregular linear difference equations;Birkhoff, George D.;Acta Math.,1930
3. Analytic theory of singular difference equations;Birkhoff, George D.;Acta Math.,1933
4. Linear systems of difference equations;Harris, W. A., Jr.;Contributions to Differential Equations,1963
5. Equivalent classes of difference equations;Harris, W. A., Jr.;Contributions to Differential Equations,1963