1. M. G. Krein, “On a generalization of investigations of Stieltjes,” Dokl. Akad. Nauk SSSR 87 (6), 881–884 (1952).
2. M. G. Krein and I. S. Kats, “On the spectral functions of the string,” Amer. Math. Soc. Transl. 103 (2), 19–102 (1974).
3. W. Feller, “Generalized second-order differential operators and their lateral conditions,” Illinois J. Math. 1, 459–504 (1957).
4. Y. Kasahara, “Spectral theory of generalized second-order differential operators and its applications to Markov processes,” Japan J. Math. (N. S.) 1 (1), 67–84 (1975).
5. H. Dym and H. P. McKean, Gaussian Processes, Function Theory, and the Inverse Spectral Problem, in Probab. and Math. Stat. (Academic Press, New York, 1976), Vol. 31.