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Proceedings of the American Mathematical Society

ISSN 1088-6826(online) ISSN 0002-9939(print)



A class of regular matrices

Author: Godfrey L. Isaacs
Journal: Proc. Amer. Math. Soc. 60 (1976), 211-214
MSC: Primary 40C05
MathSciNet review: 0430589
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Abstract: Let m be the space of real, bounded sequences $ x = \{ {x_k}\} $ with the sup norm, and let $ A = ({a_{n,k}})$ be a regular (i.e., Toeplitz) matrix. We consider the following two possible conditions for A: (1) $ \Sigma _{k = 1}^\infty \vert{a_{n,k}}\vert \to 1$ as $ n \to \infty $, (2) $ \Sigma _{k = 1}^\infty \vert{a_{n,k}} - {a_{n,k + 1}}\vert \to 0$ as $ n \to \infty $. G. Das [J. London Math. Soc. (2) 7 (1974), 501-507] proved that if a regular matrix A satisfies both (1) and (2) then (3) $ {\overline {\lim } _{n \to \infty }}{(Ax)_n} \leqslant q(x)$ for all $ x \in m$, where $ q(x) = {\inf _{{n_i},p}}{\overline {\lim } _{k \to \infty }}{p^{ - 1}}\Sigma _{i = 1}^p{x_{{n_i} + k}}$. Das used ``Banach limits'' and Hahn-Banach techniques, and stated that he thought it would be ``difficult to establish the result... by direct method". In the present paper an elementary proof of the result is given, and it is shown also that the converse holds, i.e., for a regular A, (3) implies (1) and (2). Hence (3) completely characterizes the class of regular matrices satisfying (1) and (2).

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Keywords: Regular matrices, Toeplitz matrices, Banach limits, almost convergence
Article copyright: © Copyright 1976 American Mathematical Society

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