Abstract. Let V be the set of n × n hermitian matrices, the set of n × n symmetric matrices, the set of all effects, or the set of all projections of rank one. Let c be a real number. We characterize bijective maps φ : V → V satisfying tr (AB) = c ⇐⇒ tr (φ (A)φ (B)) = c with some additional restrictions on c , depending on the underlying set of matrices.Mathematics subject classification (2010): 15A86, 15B57.
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