On the parallel sum of positive operators, forms, and functionals

The parallel sum (Formula presented.) of two bounded positive linear operators A, B on a Hilbert space H is defined to be the positive operator having the quadratic form(Formula presented.)for fixed (Formula presented.). The purpose of this paper is to provide a factorization of the parallel sum of the form (Formula presented.) where (Formula presented.) is the embedding operator of an auxiliary Hilbert space associated with A and B, and P is an orthogonal projection onto a certain linear subspace of that Hilbert space. We give similar factorizations of the parallel sum of nonnegative Hermitian forms, positive operators of a complex Banach space E into its topological anti-dual (Formula presented.), and of representable positive functionals on a (Formula presented.)-algebra. © 2015 Akadémiai Kiadó, Budapest, Hungary