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arXiv · 0909.1006

Lattices with and lattices without spectral gap

Bachir Bekka·Alexander Lubotzky
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Abstract

The following two results are shown. 1) Let $G$ be the $k$-rational points of a simple algebraic group over a local field $k$ and let $H$ be a lattice in $G.$ Then the regular representation of $G$ on $L^2(G/H)$ has a spectral gap (that is, there are no almost invariant unit vectors in the subspace of functions in $L^2(G/H)$ with zero mean). 2) There exist locally compact simple groups $G$ and lattices $H$ for which $L^2(G/H)$ has no spectral gap. This answers in the negative a question asked by Margulis. In fact, $G$ can be taken to be the group of orientation preserving automorphisms of a $k$-regular tree for $k>2.$

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Bachir Bekka, Alexander Lubotzky. 2009-09-05. Lattices with and lattices without spectral gap. https://arxiv.org/abs/0909.1006

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