1. E. W. Hobson, The Theory of Spherical and Ellipsoidal Harmonics (Cambridge University, Cambridge, 1931; Inostrannaya Literatura, Moscow, 1952).
2. H. Bateman and A. Erdelyi, Higher Transcendental Functions, vol. 1 (McGraw-Hill, New York, 1953; Nauka, Moscow, 1965).
3. V. A. Antonov, E. I. Timoshkova, and K. V. Kholshevnikov, Introduction to the Theory of Newton Potential (Nauka, Moscow, 1988) [in Russian].
4. L. Ya. Adrianova, Introduction to the Theory of Linear Systems of Differential Equations (St. Peterb. Gos. Univ., St. Petersburg, 1992) [in Russian].
5. F. Tricomi, Differential Equations (Blackie & Son, London, 1961; URSS, Moscow, 2003).