1. Commutative ordinary differential operators I;Burchnall;Proc. London Math. Soc.,1922
2. Commutative ordinary differential operators II;Burchnall;Proc. Roy. Soc. London,1928
3. Discrete calculus of variations;Cadzow;Internat. J. Control,1970
4. Modern Geometry, Methods and Applications, Part II. The Geometry and Topology of Manifolds;Dubrovin,1985
5. Differential Equations and Variational Calculus;Kartaschev,1986