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SUBROUTINE SLAQP2( M, N, OFFSET, A, LDA, JPVT, TAU, VN1, VN2,
$ WORK )
*
* -- LAPACK auxiliary routine (version 3.2.2) --
* -- LAPACK is a software package provided by Univ. of Tennessee, --
* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
* June 2010
*
* .. Scalar Arguments ..
INTEGER LDA, M, N, OFFSET
* ..
* .. Array Arguments ..
INTEGER JPVT( * )
REAL A( LDA, * ), TAU( * ), VN1( * ), VN2( * ),
$ WORK( * )
* ..
*
* Purpose
* =======
*
* SLAQP2 computes a QR factorization with column pivoting of
* the block A(OFFSET+1:M,1:N).
* The block A(1:OFFSET,1:N) is accordingly pivoted, but not factorized.
*
* 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.
*
* OFFSET (input) INTEGER
* The number of rows of the matrix A that must be pivoted
* but no factorized. OFFSET >= 0.
*
* A (input/output) REAL array, dimension (LDA,N)
* On entry, the M-by-N matrix A.
* On exit, the upper triangle of block A(OFFSET+1:M,1:N) is
* the triangular factor obtained; the elements in block
* A(OFFSET+1:M,1:N) below the diagonal, together with the
* array TAU, represent the orthogonal matrix Q as a product of
* elementary reflectors. Block A(1:OFFSET,1:N) has been
* accordingly pivoted, but no factorized.
*
* LDA (input) INTEGER
* The leading dimension of the array A. LDA >= max(1,M).
*
* JPVT (input/output) INTEGER array, dimension (N)
* On entry, if JPVT(i) .ne. 0, the i-th column of A is permuted
* to the front of A*P (a leading column); if JPVT(i) = 0,
* the i-th column of A is a free column.
* On exit, if JPVT(i) = k, then the i-th column of A*P
* was the k-th column of A.
*
* TAU (output) REAL array, dimension (min(M,N))
* The scalar factors of the elementary reflectors.
*
* VN1 (input/output) REAL array, dimension (N)
* The vector with the partial column norms.
*
* VN2 (input/output) REAL array, dimension (N)
* The vector with the exact column norms.
*
* WORK (workspace) REAL array, dimension (N)
*
* Further Details
* ===============
*
* Based on contributions by
* G. Quintana-Orti, Depto. de Informatica, Universidad Jaime I, Spain
* X. Sun, Computer Science Dept., Duke University, USA
*
* Partial column norm updating strategy modified by
* Z. Drmac and Z. Bujanovic, Dept. of Mathematics,
* University of Zagreb, Croatia.
* June 2010
* For more details see LAPACK Working Note 176.
* =====================================================================
*
* .. Parameters ..
REAL ZERO, ONE
PARAMETER ( ZERO = 0.0E+0, ONE = 1.0E+0 )
* ..
* .. Local Scalars ..
INTEGER I, ITEMP, J, MN, OFFPI, PVT
REAL AII, TEMP, TEMP2, TOL3Z
* ..
* .. External Subroutines ..
EXTERNAL SLARF, SLARFG, SSWAP
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, MAX, MIN, SQRT
* ..
* .. External Functions ..
INTEGER ISAMAX
REAL SLAMCH, SNRM2
EXTERNAL ISAMAX, SLAMCH, SNRM2
* ..
* .. Executable Statements ..
*
MN = MIN( M-OFFSET, N )
TOL3Z = SQRT(SLAMCH('Epsilon'))
*
* Compute factorization.
*
DO 20 I = 1, MN
*
OFFPI = OFFSET + I
*
* Determine ith pivot column and swap if necessary.
*
PVT = ( I-1 ) + ISAMAX( N-I+1, VN1( I ), 1 )
*
IF( PVT.NE.I ) THEN
CALL SSWAP( M, A( 1, PVT ), 1, A( 1, I ), 1 )
ITEMP = JPVT( PVT )
JPVT( PVT ) = JPVT( I )
JPVT( I ) = ITEMP
VN1( PVT ) = VN1( I )
VN2( PVT ) = VN2( I )
END IF
*
* Generate elementary reflector H(i).
*
IF( OFFPI.LT.M ) THEN
CALL SLARFG( M-OFFPI+1, A( OFFPI, I ), A( OFFPI+1, I ), 1,
$ TAU( I ) )
ELSE
CALL SLARFG( 1, A( M, I ), A( M, I ), 1, TAU( I ) )
END IF
*
IF( I.LT.N ) THEN
*
* Apply H(i)**T to A(offset+i:m,i+1:n) from the left.
*
AII = A( OFFPI, I )
A( OFFPI, I ) = ONE
CALL SLARF( 'Left', M-OFFPI+1, N-I, A( OFFPI, I ), 1,
$ TAU( I ), A( OFFPI, I+1 ), LDA, WORK( 1 ) )
A( OFFPI, I ) = AII
END IF
*
* Update partial column norms.
*
DO 10 J = I + 1, N
IF( VN1( J ).NE.ZERO ) THEN
*
* NOTE: The following 4 lines follow from the analysis in
* Lapack Working Note 176.
*
TEMP = ONE - ( ABS( A( OFFPI, J ) ) / VN1( J ) )**2
TEMP = MAX( TEMP, ZERO )
TEMP2 = TEMP*( VN1( J ) / VN2( J ) )**2
IF( TEMP2 .LE. TOL3Z ) THEN
IF( OFFPI.LT.M ) THEN
VN1( J ) = SNRM2( M-OFFPI, A( OFFPI+1, J ), 1 )
VN2( J ) = VN1( J )
ELSE
VN1( J ) = ZERO
VN2( J ) = ZERO
END IF
ELSE
VN1( J ) = VN1( J )*SQRT( TEMP )
END IF
END IF
10 CONTINUE
*
20 CONTINUE
*
RETURN
*
* End of SLAQP2
*
END
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