1. Gel’fand, I.M. and Levitan, B.M., A simple identity for the eigenvalues of a
second-order differential operator, Dokl. Akad. Nauk
SSSR, 1953, vol. 88, no. 4, pp. 593–596.
2. Dikii, L.A., Trace formulas for Sturm–Liouville differential operators,
Usp. Mat. Nauk, 1958, vol. 13, no. 3(81),
pp. 111–143.
3. Gasymov, M.G., On the sum of differences of eigenvalues of two self-adjoint
operators, Dokl. Akad. Nauk SSSR, 1963, vol. 150,
no. 6, pp. 1202–1205.
4. Gesztesy, F., Holden, H., Simon, B., and Zhao, Z., A trace formula for
multidimensional Schrödinger operators, J. Funct.
Anal., 1996, vol. 141, no. 2, pp. 449–465.
5. Guseinov, G.Sh. and Levitan, B.M., On trace formulas for Sturm–Liouville
operators, Vestn. Mosk. Univ. Ser. Mat. Mekh., 1978,
no. 1, pp. 40–49.