1. N. B. Engibaryan, Astrofizika 1, 297–302 (1965).
2. N. B. Engibaryan and A. Kh. Khachatryan, Teor. Mat. Fiz. 119, 339–342 (2000).
3. F. Spitzer, Duke Math. J. 24, 367–372 (1957).
4. N. B. Engibaryan and B. N. Engibaryan, Mat. Sb. 187(10), 53–72 (1996).
5. L. G. Arabadzhyan and N. B. Engibaryan, in Advances in Science and Engineering: Mathematical Analysis (Moscow, VINITI, 1984), Vol. 22, pp. 175–242 [in Russian].