## MB03UD

### Computation of the singular value decomposition of a real upper triangular matrix

[Specification] [Arguments] [Method] [References] [Comments] [Example]

Purpose

      To compute all, or part, of the singular value decomposition of a
      real upper triangular matrix.

      The N-by-N upper triangular matrix A is factored as  A = Q*S*P',
      where Q and P are N-by-N orthogonal matrices and S is an
      N-by-N diagonal matrix with non-negative diagonal elements,
      SV(1), SV(2), ..., SV(N), ordered such that

         SV(1) >= SV(2) >= ... >= SV(N) >= 0.

      The columns of Q are the left singular vectors of A, the diagonal
      elements of S are the singular values of A and the columns of P
      are the right singular vectors of A.

      Either or both of Q and P' may be requested.
      When P' is computed, it is returned in A.

Specification

          SUBROUTINE MB03UD( JOBQ, JOBP, N, A, LDA, Q, LDQ, SV, DWORK,
         $                   LDWORK, INFO )
    C     .. Scalar Arguments ..
          CHARACTER         JOBP, JOBQ
          INTEGER           INFO, LDA, LDQ, LDWORK, N
    C     .. Array Arguments ..
          DOUBLE PRECISION  A(LDA,*), DWORK(*), Q(LDQ,*), SV(*)

Arguments

Mode Parameters

      JOBQ    CHARACTER*1
              Specifies whether the user wishes to compute the matrix Q
              of left singular vectors as follows:
              = 'V':  Left singular vectors are computed;
              = 'N':  No left singular vectors are computed.

      JOBP    CHARACTER*1
              Specifies whether the user wishes to compute the matrix P'
              of right singular vectors as follows:
              = 'V':  Right singular vectors are computed;
              = 'N':  No right singular vectors are computed.

Input/Output Parameters

      N       (input) INTEGER
              The order of the matrix A.  N >= 0.

      A       (input/output) DOUBLE PRECISION array, dimension (LDA,N)
              On entry, the leading N-by-N upper triangular part of this
              array must contain the upper triangular matrix A.
              On exit, if JOBP = 'V', the leading N-by-N part of this
              array contains the N-by-N orthogonal matrix  P'; otherwise
              the N-by-N upper triangular part of A is used as internal
              workspace. The strictly lower triangular part of A is set
              internally to zero before the reduction to bidiagonal form
              is performed.

      LDA     INTEGER
              The leading dimension of array A.  LDA >= MAX(1,N).

      Q       (output) DOUBLE PRECISION array, dimension (LDQ,N)
              If JOBQ = 'V', the leading N-by-N part of this array
              contains the orthogonal matrix Q.
              If JOBQ = 'N', Q is not referenced.

      LDQ     INTEGER
              The leading dimension of array Q.
              LDQ >= 1,  and when JOBQ = 'V',  LDQ >= MAX(1,N).

      SV      (output) DOUBLE PRECISION array, dimension (N)
              The N singular values of the matrix A, sorted in
              descending order.

Workspace

      DWORK   DOUBLE PRECISION array, dimension (LDWORK)
              On exit, if INFO = 0, DWORK(1) returns the optimal LDWORK;
              if INFO > 0, DWORK(2:N) contains the unconverged
              superdiagonal elements of an upper bidiagonal matrix B
              whose diagonal is in SV (not necessarily sorted).
              B satisfies A = Q*B*P', so it has the same singular
              values as A, and singular vectors related by Q and P'.

      LDWORK  INTEGER
              The length of the array DWORK.
              LDWORK >= MAX(1,5*N).
              For optimum performance LDWORK should be larger.

              If LDWORK = -1, then a workspace query is assumed;
              the routine only calculates the optimal size of the
              DWORK array, returns this value as the first entry of
              the DWORK array, and no error message related to LDWORK
              is issued by XERBLA.

Error Indicator

      INFO    INTEGER
              = 0:  successful exit;
              < 0:  if INFO = -i, the i-th argument had an illegal
                    value;
              > 0:  the QR algorithm has failed to converge. In this
                    case INFO specifies how many superdiagonals did not
                    converge (see the description of DWORK).
                    This failure is not likely to occur.

Method

      The routine reduces A to bidiagonal form by means of elementary
      reflectors and then uses the QR algorithm on the bidiagonal form.

