<p>Conformal dimension is a fundamental invariant of metric spaces, particularly suited to the study of self-similar spaces, such as spaces with expanding self-coverings (e.g., Julia sets of complex rational functions). The dynamics of these systems are encoded by the associated iterated monodromy groups, which are examples of contracting self-similar groups. Their amenability is a well-known open question. We show that if <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="222_2025_1360_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="18" /> </InlineMediaObject> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> <EquationSource Format="TEX">$G$</EquationSource> </InlineEquation> is an iterated monodromy group, and if the (Ahlfors-regular) conformal dimension of the underlying space is strictly less than 2, then every symmetric random walk with finite second moment on <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="222_2025_1360_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="18" /> </InlineMediaObject> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> <EquationSource Format="TEX">$G$</EquationSource> </InlineEquation> has the Liouville property. As a corollary, every such group is amenable. This criterion applies to all examples of contracting groups previously known to be amenable, and to many new ones. In particular, it implies that for every sub-hyperbolic complex rational function <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="222_2025_1360_Article_IEq3.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="14" /> </InlineMediaObject> <EquationSource Format="MATHML"><math> <mi>f</mi> </math></EquationSource> <EquationSource Format="TEX">$f$</EquationSource> </InlineEquation> whose Julia set is not the whole sphere, the iterated monodromy group of <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="222_2025_1360_Article_IEq3.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="14" /> </InlineMediaObject> <EquationSource Format="MATHML"><math> <mi>f</mi> </math></EquationSource> <EquationSource Format="TEX">$f$</EquationSource> </InlineEquation> is amenable.</p>

Liouville property for groups and conformal dimension

期刊论文
发表日期：

2025年8月27日

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Nicolás Matte Bon,
Volodymyr Nekrashevych,
Tianyi Zheng

摘要

Conformal dimension is a fundamental invariant of metric spaces, particularly suited to the study of self-similar spaces, such as spaces with expanding self-coverings (e.g., Julia sets of complex rational functions). The dynamics of these systems are encoded by the associated iterated monodromy groups, which are examples of contracting self-similar groups. Their amenability is a well-known open question. We show that if $G$ $G$ is an iterated monodromy group, and if the (Ahlfors-regular) conformal dimension of the underlying space is strictly less than 2, then every symmetric random walk with finite second moment on $G$ $G$ has the Liouville property. As a corollary, every such group is amenable. This criterion applies to all examples of contracting groups previously known to be amenable, and to many new ones. In particular, it implies that for every sub-hyperbolic complex rational function $f$ $f$ whose Julia set is not the whole sphere, the iterated monodromy group of $f$ $f$ is amenable.