Nonnegative Moore-Penrose inverses of Gram operators

This paper is concerned with necessary and sufficient conditions for the nonnegativity of Moore-Penrose inverses of Gram operators between real Hilbert spaces. These conditions include statements on acuteness (or obtuseness) of certain closed convex cones. The main result generalizes a well known result for inverses in the finite dimensional case over the nonnegative orthant to Moore-Penrose inverses in (possibly) infinite dimensional Hilbert spaces over any general closed convex cone. © 2006 Elsevier Inc. All rights reserved.