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.