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) $\Sigma _{k = 1}^\infty |{a_{n,k}}| \to 1$ as $n \to \infty$, (2) $\Sigma _{k = 1}^\infty |{a_{n,k}} - {a_{n,k + 1}}| \to 0$ 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 âBanach limitsâ and Hahn-Banach techniques, and stated that he thought it would be âdifficult 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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