Centralizers for subsets of normed algebras
Bertram Yood (2000)
Studia Mathematica
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Let G be the set of invertible elements of a normed algebra A with an identity. For some but not all subsets H of G we have the following dichotomy. For x ∈ A either for all c ∈ H or . In that case the set of x ∈ A for which the sup is finite is the centralizer of H.