1. H. Weyl, Math. Ann., 68, 220–268 (1910).
2. E. C. Titchmarsh, “Some theorems on perturbation theory, III,” Proc. Roy. Soc., Ser. A., 207, 321–328 (1951).
3. B. M. Levitan, “On the theorem of expansion of second-order differential equations in terms of eigenfunctions,” Dokl. Akad. Nauk SSSR, 71, No. 4, 605–608 (1950).
4. V. A. Marchenko, “Certain questions of the theory of one-dimensional linear second-order differential operators,” Trudy Mosk. Mat. Obshch., 1, 327–420 (1951).
5. F. V. Atkinson, “On the asymptotic behaviour of the Titchmarsh-Weyl m-coefficient and the spectral function for scalar second-order differential expressions,” Lect. Notes Math., 964.