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A note on commutativity up to a factor of bounded operators
Authors:Jian Yang   Hong-Ke Du
Affiliation:College of Mathematics and Information Science, Shaanxi Normal University, Xi'an 710062, P. R. China ; College of Mathematics and Information Science, Shaanxi Normal University, Xi'an 710062, P. R. China
Abstract:In this note, we explore commutativity up to a factor $AB=lambda BA$ for bounded operators $A$ and $B$ in a complex Hilbert space. Conditions on possible values of the factor $lambda$ are formulated and shown to depend on spectral properties of the operators. Commutativity up to a unitary factor is considered. In some cases, we obtain some properties of the solution space of the operator equation $AX=lambda XA$ and explore the structures of $A$ and $B$ that satisfy $AB=lambda BA$ for some $lambda in mathbb{C}setminus { 0 }.$ A quantum effect is an operator $A$on a complex Hilbert space that satisfies $0leq A leq I.$ The sequential product of quantum effects $A$ and $B$ is defined by $Acirc B=A^{frac{1}{2}}BA^{frac{1}{2}}.$ We also obtain properties of the sequential product.

Keywords:Hilbert space   commutator   selfadjointness   normal operator   quantum effect
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