The extension of the scheme in §7.6 within a block Arnoldi method is straightforward. The chief difference is that an implicitly shifted QR algorithm that retains band Hessenberg form is needed. As with the standard implicitly shifted QR algorithm, a sequence of unitary matrices are constructed and applied so that an updated band Hessenberg matrix results.
Numerous choices are possible for the selection of the shifts used during the QR algorithm, including the specific choice of If the shifts are in complex conjugate pairs, the implicit double shift [198, pp. 355-358] can be used to avoid complex arithmetic.
Typically, the shifts are selected by utilizing the spectral information
contained in .
Partition the eigenvalues of so that
In Algorithm 7.12, the integer is typically set to , the number of wanted eigenvalues, during the input step. Once the iteration loop has been entered, the values of , , and thus may vary for every value of When , we cannot apply all unwanted eigenvalues as shifts. We are then faced with the question of selecting which shifts to apply and whether we should consider applying more than shifts.
For example, shifts can be applied until a Ritz pair satisfies the convergence tolerance. The Ritz pairs can then be deflated (or locked). (This is equivalent to the deflated iterative Arnoldi algorithm given by Saad [387, p. 181] and used in the implementations in [24,398].) This approach allows us to implicitly apply a polynomial filter of the maximum degree. (Application of more than shifts will require applying explicit polynomials in ) However, as more shifts are applied, the cost in computing the subsequent Arnoldi reduction increases.
A strategy that varies , (relative to ) and the shifts used during every iteration will give the best results. This is the subject of current research. The recent report [421] discusses an adaptive strategy for symmetric eigenvalue problems.