LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ sgetrf2()

recursive subroutine sgetrf2 ( integer  M,
integer  N,
real, dimension( lda, * )  A,
integer  LDA,
integer, dimension( * )  IPIV,
integer  INFO 
)

SGETRF2

Purpose:
 SGETRF2 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 recursive version of the algorithm. It divides
 the matrix into four submatrices:

        [  A11 | A12  ]  where A11 is n1 by n1 and A22 is n2 by n2
    A = [ -----|----- ]  with n1 = min(m,n)/2
        [  A21 | A22  ]       n2 = n-n1

                                       [ A11 ]
 The subroutine calls itself to factor [ --- ],
                                       [ A12 ]
                 [ A12 ]
 do the swaps on [ --- ], solve A12, update A22,
                 [ A22 ]

 then calls itself to factor A22 and do the swaps on A21.
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 REAL 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.
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

Definition at line 112 of file sgetrf2.f.

113 *
114 * -- LAPACK computational routine --
115 * -- LAPACK is a software package provided by Univ. of Tennessee, --
116 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
117 *
118 * .. Scalar Arguments ..
119  INTEGER INFO, LDA, M, N
120 * ..
121 * .. Array Arguments ..
122  INTEGER IPIV( * )
123  REAL A( LDA, * )
124 * ..
125 *
126 * =====================================================================
127 *
128 * .. Parameters ..
129  REAL ONE, ZERO
130  parameter( one = 1.0e+0, zero = 0.0e+0 )
131 * ..
132 * .. Local Scalars ..
133  REAL SFMIN, TEMP
134  INTEGER I, IINFO, n1, n2
135 * ..
136 * .. External Functions ..
137  REAL SLAMCH
138  INTEGER ISAMAX
139  EXTERNAL slamch, isamax
140 * ..
141 * .. External Subroutines ..
142  EXTERNAL sgemm, sscal, slaswp, strsm, xerbla
143 * ..
144 * .. Intrinsic Functions ..
145  INTRINSIC max, min
146 * ..
147 * .. Executable Statements ..
148 *
149 * Test the input parameters
150 *
151  info = 0
152  IF( m.LT.0 ) THEN
153  info = -1
154  ELSE IF( n.LT.0 ) THEN
155  info = -2
156  ELSE IF( lda.LT.max( 1, m ) ) THEN
157  info = -4
158  END IF
159  IF( info.NE.0 ) THEN
160  CALL xerbla( 'SGETRF2', -info )
161  RETURN
162  END IF
163 *
164 * Quick return if possible
165 *
166  IF( m.EQ.0 .OR. n.EQ.0 )
167  $ RETURN
168 
169  IF ( m.EQ.1 ) THEN
170 *
171 * Use unblocked code for one row case
172 * Just need to handle IPIV and INFO
173 *
174  ipiv( 1 ) = 1
175  IF ( a(1,1).EQ.zero )
176  $ info = 1
177 *
178  ELSE IF( n.EQ.1 ) THEN
179 *
180 * Use unblocked code for one column case
181 *
182 *
183 * Compute machine safe minimum
184 *
185  sfmin = slamch('S')
186 *
187 * Find pivot and test for singularity
188 *
189  i = isamax( m, a( 1, 1 ), 1 )
190  ipiv( 1 ) = i
191  IF( a( i, 1 ).NE.zero ) THEN
192 *
193 * Apply the interchange
194 *
195  IF( i.NE.1 ) THEN
196  temp = a( 1, 1 )
197  a( 1, 1 ) = a( i, 1 )
198  a( i, 1 ) = temp
199  END IF
200 *
201 * Compute elements 2:M of the column
202 *
203  IF( abs(a( 1, 1 )) .GE. sfmin ) THEN
204  CALL sscal( m-1, one / a( 1, 1 ), a( 2, 1 ), 1 )
205  ELSE
206  DO 10 i = 1, m-1
207  a( 1+i, 1 ) = a( 1+i, 1 ) / a( 1, 1 )
208  10 CONTINUE
209  END IF
210 *
211  ELSE
212  info = 1
213  END IF
214 *
215  ELSE
216 *
217 * Use recursive code
218 *
219  n1 = min( m, n ) / 2
220  n2 = n-n1
221 *
222 * [ A11 ]
223 * Factor [ --- ]
224 * [ A21 ]
225 *
226  CALL sgetrf2( m, n1, a, lda, ipiv, iinfo )
227 
228  IF ( info.EQ.0 .AND. iinfo.GT.0 )
229  $ info = iinfo
230 *
231 * [ A12 ]
232 * Apply interchanges to [ --- ]
233 * [ A22 ]
234 *
235  CALL slaswp( n2, a( 1, n1+1 ), lda, 1, n1, ipiv, 1 )
236 *
237 * Solve A12
238 *
239  CALL strsm( 'L', 'L', 'N', 'U', n1, n2, one, a, lda,
240  $ a( 1, n1+1 ), lda )
241 *
242 * Update A22
243 *
244  CALL sgemm( 'N', 'N', m-n1, n2, n1, -one, a( n1+1, 1 ), lda,
245  $ a( 1, n1+1 ), lda, one, a( n1+1, n1+1 ), lda )
246 *
247 * Factor A22
248 *
249  CALL sgetrf2( m-n1, n2, a( n1+1, n1+1 ), lda, ipiv( n1+1 ),
250  $ iinfo )
251 *
252 * Adjust INFO and the pivot indices
253 *
254  IF ( info.EQ.0 .AND. iinfo.GT.0 )
255  $ info = iinfo + n1
256  DO 20 i = n1+1, min( m, n )
257  ipiv( i ) = ipiv( i ) + n1
258  20 CONTINUE
259 *
260 * Apply interchanges to A21
261 *
262  CALL slaswp( n1, a( 1, 1 ), lda, n1+1, min( m, n), ipiv, 1 )
263 *
264  END IF
265  RETURN
266 *
267 * End of SGETRF2
268 *
integer function isamax(N, SX, INCX)
ISAMAX
Definition: isamax.f:71
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
recursive subroutine sgetrf2(M, N, A, LDA, IPIV, INFO)
SGETRF2
Definition: sgetrf2.f:113
subroutine slaswp(N, A, LDA, K1, K2, IPIV, INCX)
SLASWP performs a series of row interchanges on a general rectangular matrix.
Definition: slaswp.f:115
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:79
subroutine strsm(SIDE, UPLO, TRANSA, DIAG, M, N, ALPHA, A, LDA, B, LDB)
STRSM
Definition: strsm.f:181
subroutine sgemm(TRANSA, TRANSB, M, N, K, ALPHA, A, LDA, B, LDB, BETA, C, LDC)
SGEMM
Definition: sgemm.f:187
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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