1. M.Abramowitz- I. A.Stegun,Handbook of mathematical functions, NBS Applied Math. Series 55, U. S. Department of Commerce, 1964.
2. A. G. Alenitsyn,Asymptotic properties of the spectrum of a Sturm-Liouville operator in the case of a limit circle, Differential Equations,12, no. 2 (1977), pp. 298–305. (Differentsial'nye Uravneniya,12, no. 3 (1976), pp. 428–437).
3. F. V. Atkinson,On second-order linear oscillators, Revista Tucuman,8 (1951), pp. 71–87.
4. F. V. Atkinson,Asymptotic formulae for linear oscillations, Proc. Glasgow Math. Association,3 (1957), pp. 105–111.
5. V. P. Belogrud -A. G. Kostyuchenko, Usp. Mat. Nauk.,28, no. 2 (170) (1973), pp. 227–228.