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Convexity and concavity of eigenvalue sums
Authors:Elliott H Lieb  Heinz Siedentop
Institution:(1) Departments of Mathematics and Physics, Princeton University, 08544 Princeton, New Jersey;(2) Department of Mathematics, University of Alabama at Birmingham, 35294 Birmingham, Alabama
Abstract:It is well known thatsgr(H), the sum of the negative eigenvalues of a Hermitian matrixH, is a concave and increasing function ofH. In contrast to this, we prove that forA nonsingular Hermitian andP positive definite, the functionPmapsgr(AP)=sgr(P 1/2 AP 1/2) is convex and decreasing. Several other results of this nature are also proved.
Keywords:Convexity  concavity  eigenvalues
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