1. I.M. Gelfand, B.M. Levitan, On a Simple Identity for the Eigenvalues of a Differential Equation. Dokl. Akad. Nauk SSSR, 88 (1953) N. 4, 593–596, (in Russian).
2. V.B. Lidskii, V.A. Sadovnichii, Regularized Sums of Roots of One Class of Entire Functions. Func. anal. and its apps., 1 (1967), N. 2, 52–59, (in Russian); English translation in Functional Anal. Appl. 1 (1967).
3. V.A. Ljubishkin, Regularized Sums of Roots of Generalized Quasipolynomials. Matematicheskii Sbornik v. 135 (1988), (177), N. 4, 463–471, 1988 (in Russian).
4. V.A. Ljubishkin, V.A. Sadovnicii, Regularized Sums of Roots of One Class of Entire Functions of Exponential Type. Dokl. Akad. Nauk SSSR 256 (1981), N. 4, 794–798, (in Russian).
5. F.W. Olver, Asymptotics and Special functions. N.Y., Academic Press, 1974.