LAPACK 3.11.0 LAPACK: Linear Algebra PACKage
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## ◆ dgetrf()

 subroutine dgetrf ( integer M, integer N, double precision, dimension( lda, * ) A, integer LDA, integer, dimension( * ) IPIV, integer INFO )

DGETRF

DGETRF VARIANT: iterative version of Sivan Toledo's recursive LU algorithm

DGETRF VARIANT: left-looking Level 3 BLAS version of the algorithm.

Purpose:
``` DGETRF computes an LU factorization of a general M-by-N matrix A
using partial pivoting with row interchanges.

The factorization has the form
A = P * L * U
where P is a permutation matrix, L is lower triangular with unit
diagonal elements (lower trapezoidal if m > n), and U is upper
triangular (upper trapezoidal if m < n).

This is the right-looking Level 3 BLAS version of the algorithm.```
Parameters
 [in] M ``` M is INTEGER The number of rows of the matrix A. M >= 0.``` [in] N ``` N is INTEGER The number of columns of the matrix A. N >= 0.``` [in,out] A ``` A is DOUBLE PRECISION array, dimension (LDA,N) On entry, the M-by-N matrix to be factored. On exit, the factors L and U from the factorization A = P*L*U; the unit diagonal elements of L are not stored.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,M).``` [out] IPIV ``` IPIV is INTEGER array, dimension (min(M,N)) The pivot indices; for 1 <= i <= min(M,N), row i of the matrix was interchanged with row IPIV(i).``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: if INFO = i, U(i,i) is exactly zero. The factorization has been completed, but the factor U is exactly singular, and division by zero will occur if it is used to solve a system of equations.```

Purpose:

``` DGETRF computes an LU factorization of a general M-by-N matrix A
using partial pivoting with row interchanges.

The factorization has the form
A = P * L * U
where P is a permutation matrix, L is lower triangular with unit
diagonal elements (lower trapezoidal if m > n), and U is upper
triangular (upper trapezoidal if m < n).

This is the left-looking Level 3 BLAS version of the algorithm.```
Parameters
 [in] M ``` M is INTEGER The number of rows of the matrix A. M >= 0.``` [in] N ``` N is INTEGER The number of columns of the matrix A. N >= 0.``` [in,out] A ``` A is DOUBLE PRECISION array, dimension (LDA,N) On entry, the M-by-N matrix to be factored. On exit, the factors L and U from the factorization A = P*L*U; the unit diagonal elements of L are not stored.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,M).``` [out] IPIV ``` IPIV is INTEGER array, dimension (min(M,N)) The pivot indices; for 1 <= i <= min(M,N), row i of the matrix was interchanged with row IPIV(i).``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: if INFO = i, U(i,i) is exactly zero. The factorization has been completed, but the factor U is exactly singular, and division by zero will occur if it is used to solve a system of equations.```
Date
December 2016

