# Analysis of the Cholesky method with iterative refinement for solving the symmetric definite generalized eigenproblem

Prof Nick Higham
Abstract
The Cholesky factorization approach to solving the symmetric definite generalized eigenvalue problem $Ax = \lambda Bx$, where $A$ is symmetric and $B$ is symmetric positive definite, computes a Cholesky factorization $B = LL^T$ and solves the equivalent standard symmetric eigenvalue problem $C y = \l y$ where $C = L^{-1} A L^{-T}$. Provided that a stable eigensolver is used, standard error analysis says that the computed eigenvalues are exact for $A+\dA$ and $B+\dB$ with $\max( \normt{\dA}/\normt{A}, \normt{\dB}/\normt{B} )$ bounded by a multiple of $\kappa_2(B)u$, where $u$ is the unit roundoff. We take the Jacobi method as the eigensolver and explain how backward error bounds potentially much smaller than $\kappa_2(B)u$ can be obtained. To show the practical utility of our bounds we describe a vibration problem from structural engineering in which $B$ is ill conditioned yet the error bounds are small. We show how, in cases of instability, iterative refinement based on Newton's method can be used to produce eigenpairs with small backward errors. Our analysis and experiments also give insight into the popular Cholesky--QR method used in LAPACK, in which the QR method is used as the eigensolver.
• Computational Mathematics and Applications Seminar