1. General theory of linear difference equations;Birkhoff;Trans. Amer. Math. Soc.,1911
2. The generalized Riemann problem for linear differential equations and the allied problems for linear difference and q-difference equations;Birkhoff,1913
3. Note on linear difference and differential equations;Birkhoff,1941
4. Linear difference equations and their analytic solutions;Carmichael;Trans. Amer. Math. Soc.,1911
5. The general theory of linear q-difference equations;Carmichael;Amer. Math. Soc.,1912