dqrt14.f

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      DOUBLE PRECISION FUNCTION dqrt14( TRANS, M, N, NRHS, A, LDA, X,
     $                 ldx, work, lwork )
*
*  -- LAPACK test routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     .. Scalar Arguments ..
      CHARACTER          trans
      INTEGER            lda, ldx, lwork, m, n, nrhs
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   a( lda, * ), work( lwork ), x( ldx, * )
*     ..
*
*  Purpose
*  =======
*
*  DQRT14 checks whether X is in the row space of A or A'.  It does so
*  by scaling both X and A such that their norms are in the range
*  [sqrt(eps), 1/sqrt(eps)], then computing a QR factorization of [A,X]
*  (if TRANS = 'T') or an LQ factorization of [A',X]' (if TRANS = 'N'),
*  and returning the norm of the trailing triangle, scaled by
*  MAX(M,N,NRHS)*eps.
*
*  Arguments
*  =========
*
*  TRANS   (input) CHARACTER*1
*          = 'N':  No transpose, check for X in the row space of A
*          = 'T':  Transpose, check for X in the row space of A'.
*
*  M       (input) INTEGER
*          The number of rows of the matrix A.
*
*  N       (input) INTEGER
*          The number of columns of the matrix A.
*
*  NRHS    (input) INTEGER
*          The number of right hand sides, i.e., the number of columns
*          of X.
*
*  A       (input) DOUBLE PRECISION array, dimension (LDA,N)
*          The M-by-N matrix A.
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A.
*
*  X       (input) DOUBLE PRECISION array, dimension (LDX,NRHS)
*          If TRANS = 'N', the N-by-NRHS matrix X.
*          IF TRANS = 'T', the M-by-NRHS matrix X.
*
*  LDX     (input) INTEGER
*          The leading dimension of the array X.
*
*  WORK    (workspace) DOUBLE PRECISION array dimension (LWORK)
*
*  LWORK   (input) INTEGER
*          length of workspace array required
*          If TRANS = 'N', LWORK >= (M+NRHS)*(N+2);
*          if TRANS = 'T', LWORK >= (N+NRHS)*(M+2).
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   zero, one
      parameter( zero = 0.0d0, one = 1.0d0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            tpsd
      INTEGER            i, info, j, ldwork
      DOUBLE PRECISION   anrm, err, xnrm
*     ..
*     .. Local Arrays ..
      DOUBLE PRECISION   rwork( 1 )
*     ..
*     .. External Functions ..
      LOGICAL            lsame
      DOUBLE PRECISION   dlamch, dlange
      EXTERNAL           lsame, dlamch, dlange
*     ..
*     .. External Subroutines ..
      EXTERNAL           dgelq2, dgeqr2, dlacpy, dlascl, xerbla
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs, dble, max, min
*     ..
*     .. Executable Statements ..
*
      dqrt14 = zero
      IF( lsame( trans, 'N' ) ) THEN
         ldwork = m + nrhs
         tpsd = .false.
         IF( lwork.LT.( m+nrhs )*( n+2 ) ) THEN
            CALL xerbla( 'DQRT14', 10 )
            return
         ELSE IF( n.LE.0 .OR. nrhs.LE.0 ) THEN
            return
         END IF
      ELSE IF( lsame( trans, 'T' ) ) THEN
         ldwork = m
         tpsd = .true.
         IF( lwork.LT.( n+nrhs )*( m+2 ) ) THEN
            CALL xerbla( 'DQRT14', 10 )
            return
         ELSE IF( m.LE.0 .OR. nrhs.LE.0 ) THEN
            return
         END IF
      ELSE
         CALL xerbla( 'DQRT14', 1 )
         return
      END IF
*
*     Copy and scale A
*
      CALL dlacpy( 'All', m, n, a, lda, work, ldwork )
      anrm = dlange( 'M', m, n, work, ldwork, rwork )
      IF( anrm.NE.zero )
     $   CALL dlascl( 'G', 0, 0, anrm, one, m, n, work, ldwork, info )
*
*     Copy X or X' into the right place and scale it
*
      IF( tpsd ) THEN
*
*        Copy X into columns n+1:n+nrhs of work
*
         CALL dlacpy( 'All', m, nrhs, x, ldx, work( n*ldwork+1 ),
     $                ldwork )
         xnrm = dlange( 'M', m, nrhs, work( n*ldwork+1 ), ldwork,
     $          rwork )
         IF( xnrm.NE.zero )
     $      CALL dlascl( 'G', 0, 0, xnrm, one, m, nrhs,
     $                   work( n*ldwork+1 ), ldwork, info )
         anrm = dlange( 'One-norm', m, n+nrhs, work, ldwork, rwork )
*
*        Compute QR factorization of X
*
         CALL dgeqr2( m, n+nrhs, work, ldwork,
     $                work( ldwork*( n+nrhs )+1 ),
     $                work( ldwork*( n+nrhs )+min( m, n+nrhs )+1 ),
     $                info )
*
*        Compute largest entry in upper triangle of
*        work(n+1:m,n+1:n+nrhs)
*
         err = zero
         DO 20 j = n + 1, n + nrhs
            DO 10 i = n + 1, min( m, j )
               err = max( err, abs( work( i+( j-1 )*m ) ) )
   10       continue
   20    continue
*
      ELSE
*
*        Copy X' into rows m+1:m+nrhs of work
*
         DO 40 i = 1, n
            DO 30 j = 1, nrhs
               work( m+j+( i-1 )*ldwork ) = x( i, j )
   30       continue
   40    continue
*
         xnrm = dlange( 'M', nrhs, n, work( m+1 ), ldwork, rwork )
         IF( xnrm.NE.zero )
     $      CALL dlascl( 'G', 0, 0, xnrm, one, nrhs, n, work( m+1 ),
     $                   ldwork, info )
*
*        Compute LQ factorization of work
*
         CALL dgelq2( ldwork, n, work, ldwork, work( ldwork*n+1 ),
     $                work( ldwork*( n+1 )+1 ), info )
*
*        Compute largest entry in lower triangle in
*        work(m+1:m+nrhs,m+1:n)
*
         err = zero
         DO 60 j = m + 1, n
            DO 50 i = j, ldwork
               err = max( err, abs( work( i+( j-1 )*ldwork ) ) )
   50       continue
   60    continue
*
      END IF
*
      dqrt14 = err / ( dble( max( m, n, nrhs ) )*dlamch( 'Epsilon' ) )
*
      return
*
*     End of DQRT14
*
      END