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http://journals.cambridge.org/action/displayAbstract?aid=1203216
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Q. Kong,
H. Wu,
A. Zettl
and M. Möller
(2003).
Indefinite Sturm–Liouville problems. Proceedings of the Royal Society of Edinburgh: Section A Mathematics, 133
,
pp 639-652
doi:10.1017/S0308210500002584