1. P. Hartman, Ordinary Differential Equations (Wiley, New York, 1964; Mir, Moscow, 1970).
2. B. Riemann, Works (Gostekhteorizdat, Moscow, 1948).
3. D. Hilbert, “Mathematische Probleme,” in Nachrichten von der Königlichen Gesellschaft der Wissenschaften zu Göttingen, Math.-Phys. Klasse (Paris, 1900), pp. 253–297.
4. N. P. Vekua, Systems of Singular Integral Equations and Some Boundary Value Problems (Nauka, Moscow, 1970) [in Russian].
5. J. Plemelj, Problems in the Sense of Riemann and Klein (Interscience, New York, 1964).