Cohomological obstructions to group stability
For countable discrete groups Γ, we consider ϵ-representations of Γ into unitary groups U(n). These are unital maps ρ:Γ→U(n) such that ‖ρ(st)−ρ(s)ρ(t)‖<ϵ for all s,t∈Γ. Kazhdan has shown that the surface groups of genus >1 admit ϵ-representations which are far from genuine representations in the point-norm topology. We exhibit new classes of hyperbolic groups Γ which have the same instability features. More precisely, there exist a finite subset F⊂Γ and C>0 with the following property. For any ϵ>0 there is an ϵ-representation ρ:Γ→U(n) such that for any representation π:Γ→U(n), maxs∈F‖ρ(s)−π(s)‖>C.