1. Gelfand, I.M. and Levitan, B.M., On a Simple Identity for the Characteristic Values of a Differential Operator of Second Order, Dokl. Akad. Nauk SSSR, 1953, vol. 88, pp. 593–596.
2. Gelfand, I.M., On Identities for Eigenvalues of a Differential Operator of Second Order, Uspekhi Mat. Nauk, 1956, vol. 11, no. 1, pp. 191–198.
3. Dikii, L.A., On a Formula of Gelfand-Levitan, Uspekhi Mat. Nauk, 1953, vol. 8, no. 2, pp. 119–123.
4. Dikii, L.A., The Zeta Function of an Ordinary Differential Equation on a Finite Interval, Izv. Akad. Nauk SSSR, 1955, vol. 19, no. 4, pp. 187–200.
5. Levitan, B.M., Calculation of the Regularized Trace for the Sturm-Liouville Operator, Uspekhi Mat. Nauk, 1964, vol. 19, no. 1, pp. 161–165.