1. Borg, G.: Eine Umkehrung der Sturm-Liouvilleschen Eigenwertaufgabe. Acta Math.78, 1?96 (1945)
2. Buslaev, V., Faddeev, L.: On trace formulas for singular Sturm-Liouville operators. Dokl. Akad. Nauk132, 13?16 (1960)
3. Copson, E.T.: An Introduction to the Theory of Functions of a Complex Variable. London: Oxford University Press 1960
4. Dikii, L.A.: Zeta functions for ordinary differential operators on a finite interval. Izv. Akad. Nauk19, 187?200 (1955)
5. Dikii, L.A.: Trace formulas for Sturm-Liouville differential opertors. AMST18, 81?115 (1958)