1. Hardy, G. and Littlewood, J., Some Properties of Fractional Integrals, Math. Z., 1928, vol. 27, no. 4, pp. 565–606.
2. Kumykova, S.K., A Problem with Nonlocal Boundary Conditions on the Characteristics for an Equation of Mixed Type, Differ. Uravn., 1974, vol. 10, no. 1, pp. 78–88.
3. Denisova, Z.G., A Boundary Value Problem with Shift for the Equation sgn y × |y|m u xx + u yy = 0 in an Unbounded Domain, Differ. Uravn., 1978, vol. 14, no. 1, pp. 170–173.
4. Nakhushev, A.M., On Some Boundary Value Problems for Hyperbolic Equations and Equations of the Mixed Type, Differ. Uravn., 1969, vol. 5, no. 1, pp. 44–59.
5. Tricomi, F., Lezioni sulle equazioni a derivate parziali, Turin, 1954. Translated under the title Lektsii po uravneniyam v chastnykh proizvodnykh, Moscow: Inostrannaya literatura, 1957.