1. Eine Umkehrung der Sturm-Liouvillechen Eigenwertaufgabe;Borg;Acta Math.,1946
2. Determination of a differential equation by two spectrum;Gasymov;Usp. Mat. Nauk.,1964
3. Asymptotic estimates of the Sturm-Liouville spectrum;Hockstead;Comm. Pure Appl. Math.,1961
4. Asymptotic Approximations;Jeffreys,1962
5. G. Birkhoff and G. Fix. Rayleigh-Ritz approximation with trigonometric polynominals. Indian J. Math., to appear.