algebra / Group theory; stability of approximate representations

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 GG and δ>0\delta>0 there is a separable HH and a normalized μ:G→U(H)\mu:G\to U(H) with defect below δ\delta that stays at uniform distance more than 1/101/10 from every unitary representation on HH. Not shown: anything about finite-dimensional (ordinary) Ulam stability, or uncountable groups.

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algebraOct 5, 2026Significance 25/100Registry: unreviewed

The Burger-Ozawa-Thom question: does strong Ulam stability characterize amenability?

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Theorem 1.1: a countable discrete group is strongly Ulam stable if and only if it is amenable. For every nonamenable countable GG and δ>0\delta>0 there is a separable HH and a normalized μ:G→U(H)\mu:G\to U(H) with defect below δ\delta that stays at uniform distance more than 1/101/10 from every unitary representation on HH. Not shown: anything about finite-dimensional (ordinary) Ulam stability, or uncountable groups.

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Theorem 1.1: a countable discrete group is strongly Ulam stable if and only if it is amenable. For every nonamenable countable GG and δ>0\delta>0 there is a separable HH and a normalized μ:G→U(H)\mu:G\to U(H) with defect below δ\delta that stays at uniform distance more than 1/101/10 from every unitary representation on HH. Not shown: anything about finite-dimensional (ordinary) Ulam stability, or uncountable groups.

Theorem 1.1: a countable discrete group is strongly Ulam stable if and only if it is amenable. For every nonamenable countable GG and δ>0\delta>0 there is a separable HH and a normalized μ:G→U(H)\mu:G\to U(H) with defect below δ\delta that stays at uniform distance more than 1/101/10 from every unitary representation on HH. Not shown: anything about finite-dimensional (ordinary) Ulam stability, or uncountable groups.

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