a1 Département de Mathématiques et de Statistique, Faculté des Sciences et de Genie, Université Laval, Québec, Canada G1K 7P4
If a is a n × n matrix such that a + m is invertible for every invertible a + m matrix m, then a = 0, by a classical result of Perlis . Unfortunately the same result is not true in general for semi-simple rings as shown by T. Laffey. In the general situation of Banach algebras, Zemánek has proved that a is in the Jacobson radical of A if and only if ρ(a+x) = ρ(x), for every x in A, where ρ denotes the spectral radius. More sophisticated characterizations of the radical were given in  and , theorem 5·3·1. The arguments used in all these situations depend heavily on representation theory.
(Received June 04 1992)
(Revised October 21 1992)