An algorithm and a stability theory for downdating the ULV decomposition

An alternative to performing the singular value decomposition is to factor a matrixA into , whereU andV are orthogonal matrices andC is a lower triangular matrix which indicates a separation between two subspaces by the size of its columns. These subspaces are denoted byV = (V ₁,V ₂), where the columns ofC are partitioned conformally intoC = (C ₁,C ₂) with C _{2 F} . Here is some tolerance. In recent years, this has been called the ULV decomposition (ULVD).If the matrixA results from statistical observations, it is often desired to remove old observations, thus deleting a row fromA and its ULVD. In matrix terms, this is called a downdate. A downdating algorithm is proposed that preserves the structure in the downdated matrix to the extent possible. Strong stability results are proven for these algorithms based upon a new perturbation theory.The research of Jesse L. Barlow and Hongyuan Zha was supported by the National Science Foundation under grant no. CCR-9201612(Barlow) and CCR-9308399(Zha). The research of Peter A. Yoon was supported by the Office of Naval Research under the Fundamental Research Initiatives Program. Peter A. Yoon also has an appointment with the Applied Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA