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EB22 Sparse unsymmetric: subspace iteration

Given a real unsymmetric $n\times n$ matrix $A=\left\{a_{ij}\right\}$, this routine uses subspace iteration to calculate the $r$ eigenvalues ${\lambda }_i$, $i=1,2,...,r$, that are right-most, left-most, or are of largest modulus. The right-most (respectively, left-most) eigenvalues are the eigenvalues with the most positive (respectively, negative) real part. A second entry will return the associated eigenvectors $y_i$, $i=1,2,...,r$, where ${Ay}_i={\lambda }_i{y}_i$. The routine may also be used to calculate a group of eigensolutions elsewhere in the spectrum.