McKean, H. P.; van Moerbeke, P. The spectrum of Hill’s equation. (English) Zbl 0319.34024 Invent. Math. 30, 217-274 (1975). MSC: 34L99 34B30 33E05 PDFBibTeX XMLCite \textit{H. P. McKean} and \textit{P. van Moerbeke}, Invent. Math. 30, 217--274 (1975; Zbl 0319.34024) Full Text: DOI EuDML
Gilbert, R. C. An asymptotic phase for a Sturm-Liouville operator with unbounded potential. (English) Zbl 0159.11603 Proc. Am. Math. Soc. 18, 967-973 (1967). PDFBibTeX XMLCite \textit{R. C. Gilbert}, Proc. Am. Math. Soc. 18, 967--973 (1967; Zbl 0159.11603) Full Text: DOI