SUBROUTINE HQR2(NM,N,LOW,IGH,H,WR,WI,Z,IERR)
C***BEGIN PROLOGUE  HQR2
C***DATE WRITTEN   760101   (YYMMDD)
C***REVISION DATE  830518   (YYMMDD)
C***CATEGORY NO.  D4C2B
C***KEYWORDS  EIGENVALUES,EIGENVECTORS,EISPACK
C***AUTHOR  SMITH, B. T., ET AL.
C***PURPOSE  Computes eigenvalues and eigenvectors of real upper
C            Hessenberg matrix using QR method.
C***DESCRIPTION
C
C     This subroutine is a translation of the ALGOL procedure HQR2,
C     NUM. MATH. 16, 181-204(1970) by Peters and Wilkinson.
C     HANDBOOK FOR AUTO. COMP., VOL.II-LINEAR ALGEBRA, 372-395(1971).
C
C     This subroutine finds the eigenvalues and eigenvectors
C     of a REAL UPPER Hessenberg matrix by the QR method.  The
C     eigenvectors of a REAL GENERAL matrix can also be found
C     if  ELMHES  and  ELTRAN  or  ORTHES  and  ORTRAN  have
C     been used to reduce this general matrix to Hessenberg form
C     and to accumulate the similarity transformations.
C
C     On INPUT
C
C        NM must be set to the row dimension of two-dimensional
C          array parameters as declared in the calling program
C          dimension statement.
C
C        N is the order of the matrix.
C
C        LOW and IGH are integers determined by the balancing
C          subroutine  BALANC.  If  BALANC  has not been used,
C          set LOW=1, IGH=N.
C
C        H contains the upper Hessenberg matrix.
C
C        Z contains the transformation matrix produced by  ELTRAN
C          after the reduction by  ELMHES, or by  ORTRAN  after the
C          reduction by  ORTHES, if performed.  If the eigenvectors
C          of the Hessenberg matrix are desired, Z must contain the
C          identity matrix.
C
C     On OUTPUT
C
C        H has been destroyed.
C
C        WR and WI contain the real and imaginary parts,
C          respectively, of the eigenvalues.  The eigenvalues
C          are unordered except that complex conjugate pairs
C          of values appear consecutively with the eigenvalue
C          having the positive imaginary part first.  If an
C          error exit is made, the eigenvalues should be correct
C          for indices IERR+1,...,N.
C
C        Z contains the real and imaginary parts of the eigenvectors.
C          If the I-th eigenvalue is real, the I-th column of Z
C          contains its eigenvector.  If the I-th eigenvalue is complex
C          with positive imaginary part, the I-th and (I+1)-th
C          columns of Z contain the real and imaginary parts of its
C          eigenvector.  The eigenvectors are unnormalized.  If an
C          error exit is made, none of the eigenvectors has been found.
C
C        IERR is set to
C          Zero       for normal return,
C          J          if the J-th eigenvalue has not been
C                     determined after a total of 30*N iterations.
C
C     Calls CDIV for complex division.
C
C     Questions and comments should be directed to B. S. Garbow,
C     APPLIED MATHEMATICS DIVISION, ARGONNE NATIONAL LABORATORY
C     ------------------------------------------------------------------
C***REFERENCES  B. T. SMITH, J. M. BOYLE, J. J. DONGARRA, B. S. GARBOW,
C                 Y. IKEBE, V. C. KLEMA, C. B. MOLER, *MATRIX EIGEN-
C                 SYSTEM ROUTINES - EISPACK GUIDE*, SPRINGER-VERLAG,
C                 1976.
C***ROUTINES CALLED  CDIV
C***END PROLOGUE  HQR2