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2. Über die freien Schwingungen in Kondensatorkreisen mit periodisch veränderlicher Kapazität
3. Asymptotic estimates for the Sturm-Liouville spectrum
4. and , Hill's equation, I. General theory, New York Univ., Courant Inst. Math. Sci., Div. Electromagnetic Res., Rep. BR-32, 1957.
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