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+ SUBROUTINE DGELQF( M, N, A, LDA, TAU, WORK, LWORK, INFO )
+*
+* -- LAPACK routine (version 3.1) --
+* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
+* November 2006
+*
+* .. Scalar Arguments ..
+ INTEGER INFO, LDA, LWORK, M, N
+* ..
+* .. Array Arguments ..
+ DOUBLE PRECISION A( LDA, * ), TAU( * ), WORK( * )
+* ..
+*
+* Purpose
+* =======
+*
+* DGELQF computes an LQ factorization of a real M-by-N matrix A:
+* A = L * Q.
+*
+* Arguments
+* =========
+*
+* M (input) INTEGER
+* The number of rows of the matrix A. M >= 0.
+*
+* N (input) INTEGER
+* The number of columns of the matrix A. N >= 0.
+*
+* A (input/output) DOUBLE PRECISION array, dimension (LDA,N)
+* On entry, the M-by-N matrix A.
+* On exit, the elements on and below the diagonal of the array
+* contain the m-by-min(m,n) lower trapezoidal matrix L (L is
+* lower triangular if m <= n); the elements above the diagonal,
+* with the array TAU, represent the orthogonal matrix Q as a
+* product of elementary reflectors (see Further Details).
+*
+* LDA (input) INTEGER
+* The leading dimension of the array A. LDA >= max(1,M).
+*
+* TAU (output) DOUBLE PRECISION array, dimension (min(M,N))
+* The scalar factors of the elementary reflectors (see Further
+* Details).
+*
+* WORK (workspace/output) DOUBLE PRECISION array, dimension (MAX(1,LWORK))
+* On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
+*
+* LWORK (input) INTEGER
+* The dimension of the array WORK. LWORK >= max(1,M).
+* For optimum performance LWORK >= M*NB, where NB is the
+* optimal blocksize.
+*
+* 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.
+*
+* INFO (output) INTEGER
+* = 0: successful exit
+* < 0: if INFO = -i, the i-th argument had an illegal value
+*
+* Further Details
+* ===============
+*
+* The matrix Q is represented as a product of elementary reflectors
+*
+* Q = H(k) . . . H(2) H(1), where k = min(m,n).
+*
+* Each H(i) has the form
+*
+* H(i) = I - tau * v * v'
+*
+* where tau is a real scalar, and v is a real vector with
+* v(1:i-1) = 0 and v(i) = 1; v(i+1:n) is stored on exit in A(i,i+1:n),
+* and tau in TAU(i).
+*
+* =====================================================================
+*
+* .. Local Scalars ..
+ LOGICAL LQUERY
+ INTEGER I, IB, IINFO, IWS, K, LDWORK, LWKOPT, NB,
+ $ NBMIN, NX
+* ..
+* .. External Subroutines ..
+ EXTERNAL DGELQ2, DLARFB, DLARFT, XERBLA
+* ..
+* .. Intrinsic Functions ..
+ INTRINSIC MAX, MIN
+* ..
+* .. External Functions ..
+ INTEGER ILAENV
+ EXTERNAL ILAENV
+* ..
+* .. Executable Statements ..
+*
+* Test the input arguments
+*
+ INFO = 0
+ NB = ILAENV( 1, 'DGELQF', ' ', M, N, -1, -1 )
+ LWKOPT = M*NB
+ WORK( 1 ) = LWKOPT
+ LQUERY = ( LWORK.EQ.-1 )
+ IF( M.LT.0 ) THEN
+ INFO = -1
+ ELSE IF( N.LT.0 ) THEN
+ INFO = -2
+ ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
+ INFO = -4
+ ELSE IF( LWORK.LT.MAX( 1, M ) .AND. .NOT.LQUERY ) THEN
+ INFO = -7
+ END IF
+ IF( INFO.NE.0 ) THEN
+ CALL XERBLA( 'DGELQF', -INFO )
+ RETURN
+ ELSE IF( LQUERY ) THEN
+ RETURN
+ END IF
+*
+* Quick return if possible
+*
+ K = MIN( M, N )
+ IF( K.EQ.0 ) THEN
+ WORK( 1 ) = 1
+ RETURN
+ END IF
+*
+ NBMIN = 2
+ NX = 0
+ IWS = M
+ IF( NB.GT.1 .AND. NB.LT.K ) THEN
+*
+* Determine when to cross over from blocked to unblocked code.
+*
+ NX = MAX( 0, ILAENV( 3, 'DGELQF', ' ', M, N, -1, -1 ) )
+ IF( NX.LT.K ) THEN
+*
+* Determine if workspace is large enough for blocked code.
+*
+ LDWORK = M
+ IWS = LDWORK*NB
+ IF( LWORK.LT.IWS ) THEN
+*
+* Not enough workspace to use optimal NB: reduce NB and
+* determine the minimum value of NB.
+*
+ NB = LWORK / LDWORK
+ NBMIN = MAX( 2, ILAENV( 2, 'DGELQF', ' ', M, N, -1,
+ $ -1 ) )
+ END IF
+ END IF
+ END IF
+*
+ IF( NB.GE.NBMIN .AND. NB.LT.K .AND. NX.LT.K ) THEN
+*
+* Use blocked code initially
+*
+ DO 10 I = 1, K - NX, NB
+ IB = MIN( K-I+1, NB )
+*
+* Compute the LQ factorization of the current block
+* A(i:i+ib-1,i:n)
+*
+ CALL DGELQ2( IB, N-I+1, A( I, I ), LDA, TAU( I ), WORK,
+ $ IINFO )
+ IF( I+IB.LE.M ) THEN
+*
+* Form the triangular factor of the block reflector
+* H = H(i) H(i+1) . . . H(i+ib-1)
+*
+ CALL DLARFT( 'Forward', 'Rowwise', N-I+1, IB, A( I, I ),
+ $ LDA, TAU( I ), WORK, LDWORK )
+*
+* Apply H to A(i+ib:m,i:n) from the right
+*
+ CALL DLARFB( 'Right', 'No transpose', 'Forward',
+ $ 'Rowwise', M-I-IB+1, N-I+1, IB, A( I, I ),
+ $ LDA, WORK, LDWORK, A( I+IB, I ), LDA,
+ $ WORK( IB+1 ), LDWORK )
+ END IF
+ 10 CONTINUE
+ ELSE
+ I = 1
+ END IF
+*
+* Use unblocked code to factor the last or only block.
+*
+ IF( I.LE.K )
+ $ CALL DGELQ2( M-I+1, N-I+1, A( I, I ), LDA, TAU( I ), WORK,
+ $ IINFO )
+*
+ WORK( 1 ) = IWS
+ RETURN
+*
+* End of DGELQF
+*
+ END