1. Mokeichev V.S. Differentsial’nye uravneniya s otklonyayushchimisya argumentami [Differential Equations with Deviating Arguments]. Kazan, Izd. Kazan. Univ., 1985. 221 p. (In Russian)
2. Adams C.R. Linear q-difference equation. Bull. Am. Math. Soc., 1931, vol. 37, no. 6, pp. 361–400.
3. Flamant P. Sur la forme des solutions d'une ´equation diff´erentielle fonctionelle. C. R. Acad. Sci., Paris, 1924, vol. 137, pp. 1595–1597. (In French)
4. Ambartsumyan V.A. Nauchnye trudy [Research Works]. Vol. 1. Yerevan, Izd. Akad. Nauk Arm. SSR, 1960. 430 p. (In Russian)
5. Rusakov G.I. Fluctuations in brightness of the Milky Way. Uch. Zap. Leningr. Univ. Ser. Mat. Nauk, 1949, no. 18, pp. 53–79. (In Russian)