We resolve a basic problem on subspace distances that often arises in applications: How can the usual Grassmann distance between equidimensional subspaces be extended to subspaces of different dimensions? We show that a natural solution is given by the distance of a point to a Schubert variety within the Grassmannian. This distance reduces to the Grassmann distance when the subspaces are equidimensional and does not depend on any embedding into a larger ambient space. Furthermore, it has a concrete expression involving principal angles, and is efficiently computable in numerically stable ways. Our results are largely independent of the Grassmann distance --- if desired, it may be substituted by any other common distances between subspaces. Our approach depends on a concrete algebraic geometric view of the Grassmannian that parallels the differential geometric perspective that is well-established in applied and computational mathematics.
Schubert Varieties and Distances between Subspaces of Different Dimensions
Published 2014 in SIAM Journal on Matrix Analysis and Applications
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- Publication year
2014
- Venue
SIAM Journal on Matrix Analysis and Applications
- Publication date
2014-07-03
- Fields of study
Mathematics, Computer Science
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