Simple Lie groups without the Approximation Property II

TL;DR

This paper proves that certain simple Lie groups, including their universal covers, lack the Approximation Property, completing the classification of simple Lie groups based on this property and extending results to associated noncommutative Lp-spaces.

Contribution

It establishes the absence of the Approximation Property for the universal cover of Sp(2,R), completing the classification for all connected simple Lie groups based on their real rank.

Findings

Universal cover of Sp(2,R) does not have the AP.

Connected simple Lie groups have the AP iff their real rank is 0 or 1.

Results extend to approximation properties of noncommutative Lp-spaces associated with lattices.

Abstract

We prove that the universal covering group $\widetilde{\mathrm{Sp}}(2,\mathbb{R})$ of $\mathrm{Sp}(2,\mathbb{R})$ does not have the Approximation Property (AP). Together with the fact that $\mathrm{SL}(3,\mathbb{R})$ does not have the AP, which was proved by Lafforgue and de la Salle, and the fact that $\mathrm{Sp}(2,\mathbb{R})$ does not have the AP, which was proved by the authors of this article, this finishes the description of the AP for connected simple Lie groups. Indeed, it follows that a connected simple Lie group has the AP if and only if its real rank is zero or one. By an adaptation of the methods we use to study the AP, we obtain results on approximation properties for noncommutative $L^p$-spaces associated with lattices in $\widetilde{\mathrm{Sp}}(2,\mathbb{R})$. Combining this with earlier results of Lafforgue and de la Salle and results of the second named author of this article, this gives rise to results on approximation properties of noncommutative $L^p$-spaces associated with lattices in any connected simple Lie group.