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Counting topologically invariant means on $L_\infty(G)$ and $VN(G)$ with ultrafilters

Authors :
John Hopfensperger
Source :
Rocky Mountain J. Math. 50, no. 6 (2020), 2103-2115
Publication Year :
2020
Publisher :
Rocky Mountain Mathematics Consortium, 2020.

Abstract

In 1970, Chou showed there are $|\mathbb{N}^*| = 2^{2^\mathbb{N}}$ topologically invariant means on $L_\infty(G)$ for any noncompact, $\sigma$-compact amenable group. Over the following 25 years, the sizes of the sets of topologically invariant means on $L_\infty(G)$ and $VN(G)$ were determined for any locally compact group. Each paper on a new case reached the same conclusion -- "the cardinality is as large as possible" -- but a unified proof never emerged. In this paper, I show $L_1(G)$ and $A(G)$ always contain orthogonal nets converging to invariance. An orthogonal net indexed by $\Gamma$ has $|\Gamma^*|$ accumulation points, where $|\Gamma^*|$ is determined by ultrafilter theory. Among a smattering of other results, I prove Paterson's conjecture that left and right topologically invariant means on $L_\infty(G)$ coincide iff $G$ has precompact conjugacy classes.<br />Comment: 10 pages, completely rewritten from v2

Details

Language :
English
Database :
OpenAIRE
Journal :
Rocky Mountain J. Math. 50, no. 6 (2020), 2103-2115
Accession number :
edsair.doi.dedup.....c904c3c7fddca367502bc4e973cdca4e