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2. “Bateman Project” (Erdélyi, Magnus, Obehettinger, Tricomi), Higher Transcendental Functions. III. New York-Toronto-London: McGraw-Hill 1955.
3. Coddington, A., & N. Levison, Theory of Ordinary Differential Equations. New York-Toronto-London: McGraw-Hill 1955.
4. Ince, E.L., Ordinary Differential Equations. New York: Dover Publications 1956.
5. Magnus, W., & S. Winkler, Hill's Equation. New York: Interscience Publishers 1966.