Super-approximation I
Abstract: Let Γ be a finitely generated subgroup of a compact group G.
1. I will recall what it means to say the action of Γ on G by left translation has spectral
gap, and mention some examples and applications, e.g. Banach-Ruziewicz problem, explicit
construction of expanders.
2. I will give the precise formulation of super-approximation conjecture and present the
best known results.
3. I will mention some new applications, e.g. orbit equivalence rigidity, variation of
Galois representations.