1. The theory of linear differential equations with periodic coefficients and time lag;Shimanov;Prikl. Mat. i mekh.,1963
2. A Floquet theory for functional differential equations;Stokes,1962
3. On difference differential equations with periodic coefficients;Hahn;J. Math. Anal. Appl.,1961
4. On the completeness of the eigensolutions of an equation with lags and periodic coefficients;Zverkin,1963
5. Periodic differential difference equations;Lillo;J. Math. Anal. Appl.,1966