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Subroutine [s,d]neigh  calls the ARPACK subroutine [s,d]laqrb . This routine is a modified version of the LAPACK subroutine [s,d]lahqr  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 subroutine [c,z]trevc  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.

Chao Yang