1. [ 1] N.I. Achieser, I.M. Glasmann, Theorie der linearen Operatoren im Hilbert-Raum (Theory of
Linear Operators in Hilbert Space; German Translation of the Russian Original), Akademie-
Verlag, Berlin, 1968.
2. [ 2] W.E. Boyce, R.C. DiPrima, Elementary Differential Equations and Boundary Value Problems,
8th Edition, John Wiley & Sons, Inc., Hoboke, 2005.
3. [ 3] E.A. Coddington, N. Levinson, Theory of Ordinary Differential Equations, McGraw-Hill Book
Company, New York Toronto London, 1955.
4. [ 4] L. Collatz, Eigenwertaufgaben mit technischen Anwendungen (Eigenvalue Problems with Applications
in Engineering), Geest & Portig K.-G., Leipzig, 1949.
5. [ 5] R. Courant, D. Hilbert, Mathematische Methoden der Physik, Band 1 (Methods of Mathematical
Physics, Vol. 1), Springer-Verlag, Berlin, Heidelberg, New York, 1968.