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A quasi-differential generalization of operators of the form lnu = w"' + px w"~ + ■ ■ ■ + p"u is considered. This type of generalization was first formulated by M. Bôcher (1913). A result of A. Zettl (1971) giving a necessary and sufficient condition that a differential operator / be factorable into a product of lower order differential operators is extended to quasi-differential expressions.doi:10.1090/s0002-9939-1991-1045592-1 fatcat:7xhp4aq6pfexjfpxndfduvvx6i