Abstract
When the matrix in question is unsymmetric, the approximate eigenvectors or Ritz vectors obtained by orthogonal projection methods including Arnoldi's method and the block Arnoldi method cannot be guaranteed to converge in theory even if the corresponding approximate eigenvalues or Ritz values do. In order to circumvent this danger, a new strategy has been proposed by the author for Arnoldi's method. The strategy used is generalized to the block Arnoldi method in this paper. It discards Ritz vectors and instead computes refined approximate eigenvectors by small-sized singular-value decompositions. It is proved that the new strategy can guarantee the convergence of refined approximate eigenvectors if the corresponding Ritz values do. The resulting refined iterative algorithm is realized by the block Arnoldi process. Numerical experiments show that the refined algorithm is much more efficient than the iterative block Arnoldi algorithm.
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