LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ sorgtsqr_row()

subroutine sorgtsqr_row ( integer  M,
integer  N,
integer  MB,
integer  NB,
real, dimension( lda, * )  A,
integer  LDA,
real, dimension( ldt, * )  T,
integer  LDT,
real, dimension( * )  WORK,
integer  LWORK,
integer  INFO 
)

SORGTSQR_ROW

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Purpose:
 SORGTSQR_ROW generates an M-by-N real matrix Q_out with
 orthonormal columns from the output of SLATSQR. These N orthonormal
 columns are the first N columns of a product of complex unitary
 matrices Q(k)_in of order M, which are returned by SLATSQR in
 a special format.

      Q_out = first_N_columns_of( Q(1)_in * Q(2)_in * ... * Q(k)_in ).

 The input matrices Q(k)_in are stored in row and column blocks in A.
 See the documentation of SLATSQR for more details on the format of
 Q(k)_in, where each Q(k)_in is represented by block Householder
 transformations. This routine calls an auxiliary routine SLARFB_GETT,
 where the computation is performed on each individual block. The
 algorithm first sweeps NB-sized column blocks from the right to left
 starting in the bottom row block and continues to the top row block
 (hence _ROW in the routine name). This sweep is in reverse order of
 the order in which SLATSQR generates the output blocks.
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. M >= N >= 0.
[in]MB
          MB is INTEGER
          The row block size used by SLATSQR to return
          arrays A and T. MB > N.
          (Note that if MB > M, then M is used instead of MB
          as the row block size).
[in]NB
          NB is INTEGER
          The column block size used by SLATSQR to return
          arrays A and T. NB >= 1.
          (Note that if NB > N, then N is used instead of NB
          as the column block size).
[in,out]A
          A is REAL array, dimension (LDA,N)

          On entry:

             The elements on and above the diagonal are not used as
             input. The elements below the diagonal represent the unit
             lower-trapezoidal blocked matrix V computed by SLATSQR
             that defines the input matrices Q_in(k) (ones on the
             diagonal are not stored). See SLATSQR for more details.

          On exit:

             The array A contains an M-by-N orthonormal matrix Q_out,
             i.e the columns of A are orthogonal unit vectors.
[in]LDA
          LDA is INTEGER
          The leading dimension of the array A.  LDA >= max(1,M).
[in]T
          T is REAL array,
          dimension (LDT, N * NIRB)
          where NIRB = Number_of_input_row_blocks
                     = MAX( 1, CEIL((M-N)/(MB-N)) )
          Let NICB = Number_of_input_col_blocks
                   = CEIL(N/NB)

          The upper-triangular block reflectors used to define the
          input matrices Q_in(k), k=(1:NIRB*NICB). The block
          reflectors are stored in compact form in NIRB block
          reflector sequences. Each of the NIRB block reflector
          sequences is stored in a larger NB-by-N column block of T
          and consists of NICB smaller NB-by-NB upper-triangular
          column blocks. See SLATSQR for more details on the format
          of T.
[in]LDT
          LDT is INTEGER
          The leading dimension of the array T.
          LDT >= max(1,min(NB,N)).
[out]WORK
          (workspace) REAL array, dimension (MAX(1,LWORK))
          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
[in]LWORK
          The dimension of the array WORK.
          LWORK >= NBLOCAL * MAX(NBLOCAL,(N-NBLOCAL)),
          where NBLOCAL=MIN(NB,N).
          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.
[out]INFO
          INFO is INTEGER
          = 0:  successful exit
          < 0:  if INFO = -i, the i-th argument had an illegal value
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Contributors:
 November 2020, Igor Kozachenko,
                Computer Science Division,
                University of California, Berkeley

Definition at line 186 of file sorgtsqr_row.f.