Purpose:

``` DGETRF computes an LU factorization of a general M-by-N matrix A
using partial pivoting with row interchanges.

The factorization has the form
A = P * L * U
where P is a permutation matrix, L is lower triangular with unit
diagonal elements (lower trapezoidal if m > n), and U is upper
triangular (upper trapezoidal if m < n).

This code implements an iterative version of Sivan Toledo's recursive
LU algorithm[1].  For square matrices, this iterative versions should
be within a factor of two of the optimum number of memory transfers.

The pattern is as follows, with the large blocks of U being updated
in one call to DTRSM, and the dotted lines denoting sections that
have had all pending permutations applied:

1 2 3 4 5 6 7 8
+-+-+---+-------+------
| |1|   |       |
|.+-+ 2 |       |
| | |   |       |
|.|.+-+-+   4   |
| | | |1|       |
| | |.+-+       |
| | | | |       |
|.|.|.|.+-+-+---+  8
| | | | | |1|   |
| | | | |.+-+ 2 |
| | | | | | |   |
| | | | |.|.+-+-+
| | | | | | | |1|
| | | | | | |.+-+
| | | | | | | | |
|.|.|.|.|.|.|.|.+-----
| | | | | | | | |

The 1-2-1-4-1-2-1-8-... pattern is the position of the last 1 bit in
the binary expansion of the current column.  Each Schur update is
applied as soon as the necessary portion of U is available.

[1] Toledo, S. 1997. Locality of Reference in LU Decomposition with
Partial Pivoting. SIAM J. Matrix Anal. Appl. 18, 4 (Oct. 1997),
1065-1081. http://dx.doi.org/10.1137/S0895479896297744```
Parameters
 [in] M ``` M is INTEGER The number of rows of the matrix A. M >= 0.``` [in] N ``` N is INTEGER The number of columns of the matrix A. N >= 0.``` [in,out] A ``` A is DOUBLE PRECISION array, dimension (LDA,N) On entry, the M-by-N matrix to be factored. On exit, the factors L and U from the factorization A = P*L*U; the unit diagonal elements of L are not stored.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,M).``` [out] IPIV ``` IPIV is INTEGER array, dimension (min(M,N)) The pivot indices; for 1 <= i <= min(M,N), row i of the matrix was interchanged with row IPIV(i).``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: if INFO = i, U(i,i) is exactly zero. The factorization has been completed, but the factor U is exactly singular, and division by zero will occur if it is used to solve a system of equations.```
Date
December 2016

Definition at line 107 of file dgetrf.f.

108*
109* -- LAPACK computational routine --
110* -- LAPACK is a software package provided by Univ. of Tennessee, --
111* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
112*
113* .. Scalar Arguments ..
114 INTEGER INFO, LDA, M, N
115* ..
116* .. Array Arguments ..
117 INTEGER IPIV( * )
118 DOUBLE PRECISION A( LDA, * )
119* ..
120*
121* =====================================================================
122*
123* .. Parameters ..
124 DOUBLE PRECISION ONE
125 parameter( one = 1.0d+0 )
126* ..
127* .. Local Scalars ..
128 INTEGER I, IINFO, J, JB, NB
129* ..
130* .. External Subroutines ..
131 EXTERNAL dgemm, dgetrf2, dlaswp, dtrsm, xerbla
132* ..
133* .. External Functions ..
134 INTEGER ILAENV
135 EXTERNAL ilaenv
136* ..
137* .. Intrinsic Functions ..
138 INTRINSIC max, min
139* ..
140* .. Executable Statements ..
141*
142* Test the input parameters.
143*
144 info = 0
145 IF( m.LT.0 ) THEN
146 info = -1
147 ELSE IF( n.LT.0 ) THEN
148 info = -2
149 ELSE IF( lda.LT.max( 1, m ) ) THEN
150 info = -4
151 END IF
152 IF( info.NE.0 ) THEN
153 CALL xerbla( 'DGETRF', -info )
154 RETURN
155 END IF
156*
157* Quick return if possible
158*
159 IF( m.EQ.0 .OR. n.EQ.0 )
160 \$ RETURN
161*
162* Determine the block size for this environment.
163*
164 nb = ilaenv( 1, 'DGETRF', ' ', m, n, -1, -1 )
165 IF( nb.LE.1 .OR. nb.GE.min( m, n ) ) THEN
166*
167* Use unblocked code.
168*
169 CALL dgetrf2( m, n, a, lda, ipiv, info )
170 ELSE
171*
172* Use blocked code.
173*
174 DO 20 j = 1, min( m, n ), nb
175 jb = min( min( m, n )-j+1, nb )
176*
177* Factor diagonal and subdiagonal blocks and test for exact
178* singularity.
179*
180 CALL dgetrf2( m-j+1, jb, a( j, j ), lda, ipiv( j ), iinfo )
181*
182* Adjust INFO and the pivot indices.
183*
184 IF( info.EQ.0 .AND. iinfo.GT.0 )
185 \$ info = iinfo + j - 1
186 DO 10 i = j, min( m, j+jb-1 )
187 ipiv( i ) = j - 1 + ipiv( i )
188 10 CONTINUE
189*
190* Apply interchanges to columns 1:J-1.
191*
192 CALL dlaswp( j-1, a, lda, j, j+jb-1, ipiv, 1 )
193*
194 IF( j+jb.LE.n ) THEN
195*
196* Apply interchanges to columns J+JB:N.
197*
198 CALL dlaswp( n-j-jb+1, a( 1, j+jb ), lda, j, j+jb-1,
199 \$ ipiv, 1 )
200*
201* Compute block row of U.
202*
203 CALL dtrsm( 'Left', 'Lower', 'No transpose', 'Unit', jb,
204 \$ n-j-jb+1, one, a( j, j ), lda, a( j, j+jb ),
205 \$ lda )
206 IF( j+jb.LE.m ) THEN
207*
208* Update trailing submatrix.
209*
210 CALL dgemm( 'No transpose', 'No transpose', m-j-jb+1,
211 \$ n-j-jb+1, jb, -one, a( j+jb, j ), lda,
212 \$ a( j, j+jb ), lda, one, a( j+jb, j+jb ),
213 \$ lda )
214 END IF
215 END IF
216 20 CONTINUE
217 END IF
218 RETURN
219*
220* End of DGETRF
221*
integer function ilaenv(ISPEC, NAME, OPTS, N1, N2, N3, N4)
ILAENV
Definition: ilaenv.f:162
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
subroutine dtrsm(SIDE, UPLO, TRANSA, DIAG, M, N, ALPHA, A, LDA, B, LDB)
DTRSM
Definition: dtrsm.f:181
subroutine dgemm(TRANSA, TRANSB, M, N, K, ALPHA, A, LDA, B, LDB, BETA, C, LDC)
DGEMM
Definition: dgemm.f:187
recursive subroutine dgetrf2(M, N, A, LDA, IPIV, INFO)
DGETRF2
Definition: dgetrf2.f:113
subroutine dlaswp(N, A, LDA, K1, K2, IPIV, INCX)
DLASWP performs a series of row interchanges on a general rectangular matrix.
Definition: dlaswp.f:115
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