zgels.f

Go to the documentation of this file.

*> \brief <b> ZGELS solves overdetermined or underdetermined systems for GE matrices</b>
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
*> \htmlonly
*> Download ZGELS + dependencies
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/zgels.f">
*> [TGZ]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/zgels.f">
*> [ZIP]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/zgels.f">
*> [TXT]</a>
*> \endhtmlonly
*
*  Definition:
*  ===========
*
*       SUBROUTINE ZGELS( TRANS, M, N, NRHS, A, LDA, B, LDB, WORK, LWORK,
*                         INFO )
*
*       .. Scalar Arguments ..
*       CHARACTER          TRANS
*       INTEGER            INFO, LDA, LDB, LWORK, M, N, NRHS
*       ..
*       .. Array Arguments ..
*       COMPLEX*16         A( LDA, * ), B( LDB, * ), WORK( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> ZGELS solves overdetermined or underdetermined complex linear systems
*> involving an M-by-N matrix A, or its conjugate-transpose, using a QR
*> or LQ factorization of A.  It is assumed that A has full rank.
*>
*> The following options are provided:
*>
*> 1. If TRANS = 'N' and m >= n:  find the least squares solution of
*>    an overdetermined system, i.e., solve the least squares problem
*>                 minimize || B - A*X ||.
*>
*> 2. If TRANS = 'N' and m < n:  find the minimum norm solution of
*>    an underdetermined system A * X = B.
*>
*> 3. If TRANS = 'C' and m >= n:  find the minimum norm solution of
*>    an underdetermined system A**H * X = B.
*>
*> 4. If TRANS = 'C' and m < n:  find the least squares solution of
*>    an overdetermined system, i.e., solve the least squares problem
*>                 minimize || B - A**H * X ||.
*>
*> Several right hand side vectors b and solution vectors x can be
*> handled in a single call; they are stored as the columns of the
*> M-by-NRHS right hand side matrix B and the N-by-NRHS solution
*> matrix X.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] TRANS
*> \verbatim
*>          TRANS is CHARACTER*1
*>          = 'N': the linear system involves A;
*>          = 'C': the linear system involves A**H.
*> \endverbatim
*>
*> \param[in] M
*> \verbatim
*>          M is INTEGER
*>          The number of rows of the matrix A.  M >= 0.
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>          The number of columns of the matrix A.  N >= 0.
*> \endverbatim
*>
*> \param[in] NRHS
*> \verbatim
*>          NRHS is INTEGER
*>          The number of right hand sides, i.e., the number of
*>          columns of the matrices B and X. NRHS >= 0.
*> \endverbatim
*>
*> \param[in,out] A
*> \verbatim
*>          A is COMPLEX*16 array, dimension (LDA,N)
*>          On entry, the M-by-N matrix A.
*>            if M >= N, A is overwritten by details of its QR
*>                       factorization as returned by ZGEQRF;
*>            if M <  N, A is overwritten by details of its LQ
*>                       factorization as returned by ZGELQF.
*> \endverbatim
*>
*> \param[in] LDA
*> \verbatim
*>          LDA is INTEGER
*>          The leading dimension of the array A.  LDA >= max(1,M).
*> \endverbatim
*>
*> \param[in,out] B
*> \verbatim
*>          B is COMPLEX*16 array, dimension (LDB,NRHS)
*>          On entry, the matrix B of right hand side vectors, stored
*>          columnwise; B is M-by-NRHS if TRANS = 'N', or N-by-NRHS
*>          if TRANS = 'C'.
*>          On exit, if INFO = 0, B is overwritten by the solution
*>          vectors, stored columnwise:
*>          if TRANS = 'N' and m >= n, rows 1 to n of B contain the least
*>          squares solution vectors; the residual sum of squares for the
*>          solution in each column is given by the sum of squares of the
*>          modulus of elements N+1 to M in that column;
*>          if TRANS = 'N' and m < n, rows 1 to N of B contain the
*>          minimum norm solution vectors;
*>          if TRANS = 'C' and m >= n, rows 1 to M of B contain the
*>          minimum norm solution vectors;
*>          if TRANS = 'C' and m < n, rows 1 to M of B contain the
*>          least squares solution vectors; the residual sum of squares
*>          for the solution in each column is given by the sum of
*>          squares of the modulus of elements M+1 to N in that column.
