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15 June 2003 Microstates free entropy and cost of equivalence relations
Dimitri Shlyakhtenko
Duke Math. J. 118(3): 375-425 (15 June 2003). DOI: 10.1215/S0012-7094-03-11831-1


We define an analog of Voiculescu's free entropy for $n$-tuples of unitaries $u\sb 1,\ldots u\sb n$ in a tracial von Neumann algebra $M$ which normalize a unital subalgebra $L\sp \infty[0,1]=B\subset M$. Using this quantity, we define the free dimension $\delta\sb 0(u\sb 1,\ldots,u\sb n\between B)$. This number depends on $u\sb 1,\ldots u\sb n$ only up to orbit equivalence over $B$. In particular, if $R$ is a measurable equivalence relation on $[0,1]$ generated by $n$ automorphisms $\alpha\sb 1,\ldots \alpha\sb n$, let $u\sb 1,\ldots u\sb n$ be the unitaries implementing $\alpha\sb 1,\ldots \alpha\sb n$ in the Feldman-Moore crossed product algebra $M=W\sp \ast([0,1],R)\supset B=L\sp \infty[0,1]$. Then the number $\delta(R)=\delta\sb 0(u\sb 1,\ldots u\sb n\between B)$ is an invariant of the equivalence relation $R$. If $R$ is treeable, $\delta(R)$ coincides with the cost $C(R)$ of $R$ in the sense of D. Gaboriau. In particular, it is $n$ for an equivalence relation induced by a free action of the free group $\mathbb {F}\sb n$. For a general equivalence relation $R$ possessing a finite graphing of finite cost, $\delta(R)\leq C(R)$. Using the notion of free dimension, we define a dynamical entropy invariant for an automorphism of a measurable equivalence relation (or, more generally, of an $r$-discrete measure groupoid) and give examples.


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Dimitri Shlyakhtenko. "Microstates free entropy and cost of equivalence relations." Duke Math. J. 118 (3) 375 - 425, 15 June 2003.


Published: 15 June 2003
First available in Project Euclid: 23 April 2004

zbMATH: 1032.37003
MathSciNet: MR1983036
Digital Object Identifier: 10.1215/S0012-7094-03-11831-1

Primary: 46Lxx‎

Rights: Copyright © 2003 Duke University Press


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Vol.118 • No. 3 • 15 June 2003
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