188  IMPLICIT NONE
189 *
190 * -- LAPACK computational routine --
191 * -- LAPACK is a software package provided by Univ. of Tennessee, --
192 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
193 *
194 * .. Scalar Arguments ..
195  INTEGER INFO, LDA, LDT, LWORK, M, N, MB, NB
196 * ..
197 * .. Array Arguments ..
198  REAL A( LDA, * ), T( LDT, * ), WORK( * )
199 * ..
200 *
201 * =====================================================================
202 *
203 * .. Parameters ..
204  REAL ONE, ZERO
205  parameter( one = 1.0e+0, zero = 0.0e+0 )
206 * ..
207 * .. Local Scalars ..
208  LOGICAL LQUERY
209  INTEGER NBLOCAL, MB2, M_PLUS_ONE, ITMP, IB_BOTTOM,
210  $ LWORKOPT, NUM_ALL_ROW_BLOCKS, JB_T, IB, IMB,
211  $ KB, KB_LAST, KNB, MB1
212 * ..
213 * .. Local Arrays ..
214  REAL DUMMY( 1, 1 )
215 * ..
216 * .. External Subroutines ..
217  EXTERNAL slarfb_gett, slaset, xerbla
218 * ..
219 * .. Intrinsic Functions ..
220  INTRINSIC real, max, min
221 * ..
222 * .. Executable Statements ..
223 *
224 * Test the input parameters
225 *
226  info = 0
227  lquery = lwork.EQ.-1
228  IF( m.LT.0 ) THEN
229  info = -1
230  ELSE IF( n.LT.0 .OR. m.LT.n ) THEN
231  info = -2
232  ELSE IF( mb.LE.n ) THEN
233  info = -3
234  ELSE IF( nb.LT.1 ) THEN
235  info = -4
236  ELSE IF( lda.LT.max( 1, m ) ) THEN
237  info = -6
238  ELSE IF( ldt.LT.max( 1, min( nb, n ) ) ) THEN
239  info = -8
240  ELSE IF( lwork.LT.1 .AND. .NOT.lquery ) THEN
241  info = -10
242  END IF
243 *
244  nblocal = min( nb, n )
245 *
246 * Determine the workspace size.
247 *
248  IF( info.EQ.0 ) THEN
249  lworkopt = nblocal * max( nblocal, ( n - nblocal ) )
250  END IF
251 *
252 * Handle error in the input parameters and handle the workspace query.
253 *
254  IF( info.NE.0 ) THEN
255  CALL xerbla( 'SORGTSQR_ROW', -info )
256  RETURN
257  ELSE IF ( lquery ) THEN
258  work( 1 ) = real( lworkopt )
259  RETURN
260  END IF
261 *
262 * Quick return if possible
263 *
264  IF( min( m, n ).EQ.0 ) THEN
265  work( 1 ) = real( lworkopt )
266  RETURN
267  END IF
268 *
269 * (0) Set the upper-triangular part of the matrix A to zero and
270 * its diagonal elements to one.
271 *
272  CALL slaset('U', m, n, zero, one, a, lda )
273 *
274 * KB_LAST is the column index of the last column block reflector
275 * in the matrices T and V.
276 *
277  kb_last = ( ( n-1 ) / nblocal ) * nblocal + 1
278 *
279 *
280 * (1) Bottom-up loop over row blocks of A, except the top row block.
281 * NOTE: If MB>=M, then the loop is never executed.
282 *
283  IF ( mb.LT.m ) THEN
284 *
285 * MB2 is the row blocking size for the row blocks before the
286 * first top row block in the matrix A. IB is the row index for
287 * the row blocks in the matrix A before the first top row block.
288 * IB_BOTTOM is the row index for the last bottom row block
289 * in the matrix A. JB_T is the column index of the corresponding
290 * column block in the matrix T.
291 *
292 * Initialize variables.
293 *
294 * NUM_ALL_ROW_BLOCKS is the number of row blocks in the matrix A
295 * including the first row block.
296 *
297  mb2 = mb - n
298  m_plus_one = m + 1
299  itmp = ( m - mb - 1 ) / mb2
300  ib_bottom = itmp * mb2 + mb + 1
301  num_all_row_blocks = itmp + 2
302  jb_t = num_all_row_blocks * n + 1
303 *
304  DO ib = ib_bottom, mb+1, -mb2
305 *
306 * Determine the block size IMB for the current row block
307 * in the matrix A.
308 *
309  imb = min( m_plus_one - ib, mb2 )
310 *
311 * Determine the column index JB_T for the current column block
312 * in the matrix T.
313 *
314  jb_t = jb_t - n
315 *
316 * Apply column blocks of H in the row block from right to left.
317 *
318 * KB is the column index of the current column block reflector
319 * in the matrices T and V.
320 *
321  DO kb = kb_last, 1, -nblocal
322 *
323 * Determine the size of the current column block KNB in
324 * the matrices T and V.
325 *
326  knb = min( nblocal, n - kb + 1 )
327 *
328  CALL slarfb_gett( 'I', imb, n-kb+1, knb,
329  $ t( 1, jb_t+kb-1 ), ldt, a( kb, kb ), lda,
330  $ a( ib, kb ), lda, work, knb )
331 *
332  END DO
333 *
334  END DO
335 *
336  END IF
337 *
338 * (2) Top row block of A.
339 * NOTE: If MB>=M, then we have only one row block of A of size M
340 * and we work on the entire matrix A.
341 *
342  mb1 = min( mb, m )
343 *
344 * Apply column blocks of H in the top row block from right to left.
345 *
346 * KB is the column index of the current block reflector in
347 * the matrices T and V.
348 *
349  DO kb = kb_last, 1, -nblocal
350 *
351 * Determine the size of the current column block KNB in
352 * the matrices T and V.
353 *
354  knb = min( nblocal, n - kb + 1 )
355 *
356  IF( mb1-kb-knb+1.EQ.0 ) THEN
357 *
358 * In SLARFB_GETT parameters, when M=0, then the matrix B
359 * does not exist, hence we need to pass a dummy array
360 * reference DUMMY(1,1) to B with LDDUMMY=1.
361 *
362  CALL slarfb_gett( 'N', 0, n-kb+1, knb,
363  $ t( 1, kb ), ldt, a( kb, kb ), lda,
364  $ dummy( 1, 1 ), 1, work, knb )
365  ELSE
366  CALL slarfb_gett( 'N', mb1-kb-knb+1, n-kb+1, knb,
367  $ t( 1, kb ), ldt, a( kb, kb ), lda,
368  $ a( kb+knb, kb), lda, work, knb )
369 
370  END IF
371 *
372  END DO
373 *
374  work( 1 ) = real( lworkopt )
375  RETURN
376 *
377 * End of SORGTSQR_ROW
378 *
subroutine slaset(UPLO, M, N, ALPHA, BETA, A, LDA)
SLASET initializes the off-diagonal elements and the diagonal elements of a matrix to given values.
Definition: slaset.f:110
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
subroutine slarfb_gett(IDENT, M, N, K, T, LDT, A, LDA, B, LDB, WORK, LDWORK)
SLARFB_GETT
Definition: slarfb_gett.f:392
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