1. Ul’yanov, P.L. (1977) Matematicheskaya entsyklopedia, Tom 1 (from 5 volumes). “Sovetskaya Encyclopedia” Publishing House, 958-959.
2. Sommerfeld, A. (1958) Partielle Differentialgleichungen der Physik, 4. Sauter, Akademische Verlagsgesellschaft Geest und Portig.
3. Fikhtengol’ts, G.M. (1960) Kurs differentialnovo i integral’novo is’chisleniya, tom III. Fizmatgiz.
4. Arfken, G. (1985) Mathematical Methods for Physicists. Third Edition, Academic Press.
5. Fourier Analysis