Generalizing block LU factorization: A lower–upper–lower block triangular decomposition with minimal off-diagonal ranks

• Published in: Linear algebra and its applications Vol. 509; pp. 114 - 142
• Main Authors: Serre, François, Püschel, Markus
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 15-11-2016
• Subjects: Block matrices; Block triangular decomposition; Circuit complexity; LU factorization; Rank minimization; Schur complement; Block triangular decomposition; LU factorization; 15A23; Rank minimization; 15A03; Schur complement; Circuit complexity; Block matrices
• ISSN: 0024-3795; 1873-1856
• DOI: 10.1016/j.laa.2016.07.020

We propose a novel factorization of a non-singular matrix P, viewed as a 2×2-blocked matrix. The factorization decomposes P into a product of three matrices that are lower block-unitriangular, upper block-triangular, and lower block-unitriangular, respectively. Our goal is to make this factorization “as block-diagonal as possible” by minimizing the ranks of the off-diagonal blocks. We give lower bounds on these ranks and show that they are sharp by providing an algorithm that computes an optimal solution. The proposed decomposition can be viewed as a generalization of the well-known Block LU factorization using the Schur complement. Finally, we briefly explain one application of this factorization: the design of optimal circuits for a certain class of streaming permutations.