Authors: Bice, Tristan
Farah, Ilijas 
Title: Traces, ultrapowers and the pedersen-petersen C∗-algebras
Journal: Houston Journal of Mathematics
Volume: 41
Issue: 4
First page: 1175
Last page: 1190
Issue Date: 1-Jan-2015
Rank: M23
ISSN: 0362-1588
Our motivating question was whether all traces on a U-ultrapower of a C∗-algebra A, where U is a non-principal ultrafilter on N, are necessarily U-limits of traces on A. We show that this is false so long as A has infinitely many extremal traces, and even exhibit a 22ℵ0 size family of such traces on the ultrapower. For this to fail even when A has finitely many traces implies that A contains operators that can be expressed as sums of n + 1 but not n∗-commutators, for arbitrarily large n. We show that this happens for a direct sum of Pedersen-Petersen C∗-algebras, and analyze some other interesting properties of these C-algebras.
Keywords: -Algebras | C | Commutators | Traces | Ultrapowers
Publisher: University of Houston

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