Geodesic neighborhoods in unitary orbits of self-adjoint operators of K+C

Abstract

In the present paper, we study the unitary orbit of a compact Hermitian diagonal operator with spectral multiplicity one under the action of the unitary group U_K+C of the unitization of the compact operators K+C , or equivalently, the quotient U_K+C/U_D(K+C). We relate this and the action of different unitary subgroups to describe metric geodesics (using a natural distance) which join end points. As a consequence we obtain a local Hopf-Rinow theorem. We also explore cases about the uniqueness of short curves and prove that there exist some of these that cannot be parameterized using minimal anti-Hermitian operators of K+C.