Kaplansky's problem and unitary orbits in matrix amplifications

Laurent W. Marcoux; Pawel Sarkowicz; Yuanhang Zhang

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arXiv · 2508.13834

Kaplansky's problem and unitary orbits in matrix amplifications

Laurent W. Marcoux·Pawel Sarkowicz·Yuanhang Zhang

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Abstract

We study the distances between the unitary orbits of matrix amplifications of elements in certain C*-algebras. In particular, we show that the distance between unitary orbits of arbitrary elements in unital, separable, UHF-stable C*-algebras remains unchanged when amplifying to certain matrix sizes. We further exhibit examples of elements in C*-algebras where the distance between unitary orbits becomes strictly smaller after amplifying by a certain matrix size, and we demonstrate that distances between unitary orbits of amplifications are not monotone in the multiplicity of the amplifications, even in the setting of matrix algebras. Lastly, we show that topological K-theory provides obstructions in the purely infinite setting.

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Laurent W. Marcoux, Pawel Sarkowicz, Yuanhang Zhang. 2025-08-19. Kaplansky's problem and unitary orbits in matrix amplifications. https://arxiv.org/abs/2508.13834

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