*> \endverbatim
*>
*> \param[in] LDB
*> \verbatim
*>          LDB is INTEGER
*>          The leading dimension of the array B. LDB >= MAX(1,M,N).
*> \endverbatim
*>
*> \param[out] WORK
*> \verbatim
*>          WORK is COMPLEX*16 array, dimension (MAX(1,LWORK))
*>          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
*> \endverbatim
*>
*> \param[in] LWORK
*> \verbatim
*>          LWORK is INTEGER
*>          The dimension of the array WORK.
*>          LWORK >= max( 1, MN + max( MN, NRHS ) ).
*>          For optimal performance,
*>          LWORK >= max( 1, MN + max( MN, NRHS )*NB ).
*>          where MN = min(M,N) and NB is the optimum block size.
*>
*>          If LWORK = -1, then a workspace query is assumed; the routine
*>          only calculates the optimal size of the WORK array, returns
*>          this value as the first entry of the WORK array, and no error
*>          message related to LWORK is issued by XERBLA.
*> \endverbatim
*>
*> \param[out] INFO
*> \verbatim
*>          INFO is INTEGER
*>          = 0:  successful exit
*>          < 0:  if INFO = -i, the i-th argument had an illegal value
*>          > 0:  if INFO =  i, the i-th diagonal element of the
*>                triangular factor of A is zero, so that A does not have
*>                full rank; the least squares solution could not be
*>                computed.
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup complex16GEsolve
*
*  =====================================================================
      SUBROUTINE zgels( TRANS, M, N, NRHS, A, LDA, B, LDB, WORK, LWORK,
     $                  INFO )
*
*  -- LAPACK driver routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      CHARACTER          TRANS
      INTEGER            INFO, LDA, LDB, LWORK, M, N, NRHS
*     ..
*     .. Array Arguments ..
      COMPLEX*16         A( LDA, * ), B( LDB, * ), WORK( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   ZERO, ONE
      parameter( zero = 0.0d+0, one = 1.0d+0 )
      COMPLEX*16         CZERO
      parameter( czero = ( 0.0d+0, 0.0d+0 ) )
*     ..
*     .. Local Scalars ..
      LOGICAL            LQUERY, TPSD
      INTEGER            BROW, I, IASCL, IBSCL, J, MN, NB, SCLLEN, WSIZE
      DOUBLE PRECISION   ANRM, BIGNUM, BNRM, SMLNUM
*     ..
*     .. Local Arrays ..
      DOUBLE PRECISION   RWORK( 1 )
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      INTEGER            ILAENV
      DOUBLE PRECISION   DLAMCH, ZLANGE
      EXTERNAL           lsame, ilaenv, dlamch, zlange
*     ..
*     .. External Subroutines ..
      EXTERNAL           dlabad, xerbla, zgelqf, zgeqrf, zlascl, zlaset,
     $                   ztrtrs, zunmlq, zunmqr
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          dble, max, min
*     ..
*     .. Executable Statements ..
*
*     Test the input arguments.
*
      info = 0
      mn = min( m, n )
      lquery = ( lwork.EQ.-1 )
      IF( .NOT.( lsame( trans, 'N' ) .OR. lsame( trans, 'c' ) ) ) THEN
         INFO = -1
.LT.      ELSE IF( M0 ) THEN
         INFO = -2
.LT.      ELSE IF( N0 ) THEN
         INFO = -3
.LT.      ELSE IF( NRHS0 ) THEN
         INFO = -4
.LT.      ELSE IF( LDAMAX( 1, M ) ) THEN
         INFO = -6
.LT.      ELSE IF( LDBMAX( 1, M, N ) ) THEN
         INFO = -8
.LT..AND..NOT.      ELSE IF( LWORKMAX( 1, MN+MAX( MN, NRHS ) )  LQUERY )
     $          THEN
         INFO = -10
      END IF
*
*     Figure out optimal block size
*
.EQ..OR..EQ.      IF( INFO0  INFO-10 ) THEN
*
         TPSD = .TRUE.
         IF( LSAME( TRANS, 'n' ) )
     $      TPSD = .FALSE.
