Up: ARPACK subroutines
Subroutine [s,d]neigh calls the ARPACK subroutine
This routine is a modified version of the LAPACK subroutine
for computing a real Schur form for an upper Hessenberg matrix.
Subroutine [s,d]laqrb computes the upper quasi-triangular
Schur matrix just as [s,d]lahqr does
but it only computes the last components of the associated Schur vectors
since these are all that is needed for the error estimates.
The complex arithmetic subroutine [c,z]neigh computes
the complete Schur decomposition of the projected matrix using
LAPACK subroutine [c,z]lahqr .
The subroutine [c,z]neigh then calls the LAPACK
to compute the eigenvectors of the upper triangular Schur matrix.
However, just as in real arithmetic case, only the last components of these
eigenvectors are needed for computing error estimates.