SLATEC Routines --- HQR ---

*DECK HQR
      SUBROUTINE HQR (NM, N, LOW, IGH, H, WR, WI, IERR)
C***BEGIN PROLOGUE  HQR
C***PURPOSE  Compute the eigenvalues of a real upper Hessenberg matrix
C            using the QR method.
C***LIBRARY   SLATEC (EISPACK)
C***CATEGORY  D4C2B
C***TYPE      SINGLE PRECISION (HQR-S, COMQR-C)
C***KEYWORDS  EIGENVALUES, EIGENVECTORS, EISPACK
C***AUTHOR  Smith, B. T., et al.
C***DESCRIPTION
C
C     This subroutine is a translation of the ALGOL procedure HQR,
C     NUM. MATH. 14, 219-231(1970) by Martin, Peters, and Wilkinson.
C     HANDBOOK FOR AUTO. COMP., VOL.II-LINEAR ALGEBRA, 359-371(1971).
C
C     This subroutine finds the eigenvalues of a REAL
C     UPPER Hessenberg matrix by the QR method.
C
C     On INPUT
C
C        NM must be set to the row dimension of the two-dimensional
C          array parameter, H, as declared in the calling program
C          dimension statement.  NM is an INTEGER variable.
C
C        N is the order of the matrix H.  N is an INTEGER variable.
C          N must be less than or equal to NM.
C
C        LOW and IGH are two INTEGER variables determined by the
C          balancing subroutine  BALANC.  If  BALANC  has not been
C          used, set LOW=1 and IGH equal to the order of the matrix, N.
C
C        H contains the upper Hessenberg matrix.  Information about
C          the transformations used in the reduction to Hessenberg
C          form by  ELMHES  or  ORTHES, if performed, is stored
C          in the remaining triangle under the Hessenberg matrix.
C          H is a two-dimensional REAL array, dimensioned H(NM,N).
C
C     On OUTPUT
C
C        H has been destroyed.  Therefore, it must be saved before
C          calling  HQR  if subsequent calculation and back
C          transformation of eigenvectors is to be performed.
C
C        WR and WI contain the real and imaginary parts, respectively,
C          of the eigenvalues.  The eigenvalues are unordered except
C          that complex conjugate pairs of values appear consecutively
C          with the eigenvalue having the positive imaginary part first.
C          If an error exit is made, the eigenvalues should be correct
C          for indices IERR+1, IERR+2, ..., N.  WR and WI are one-
C          dimensional REAL arrays, dimensioned WR(N) and WI(N).
C
C        IERR is an INTEGER flag 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                     The eigenvalues should be correct for indices
C                     IERR+1, IERR+2, ..., N.
C
C     Questions and comments should be directed to B. S. Garbow,
C     APPLIED MATHEMATICS DIVISION, ARGONNE NATIONAL LABORATORY
C     ------------------------------------------------------------------
C
C***REFERENCES  B. T. Smith, J. M. Boyle, J. J. Dongarra, B. S. Garbow,
C                 Y. Ikebe, V. C. Klema and C. B. Moler, Matrix Eigen-
C                 system Routines - EISPACK Guide, Springer-Verlag,
C                 1976.
C***ROUTINES CALLED  (NONE)
C***REVISION HISTORY  (YYMMDD)
C   760101  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   890831  Modified array declarations.  (WRB)
C   890831  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   920501  Reformatted the REFERENCES section.  (WRB)
C***END PROLOGUE  HQR