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A quadrilateral inequality established by C. Sch\"otz in the context of Hilbert spaces is extended to the framework of Banach spaces. Our approach is based on the theory of majorization and a substitute for the parallelogram law associated with Clarkson's notion of von Neumann-Jordan constant. Several examples (concerning matrix theory and geometry of Banach spaces) that illustrate our new results are also included.
Constantin P. Niculescu (Thu,) studied this question.
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