Metric geometry in infinite dimensional Stiefel manifolds

Abstract

Let J be a separable Banach ideal in the space of bounded operators acting in a Hilbert space H and I the set of partial isometries in H. Fix v∈I. In this paper we study metric properties of the I-Stiefel manifold associated to v, namely. StI(v)={v0∈: v-v0∈I,j(v0*v0,v*v)=0}, where j(,) is the Fredholm index of a pair of projections. Let UI(H) be the Banach-Lie group of unitary operators which are perturbations of the identity by elements in I. Then StI(v) coincides with the orbit of v under the action of UI(H)×UI(H) on I given by (u,w)·v0=uv0w*, u,w∈UI(H) and v0∈StI(v). We endow StI(v) with a quotient Finsler metric by means of the Banach quotient norm of the Lie algebra of UI(H)×UI(H) by the Lie algebra of the isotropy group. We give a characterization of the rectifiable distance induced by this metric. In fact, we show that the rectifiable distance coincides with the quotient distance of UI(H)×UI(H) by the isotropy group. Hence this metric defines the quotient topology in StI(v).The other results concern with minimal curves in I-Stiefel manifolds when the ideal I is fixed as the compact operators in H. The initial value problem is solved when the partial isometry v has finite rank. In addition, we use a length-reducing map into the Grassmannian to find some special partial isometries that can be joined with a curve of minimal length.