MINRES-QLP: A Krylov Subspace Method for Indefinite or Singular Symmetric Systems

  • Authors:
  • Sou-Cheng T. Choi;Christopher C. Paige;Michael A. Saunders

  • Affiliations:
  • scchoi@stanford.edu;paige@cs.mcgill.ca;saunders@stanford.edu

  • Venue:
  • SIAM Journal on Scientific Computing
  • Year:
  • 2011

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Abstract

CG, SYMMLQ, and MINRES are Krylov subspace methods for solving symmetric systems of linear equations. When these methods are applied to an incompatible system (that is, a singular symmetric least-squares problem), CG could break down and SYMMLQ's solution could explode, while MINRES would give a least-squares solution but not necessarily the minimum-length (pseudoinverse) solution. This understanding motivates us to design a MINRES-like algorithm to compute minimum-length solutions to singular symmetric systems. MINRES uses QR factors of the tridiagonal matrix from the Lanczos process (where $R$ is upper-tridiagonal). MINRES-QLP uses a QLP decomposition (where rotations on the right reduce $R$ to lower-tridiagonal form). On ill-conditioned systems (singular or not), MINRES-QLP can give more accurate solutions than MINRES. We derive preconditioned MINRES-QLP, new stopping rules, and better estimates of the solution and residual norms, the matrix norm, and the condition number.