Gromov and independently Linial and Meshulam suggested two related notions of high dimensional expanders. Gromov defined "topological overlapping" while Linial and Meshulam defined "coboundary expansion". A major open problem (asked by Gromov and others) is whether *bounded degree* high dimensional expanders according to these definitions exist for d≥2. We show, for the first time, bounded degree complexes of dimension d=2 which have the topological overlapping property. Assuming a conjecture of Serre on the congruence subgroup property, these complexes are also coboundary expanders.
Joint work with David Kazhdan and Alex Lubotzky.
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