New York Journal of Mathematics
Volume 20 (2014) 133-144

  

Mohammad Sal Moslehian and Mahdi Dehghani

Operator convexity in Krein spaces

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Published: February 3, 2014
Keywords: Indefinite inner product; J-contraction; J-selfadjoint operator; Julia operator; Krein space; Krein-operator convex function.
Subject: Primary 47A63; Secondary 46C20, 47B50.

Abstract
We introduce the notion of Krein-operator convexity in the setting of Krein spaces. We present an indefinite version of the Jensen operator inequality on Krein spaces by showing that if (H,J) is a Krein space, U is an open set which is symmetric with respect to the real axis such that UR consists of a segment of real axis and f is a Krein-operator convex function on U with f(0)=0, then
f(C#AC)≦JC#f(A)C
for all J-positive operators A and all invertible J-contractions C such that the spectra of A, C#AC and D#AD are contained in U, where D is a defect operator for C#. We also show that in contrast with usual operator convex functions the converse of this implication is not true, in general.

Author information

Mohammad Sal Moslehian:
Department of Pure Mathematics, Center of Excellence in Analysis on Algebraic Structures (CEAAS), Ferdowsi University of Mashhad, P. O. Box 1159, Mashhad 91775, Iran
moslehian@member.ams.org

Mahdi Dehghani:
Department of Pure and Applied Mathematics, University of Yazd, Yazd 89195-741, Iran
Department of Pure Mathematics, Center of Excellence in Analysis on Algebraic Structures (CEAAS), Ferdowsi University of Mashhad, P. O. Box 1159, Mashhad 91775, Iran

e.g.mahdi@gmail.com