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Published online by Cambridge University Press: 14 November 2011
This paper studies the equation

where the differential operator Ln is defined by

and a necessary and sufficient condition that all oscillatory solutions of the above equation converge to zero asymptotically is presented. The results obtained extend and improve previous ones of Kusano and Onose, and Singh, even in the usual case where

where N is an integer with l≦N≦n–1.