Abstract
Dilation theory is a paradigm for studying operators by way of exhibiting an operator as a compression of another operator which is in some sense well behaved. For example, every contraction can be dilated to (i.e., is a compression of) a unitary operator, and on this simple fact a penetrating theory of non-normal operators has been developed. In the first part of this survey, I will leisurely review key classical results on dilation theory for a single operator or for several commuting operators, and sample applications of dilation theory in operator theory and in function theory. Then, in the second part, I will give a rapid account of a plethora of variants of dilation theory and their applications. In particular, I will discuss dilation theory of completely positive maps and semigroups, as well as the operator algebraic approach to dilation theory. In the last part, I will present relatively new dilation problems in the noncommutative setting which are related to the study of matrix convex sets and operator systems, and are motivated by applications in control theory. These problems include dilating tuples of noncommuting operators to tuples of commuting normal operators with a specified joint spectrum. I will also describe the recently studied problem of determining the optimal constant \(c = c_{\theta ,\theta '}\), such that every pair of unitaries U, V satisfying V U = e iθ UV can be dilated to a pair of cU′, cV ′, where U′, V ′ are unitaries that satisfy the commutation relation \(V'U' = e^{i\theta '} U'V'\). The solution of this problem gives rise to a new and surprising application of dilation theory to the continuity of the spectrum of the almost Mathieu operator from mathematical physics.
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Acknowledgements
This survey paper grew out of the talk that I gave at the International Workshop on Operator Theory and its Applications (IWOTA) that took place in the Instituto Superior Técnico, Lisbon, Portugal, in July 2019. I am grateful to the organizers of IWOTA 2019 for inviting me to speak in this incredibly successful workshop, and especially to Amélia Bastos, for inviting me to contribute to these proceedings. I used a preliminary version of this survey as lecture notes for a mini-course that I gave in the workshop Noncommutative Geometry and its Applications, which took place in January 2020, in NISER, Bhubaneswar, India. I am grateful to the organizers Bata Krishna Das, Sutanu Roy and Jaydeb Sarkar, for the wonderful hospitality and the opportunity to speak and organize my thoughts on dilation theory. I also owe thanks to Michael Skeide and to Fanciszek Szafraniec, for helpful feedback on preliminary versions. Finally, I wish to thank an anonymous referee for several useful comments and corrections.
This project was partially supported by ISF Grant no. 195/16.
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Shalit, O.M. (2021). Dilation Theory: A Guided Tour. In: Bastos, M.A., Castro, L., Karlovich, A.Y. (eds) Operator Theory, Functional Analysis and Applications. Operator Theory: Advances and Applications, vol 282. Birkhäuser, Cham. https://doi.org/10.1007/978-3-030-51945-2_28
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