E. V. . Ustilko and F. E. . Lomovtsev, “Matching conditions for values of characteristic oblique derivative at the end of a string, initial data and right-hand side of the wave equation”, Журнал Белорусского государственного университета. Математика. Информатика, no. 1, pp. 30–37, Mar. 2020, doi: 10.33581/2520-6508-2020-1-30-37.