*
.GE.         IF( MN ) THEN
            NB = ILAENV( 1, 'zgeqrf', ' ', M, N, -1, -1 )
            IF( TPSD ) THEN
               NB = MAX( NB, ILAENV( 1, 'zunmqr', 'ln', M, NRHS, N,
     $              -1 ) )
            ELSE
               NB = MAX( NB, ILAENV( 1, 'zunmqr', 'lc', M, NRHS, N,
     $              -1 ) )
            END IF
         ELSE
            NB = ILAENV( 1, 'zgelqf', ' ', m, n, -1, -1 )
            IF( tpsd ) THEN
               nb = max( nb, ilaenv( 1, 'ZUNMLQ', 'LC', n, nrhs, m,
     $              -1 ) )
            ELSE
               nb = max( nb, ilaenv( 1, 'ZUNMLQ', 'ln', N, NRHS, M,
     $              -1 ) )
            END IF
         END IF
*
         WSIZE = MAX( 1, MN+MAX( MN, NRHS )*NB )
         WORK( 1 ) = DBLE( WSIZE )
*
      END IF
*
.NE.      IF( INFO0 ) THEN
         CALL XERBLA( 'zgels ', -INFO )
         RETURN
      ELSE IF( LQUERY ) THEN
         RETURN
      END IF
*
*     Quick return if possible
*
.EQ.      IF( MIN( M, N, NRHS )0 ) THEN
         CALL ZLASET( 'full', MAX( M, N ), NRHS, CZERO, CZERO, B, LDB )
         RETURN
      END IF
*
*     Get machine parameters
*
      SMLNUM = DLAMCH( 's' ) / DLAMCH( 'p' )
      BIGNUM = ONE / SMLNUM
      CALL DLABAD( SMLNUM, BIGNUM )
*
*     Scale A, B if max element outside range [SMLNUM,BIGNUM]
*
      ANRM = ZLANGE( 'm', M, N, A, LDA, RWORK )
      IASCL = 0
.GT..AND..LT.      IF( ANRMZERO  ANRMSMLNUM ) THEN
*
*        Scale matrix norm up to SMLNUM
*
         CALL ZLASCL( 'g', 0, 0, ANRM, SMLNUM, M, N, A, LDA, INFO )
         IASCL = 1
.GT.      ELSE IF( ANRMBIGNUM ) THEN
*
*        Scale matrix norm down to BIGNUM
*
         CALL ZLASCL( 'g', 0, 0, ANRM, BIGNUM, M, N, A, LDA, INFO )
         IASCL = 2
.EQ.      ELSE IF( ANRMZERO ) THEN
*
*        Matrix all zero. Return zero solution.
*
         CALL ZLASET( 'f', MAX( M, N ), NRHS, CZERO, CZERO, B, LDB )
         GO TO 50
      END IF
*
      BROW = M
      IF( TPSD )
     $   BROW = N
      BNRM = ZLANGE( 'm', BROW, NRHS, B, LDB, RWORK )
      IBSCL = 0
.GT..AND..LT.      IF( BNRMZERO  BNRMSMLNUM ) THEN
*
*        Scale matrix norm up to SMLNUM
*
         CALL ZLASCL( 'g', 0, 0, BNRM, SMLNUM, BROW, NRHS, B, LDB,
     $                INFO )
         IBSCL = 1
.GT.      ELSE IF( BNRMBIGNUM ) THEN
*
*        Scale matrix norm down to BIGNUM
*
         CALL ZLASCL( 'g', 0, 0, BNRM, BIGNUM, BROW, NRHS, B, LDB,
     $                INFO )
         IBSCL = 2
      END IF
*
.GE.      IF( MN ) THEN
*
*        compute QR factorization of A
*
         CALL ZGEQRF( M, N, A, LDA, WORK( 1 ), WORK( MN+1 ), LWORK-MN,
     $                INFO )
*
*        workspace at least N, optimally N*NB
*
.NOT.         IF( TPSD ) THEN
*
*           Least-Squares Problem min || A * X - B ||
*
*           B(1:M,1:NRHS) := Q**H * B(1:M,1:NRHS)
*
            CALL ZUNMQR( 'left', 'conjugate transpose', M, NRHS, N, A,
     $                   LDA, WORK( 1 ), B, LDB, WORK( MN+1 ), LWORK-MN,
     $                   INFO )
*
*           workspace at least NRHS, optimally NRHS*NB
*
*           B(1:N,1:NRHS) := inv(R) * B(1:N,1:NRHS)
*
            CALL ZTRTRS( 'upper', 'no transpose', 'non-unit', N, NRHS,
     $                   A, LDA, B, LDB, INFO )
*
.GT.            IF( INFO0 ) THEN
               RETURN