Further Comments

      None

Example

Program Text

    *     MB03UD EXAMPLE PROGRAM TEXT
    *
    *     .. Parameters ..
          INTEGER          NIN, NOUT
          PARAMETER        ( NIN = 5, NOUT = 6 )
          INTEGER          NMAX
          PARAMETER        ( NMAX = 10 )
          INTEGER          LDA, LDQ
          PARAMETER        ( LDA = NMAX, LDQ = NMAX )
          INTEGER          LDWORK
          PARAMETER        ( LDWORK = MAX( 1, 5*NMAX ) )
    *     .. Local Scalars ..
          CHARACTER*1      JOBQ, JOBP
          INTEGER          I, INFO, J, N
    *     .. Local Arrays ..
          DOUBLE PRECISION A(LDA,NMAX), DWORK(LDWORK), Q(LDQ,NMAX),
         $                 SV(NMAX)
    *     .. External Functions ..
          LOGICAL          LSAME
    *     .. External Subroutines ..
          EXTERNAL         MB03UD
    *     .. Intrinsic Functions ..
          INTRINSIC        MAX
    *     .. Executable Statements ..
    *
          WRITE ( NOUT, FMT = 99999 )
    *     Skip the heading in the data file and read the data.
          READ ( NIN, FMT = '()' )
          READ ( NIN, FMT = * ) N, JOBQ, JOBP
          IF ( N.LT.0 .OR. N.GT.NMAX ) THEN
             WRITE ( NOUT, FMT = 99993 ) N
          ELSE
             READ ( NIN, FMT = * ) ( ( A(I,J), J = 1,N ), I = 1,N )
    *        Compute the singular values and vectors.
             CALL MB03UD( JOBQ, JOBP, N, A, LDA, Q, LDQ, SV, DWORK,
         $                LDWORK, INFO )
             IF ( INFO.NE.0 ) THEN
                WRITE ( NOUT, FMT = 99998 ) INFO
             ELSE
                WRITE ( NOUT, FMT = 99997 )
                WRITE ( NOUT, FMT = 99995 ) ( SV(I), I = 1,N )
                IF ( LSAME( JOBP, 'V' ) ) THEN
                   WRITE ( NOUT, FMT = 99996 )
                   DO 10 I = 1, N
                      WRITE ( NOUT, FMT = 99995 ) ( A(I,J), J = 1,N )
       10          CONTINUE
                END IF
                IF ( LSAME( JOBQ, 'V' ) ) THEN
                   WRITE ( NOUT, FMT = 99994 )
                   DO 20 I = 1, N
                      WRITE ( NOUT, FMT = 99995 ) ( Q(I,J), J = 1,N )
       20          CONTINUE
                END IF
             END IF
          END IF
    *
          STOP
    *
    99999 FORMAT (' MB03UD EXAMPLE PROGRAM RESULTS',/1X)
    99998 FORMAT (' INFO on exit from MB03UD = ',I2)
    99997 FORMAT (' Singular values are ',I5)
    99996 FORMAT (/' The transpose of the right singular vectors matrix is '
         $       )
    99995 FORMAT (8X,20(1X,F8.4))
    99994 FORMAT (/' The left singular vectors matrix is ')
    99993 FORMAT (/' N is out of range.',/' N = ',I5)
          END

Program Data

     MB03UD EXAMPLE PROGRAM DATA
       4     V      V
      -1.0  37.0 -12.0 -12.0
       0.0 -10.0   0.0   4.0
       0.0   0.0   7.0  -6.0
       0.0   0.0   0.0  -9.0

Program Results

     MB03UD EXAMPLE PROGRAM RESULTS

     Singular values are
              42.0909  11.7764   5.4420   0.2336

     The transpose of the right singular vectors matrix is
               0.0230  -0.9084   0.2759   0.3132
               0.0075  -0.1272   0.5312  -0.8376
               0.0092   0.3978   0.8009   0.4476
               0.9997   0.0182  -0.0177  -0.0050

     The left singular vectors matrix is
              -0.9671  -0.0882  -0.0501  -0.2335
               0.2456  -0.1765  -0.4020  -0.8643
               0.0012   0.7425   0.5367  -0.4008
              -0.0670   0.6401  -0.7402   0.1945

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