Computing the polar decomposition and the matrix sign decomposition in matrix groups

Nicholas J. Higham, D. Steven Mackey, Niloufer Mackey, Françoise Tisseur

    Research output: Contribution to journalArticlepeer-review


    For any matrix automorphism group double-struck G associated with a bilinear or sesquilinear form, Mackey, Mackey, and Tisseur have recently shown that the matrix sign decomposition factors of A ∈ double-struck G also lie in double-struck G; moreover, the polar factors of A lie in double-struck G if the matrix of the underlying form is unitary. Groups satisfying the latter condition include the complex orthogonal, real and complex symplectic, and pseudo-orthogonal groups. This work is concerned with exploiting the structure of double-struck G when computing the polar and matrix sign decompositions of matrices in double-struck G. We give sufficient conditions for a matrix iteration to preserve the group structure and show that a family of globally convergent rational Padé-based iterations of Kenney and Laub satisfy these conditions. The well-known scaled Newton iteration for computing the unitary polar factor does not preserve group structure, but we show that the approach of the iterates to the group is precisely tethered to the approach to unitarity, and that this forces a different and exploitable structure in the iterates. A similar relation holds for the Newton iteration for the matrix sign function. We also prove that the number of iterations needed for convergence of the structure-preserving methods can be precisely predicted by running an associated scalar iteration. Numerical experiments are given to compare the cubically and quintically converging iterations with Newton's method and to test stopping criteria. The overall conclusion is that the structure-preserving iterations and the scaled Newton iteration are all of practical interest, and which iteration is to be preferred is problem-dependent.
    Original languageEnglish
    Pages (from-to)1178-1192
    Number of pages14
    JournalSIAM Journal on Matrix Analysis and Applications
    Issue number4
    Publication statusPublished - 2004


    • Adjoint
    • Automorphism group
    • Bilinear form
    • Complex orthogonal matrix
    • Matrix iteration
    • Matrix sign decomposition
    • Newton iteration
    • Perplectic matrix
    • Polar decomposition
    • Pseudo-orthogonal matrix
    • Sesquilinear form
    • Structure preservation
    • Symplectic matrix


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