The Burger-Ozawa-Thom question: does strong Ulam stability characterize amenability?
Theorem 1.1: a countable discrete group is strongly Ulam stable if and only if it is amenable. For every nonamenable countable and there is a separable and a normalized with defect below that stays at uniform distance more than from every unitary representation on . Not shown: anything about finite-dimensional (ordinary) Ulam stability, or uncountable groups.