1. F. R. Gantmacher and M. G. Krein, Oscillation Matrices and Kernels and Small Oscillations of Mechanical Systems (Gostekhizdat, Moscow, 1950) [in Russian].
2. O. D. Kellogg, “Orthogonal function sets arising from integral equations,” Amer. J. Math 40 (2), 145–154 (1918).
3. O. D. Kellogg, “Interpolation properties of orthogonal sets of solutions of differential equations,” Amer. J. Math 40 (3), 225–234 (1918).
4. P. D. Kalafaty, “Sur les fonctions de Green des équations différentielles ordinaires,” Dokl. Akad. Nauk SSSR 26 (6), 535–539 (1940).
5. J. M. Karon, “The sign-regularity properties of a class of the Green functions for ordinary differential equations,” J. Differential Equations 6 (3), 484–502 (1969).