            END IF
*
            SCLLEN = N
*
         ELSE
*
*           Underdetermined system of equations A**T * X = B
*
*           B(1:N,1:NRHS) := inv(R**H) * B(1:N,1:NRHS)
*
            CALL ZTRTRS( 'upper', 'conjugate transpose','non-unit',
     $                   N, NRHS, A, LDA, B, LDB, INFO )
*
.GT.            IF( INFO0 ) THEN
               RETURN
            END IF
*
*           B(N+1:M,1:NRHS) = ZERO
*
            DO 20 J = 1, NRHS
               DO 10 I = N + 1, M
                  B( I, J ) = CZERO
   10          CONTINUE
   20       CONTINUE
*
*           B(1:M,1:NRHS) := Q(1:N,:) * B(1:N,1:NRHS)
*
            CALL ZUNMQR( 'left', 'no transpose', M, NRHS, N, A, LDA,
     $                   WORK( 1 ), B, LDB, WORK( MN+1 ), LWORK-MN,
     $                   INFO )
*
*           workspace at least NRHS, optimally NRHS*NB
*
            SCLLEN = M
*
         END IF
*
      ELSE
*
*        Compute LQ factorization of A
*
         CALL ZGELQF( M, N, A, LDA, WORK( 1 ), WORK( MN+1 ), LWORK-MN,
     $                INFO )
*
*        workspace at least M, optimally M*NB.
*
.NOT.         IF( TPSD ) THEN
*
*           underdetermined system of equations A * X = B
*
*           B(1:M,1:NRHS) := inv(L) * B(1:M,1:NRHS)
*
            CALL ZTRTRS( 'lower', 'no transpose', 'non-unit', M, NRHS,
     $                   A, LDA, B, LDB, INFO )
*
.GT.            IF( INFO0 ) THEN
               RETURN
            END IF
*
*           B(M+1:N,1:NRHS) = 0
*
            DO 40 J = 1, NRHS
               DO 30 I = M + 1, N
                  B( I, J ) = CZERO
   30          CONTINUE
   40       CONTINUE
*
*           B(1:N,1:NRHS) := Q(1:N,:)**H * B(1:M,1:NRHS)
*
            CALL ZUNMLQ( 'left', 'conjugate transpose', N, NRHS, M, A,
     $                   LDA, WORK( 1 ), B, LDB, WORK( MN+1 ), LWORK-MN,
     $                   INFO )
*
*           workspace at least NRHS, optimally NRHS*NB
*
            SCLLEN = N
*
         ELSE
*
*           overdetermined system min || A**H * X - B ||
*
*           B(1:N,1:NRHS) := Q * B(1:N,1:NRHS)
*
            CALL ZUNMLQ( 'left', 'no transpose', n, nrhs, m, a, lda,
     $                   work( 1 ), b, ldb, work( mn+1 ), lwork-mn,
     $                   info )
*
*           workspace at least NRHS, optimally NRHS*NB
*
*           B(1:M,1:NRHS) := inv(L**H) * B(1:M,1:NRHS)
*
            CALL ztrtrs( 'Lower', 'Conjugate transpose', 'Non-unit',
     $                   m, nrhs, a, lda, b, ldb, info )
*
            IF( info.GT.0 ) THEN
               RETURN
            END IF
*
            scllen = m
*
         END IF
*
      END IF
*
*     Undo scaling
*
      IF( iascl.EQ.1 ) THEN
         CALL zlascl( 'G', 0, 0, anrm, smlnum, scllen, nrhs, b, ldb,
     $                info )
      ELSE IF( iascl.EQ.2 ) THEN
         CALL zlascl( 'G', 0, 0, anrm, bignum, scllen, nrhs, b, ldb,
     $                info )
      END IF
      IF( ibscl.EQ.1 ) THEN
         CALL zlascl( 'G', 0, 0, smlnum, bnrm, scllen, nrhs, b, ldb,
     $                info )
      ELSE IF( ibscl.EQ.2 ) THEN
         CALL zlascl( 'G', 0, 0, bignum, bnrm, scllen, nrhs, b, ldb,
     $                info )
      END IF
*
   50 CONTINUE
      work( 1 ) = dble( wsize )
*
      RETURN
*
*     End of ZGELS
*
      END