On the absence of quantitatively critical measure equivalence couplings

Abstract:

Given a measure equivalence coupling between two finitely generated groups, Delabie, Koivisto, Le Maître and Tessera [Ann. H. Lebesgue 5 (2022), pp. 1417–1487] have found explicit upper bounds on how integrable the associated cocycles can be. These bounds are optimal in many cases but the integrability of the cocycles with respect to these critical thresholds remained unclear. For instance, a cocycle from $\mathbb {Z}^{k+\ell }$ to $\mathbb {Z}^{k}$ can be $\mathrm {L}^p$ for all $p<\frac {k}{k+\ell }$ but not for $p>\frac {k}{k+\ell }$, and the case $p=\frac {k}{k+\ell }$ was an open question which we answer by the negative. Our main result actually yields much more examples where the integrability threshold given by Delabie-Koivisto-Le Maître-Tessera Theorems cannot be reached.