relibc_openlibm/slatec/comhes.f

*DECK COMHES
      SUBROUTINE COMHES (NM, N, LOW, IGH, AR, AI, INT)
C***BEGIN PROLOGUE  COMHES
C***PURPOSE  Reduce a complex general matrix to complex upper Hessenberg
C            form using stabilized elementary similarity
C            transformations.
C***LIBRARY   SLATEC (EISPACK)
C***CATEGORY  D4C1B2
C***TYPE      COMPLEX (ELMHES-S, COMHES-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 COMHES,
C     NUM. MATH. 12, 349-368(1968) by Martin and Wilkinson.
C     HANDBOOK FOR AUTO. COMP., VOL.II-LINEAR ALGEBRA, 339-358(1971).
C
C     Given a COMPLEX GENERAL matrix, this subroutine
C     reduces a submatrix situated in rows and columns
C     LOW through IGH to upper Hessenberg form by
C     stabilized elementary similarity transformations.
C
C     On INPUT
C
C        NM must be set to the row dimension of the two-dimensional
C          array parameters, AR and AI, as declared in the calling
C          program dimension statement.  NM is an INTEGER variable.
C
C        N is the order of the matrix A=(AR,AI).  N is an INTEGER
C          variable.  N must be less than or equal to NM.
C
C        LOW and IGH are two INTEGER variables determined by the
C          balancing subroutine  CBAL.  If  CBAL  has not been used,
C          set LOW=1 and IGH equal to the order of the matrix, N.
C
C        AR and AI contain the real and imaginary parts, respectively,
C          of the complex input matrix.  AR and AI are two-dimensional
C          REAL arrays, dimensioned AR(NM,N) and AI(NM,N).
C
C     On OUTPUT
C
C        AR and AI contain the real and imaginary parts, respectively,
C          of the upper Hessenberg matrix.  The multipliers which
C          were used in the reduction are stored in the remaining
C          triangles under the Hessenberg matrix.
C
C        INT contains information on the rows and columns
C          interchanged in the reduction.  Only elements LOW through
C          IGH are used.  INT is a one-dimensional INTEGER array,
C          dimensioned INT(IGH).
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
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  CDIV
C***REVISION HISTORY  (YYMMDD)
C   760101  DATE WRITTEN
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  COMHES
C
      INTEGER I,J,M,N,LA,NM,IGH,KP1,LOW,MM1,MP1
      REAL AR(NM,*),AI(NM,*)
      REAL XR,XI,YR,YI
      INTEGER INT(*)
C
C***FIRST EXECUTABLE STATEMENT  COMHES
      LA = IGH - 1
      KP1 = LOW + 1
      IF (LA .LT. KP1) GO TO 200
C
      DO 180 M = KP1, LA
         MM1 = M - 1
         XR = 0.0E0
         XI = 0.0E0
         I = M
C
         DO 100 J = M, IGH
            IF (ABS(AR(J,MM1)) + ABS(AI(J,MM1))
     1         .LE. ABS(XR) + ABS(XI)) GO TO 100
            XR = AR(J,MM1)
            XI = AI(J,MM1)
            I = J
  100    CONTINUE
C
         INT(M) = I
         IF (I .EQ. M) GO TO 130
C     .......... INTERCHANGE ROWS AND COLUMNS OF AR AND AI ..........
         DO 110 J = MM1, N
            YR = AR(I,J)
            AR(I,J) = AR(M,J)
            AR(M,J) = YR
            YI = AI(I,J)
            AI(I,J) = AI(M,J)
            AI(M,J) = YI
  110    CONTINUE
C
         DO 120 J = 1, IGH
            YR = AR(J,I)
            AR(J,I) = AR(J,M)
            AR(J,M) = YR
            YI = AI(J,I)
            AI(J,I) = AI(J,M)
            AI(J,M) = YI
  120    CONTINUE
C     .......... END INTERCHANGE ..........
  130    IF (XR .EQ. 0.0E0 .AND. XI .EQ. 0.0E0) GO TO 180
         MP1 = M + 1
C
         DO 160 I = MP1, IGH
            YR = AR(I,MM1)
            YI = AI(I,MM1)
            IF (YR .EQ. 0.0E0 .AND. YI .EQ. 0.0E0) GO TO 160
            CALL CDIV(YR,YI,XR,XI,YR,YI)
            AR(I,MM1) = YR
            AI(I,MM1) = YI
C
            DO 140 J = M, N
               AR(I,J) = AR(I,J) - YR * AR(M,J) + YI * AI(M,J)
               AI(I,J) = AI(I,J) - YR * AI(M,J) - YI * AR(M,J)
  140       CONTINUE
C
            DO 150 J = 1, IGH
               AR(J,M) = AR(J,M) + YR * AR(J,I) - YI * AI(J,I)
               AI(J,M) = AI(J,M) + YR * AI(J,I) + YI * AR(J,I)
  150       CONTINUE
C
  160    CONTINUE
C
  180 CONTINUE
C
  200 RETURN
      END