Ebook: Property ($T$) for Groups Graded by Root Systems
- Tags: Root systems (Algebra)
- Series: Memoirs of the American Mathematical Society
- Year: 2017
- Publisher: American Mathematical Society
- City: Providence, United States
- Edition: 1
- Language: English
- pdf
The authors introduce and study the class of groups graded by root systems. They prove that if $Phi$ is an irreducible classical root system of rank $geq 2$ and $G$ is a group graded by $Phi$, then under certain natural conditions on the grading, the union of the root subgroups is a Kazhdan subset of $G$. As the main application of this theorem the authors prove that for any reduced irreducible classical root system $Phi$ of rank $geq 2$ and a finitely generated commutative ring $R$ with $1$, the Steinberg group ${mathrm St}_{Phi}(R)$ and the elementary Chevalley group $mathbb E_{Phi}(R)$ have property $(T)$. They also show that there exists a group with property $(T)$ which maps onto all finite simple groups of Lie type and rank $geq 2$, thereby providing a “unified” proof of expansion in these groups.
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