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Lookup NU author(s): Dr Michael DritschelORCiD
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
© The Author(s) 2026.Part I of the paper considered infinite orthogonal sums of regular subspaces in a Kreĭn space (that is, of subspaces which are themselves Kreĭn spaces). How precisely these sums should be defined and conditions for when such sums are themselves regular were determined. These included, for example, a boundedness condition for the sum of the corresponding orthogonal projections. The same problem is addressed here for (quasi-)pseudo-regular subspaces. Such subspaces are orthogonal direct sums of a regular space and isotropic, or neutral, subspaces. Alternate characterizations of such subspaces are given, and infinite orthogonal sums are examined via unconditional, or Moore-Smith, sums of operator ranges.
Author(s): Dritschel MA, Maestripieri A, Rovnyak J
Publication type: Article
Publication status: Published
Journal: Complex Analysis and Operator Theory
Year: 2026
Volume: 20
Issue: 3
Online publication date: 25/02/2026
Acceptance date: 24/01/2026
Date deposited: 09/03/2026
ISSN (print): 1661-8254
ISSN (electronic): 1661-8262
Publisher: Springer International Publishing
URL: https://doi.org/10.1007/s11785-026-01914-8
DOI: 10.1007/s11785-026-01914-8
Data Access Statement: No datasets were generated or analysed during the current study
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