Weak (1,1)(1,1) maximal inequality for ergodic free-group actions

Let FF be a finitely generated free group, and let wTww\mapsto T_w be an ergodic action of FF on a probability space (X,B,μ)(X,\mathscr B,\mu). For n1n\geq1, let B(id,n)B(\operatorname{id},n) be the collection of words in FF of length less than nn. Free-group maximal inequality conjecture. For every fL1(X)f\in L_1(X), one has

supn11B(id,n)wB(id,n)TwfL1,(X)fL1(X).\left\|\sup_{n\geq1}\frac{1}{|B(\operatorname{id},n)|}\sum_{w\in B(\operatorname{id},n)}|T_wf|\right\|_{L_{1,\infty}(X)}\lesssim\|f\|_{L_1(X)}.

The theorem for the infinite regular tree suggests this endpoint ergodic maximal theorem, while the corresponding LpL_p maximal theorems for p>1p>1 were already known; non-amenability prevents the standard transfer argument, so the conjecture remains open.

Sources & referencesView supporting material

Primary source

Assaf Naor and Terence Tao, “Random Martingales and localization of maximal inequalities”, arXiv:0912.1140 (2009).

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