Abstract
Let \(\{a_n\}_{n\ge 0}\) and \(\{b_n\}_{n\ge 0}\) be sequences of scalars. Suppose \(a_n \ne 0\) for all \(n \ge 0\). We consider the tridiagonal kernel (also known as band kernel with bandwidth one) as
where \(\mathbb {D} = \{z \in \mathbb {C}: |z| < 1\}\). Denote by \(M_z\) the multiplication operator on the reproducing kernel Hilbert space corresponding to the kernel k. Assume that \(M_z\) is left-invertible. We prove that \(M_z =\) compact \(+\) isometry if and only if \(|\frac{b_n}{a_n}-\frac{b_{n+1}}{a_{n+1}}|\rightarrow 0\) and \(|\frac{a_n}{a_{n+1}}| \rightarrow 1\).
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Acknowledgements
The research of the second named author is supported in part by Core Research Grant, File No: CRG/2019/000908, by the Science and Engineering Research Board (SERB), Department of Science & Technology (DST), Government of India.
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Das, S., Sarkar, J. Tridiagonal shifts as compact + isometry. Arch. Math. 119, 507–518 (2022). https://doi.org/10.1007/s00013-022-01780-8
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DOI: https://doi.org/10.1007/s00013-022-01780-8