Stable iterations for the matrix square root
Abstract
Any matrix with no nonpositive real eigenvalues has a unique square root for which every eigenvalue lies in the open right halfplane. A link between the matrix sign function and this square root is exploited to derive both old and new iterations for the square root from iterations for the sign function. One new iteration is a quadratically convergent Schulz iteration based entirely on matrix multiplication; it converges only locally, but can be used to compute the square root of any nonsingular Mmatrix. A new Padé iteration well suited to parallel implementation is also derived and its properties explained. Iterative methods for the matrix square root are notorious for suffering from numerical instability. It is shown that apparently innocuous algorithmic modifications to the Padé iteration can lead to instability, and a perturbation analysis is given to provide some explanation. Numerical experiments are included and advice is offered on the choice of iterative method for computing the matrix square root.
 Publication:

Numerical Algorithms
 Pub Date:
 September 1997
 DOI:
 10.1023/A:1019150005407
 Bibcode:
 1997NuAlg..15..227H