MR0803403 Butler, G. J. and Erbe, L. H. G. J. BUTLER and L. H. ERBE Oscillation theory for second order differential systems with functionally commutative matrix coefficients Funkcialaj Ekvacioj. Serio Internacia 28 1985 47--55 http://fe.math.kobe-u.ac.jp/FE/Free/vol28/fe28-1-4.pdf http://fe.math.kobe-u.ac.jp/FE/FE_pdf_with_bookmark/FE21-30-en_KML/fe28-047-055/fe28-047-055.pdf http://www.ams.org/mathscinet-getitem?mr=MR0803403 fe28-047-055 1985 Oscillation Theory for Second Order Differential Systems with Functionally Commutative Matrix Coefficients BUTLER, G. J.; ERBE, L. H. BUTLER, G. J.; ERBE, L. H.
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