| Register
\(\newcommand{\Cat}{{\rm Cat}} \) \(\newcommand{\A}{\mathcal A} \) \(\newcommand{\freestar}{ \framebox[7pt]{$\star$} }\)

5. Groups


    1. Add remark

      Problem 5.1.

      [S. Popa] Characterize all groups which are measure equivalent to free groups, or find new examples of such groups.


        • Add remark

          Problem 5.2.

          [A. Ioana] Find a weird example of a groups $\Gamma $ that is measure equivalent to $\mathbb{F}_2\times \mathbb{F}_2 $. Is it true that every such group $\Gamma$ is either a product of groups ME to free groups, or a lattice in a locally compact group which contains $\mathbb{F}_2\times \mathbb{F}_2$ as a lattice?
            1. Remark. The group von Neumann algebra version of this states that there are no weird examples. The same holds true in the measure equivalence setting under mild mixing assumptions.


                • Add remark

                  Problem 5.4.

                  [S. Vaes] Let $G$ be a groups with $\beta_1^{(2)}(G)>0 $. Then there is an infinite amenable subgroup, e.g. an element of infinite order, or a finite index subgroup $G_1\leqslant G $ so that $$ \beta_1^{(2)}(G)\geq \frac{1}{[G:G_1]}. $$


                    • Add remark

                      Problem 5.5.

                      [S. Vaes] Which groups $\Gamma$ admit a Bernoulli action on $\prod_{g \in \Gamma} (\{0,1\},\mu_g)$ that is nonsingular and of type III?
                        1. Remark. In https://arxiv.org/abs/1705.00439, it is proved that it is a necessary condition that $\Gamma$ is either amenable or satisfies $\beta_1^{(2)}(\Gamma) > 0$. It is also proved that for many of these groups such a nonsingular type III Bernoulli action exists, e.g. for all the groups satisfying the conclusion of Problem 5.3.


                            • Add remark

                              Problem 5.7.

                              [A. Furman] Is treeability stable under finite index, i.e. if $S\subset R $ is a finite index subequivalence relation of a treaable relation $R$, is $S$ treeable?


                                • Add remark

                                  Problem 5.8.

                                  [A. Furman] Does every equivalence relation with spectral gap or property (T) measurably contain $\mathbb{F}_2 $?

                                      Cite this as: AimPL: Classification of group von Neumann algebras, available at http://aimpl.org/groupvonneumann.