LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ slarzb()

subroutine slarzb ( character  SIDE,
character  TRANS,
character  DIRECT,
character  STOREV,
integer  M,
integer  N,
integer  K,
integer  L,
real, dimension( ldv, * )  V,
integer  LDV,
real, dimension( ldt, * )  T,
integer  LDT,
real, dimension( ldc, * )  C,
integer  LDC,
real, dimension( ldwork, * )  WORK,
integer  LDWORK 
)

SLARZB applies a block reflector or its transpose to a general matrix.

Download SLARZB + dependencies [TGZ] [ZIP] [TXT]

Purpose:
 SLARZB applies a real block reflector H or its transpose H**T to
 a real distributed M-by-N  C from the left or the right.

 Currently, only STOREV = 'R' and DIRECT = 'B' are supported.
Parameters
[in]SIDE
          SIDE is CHARACTER*1
          = 'L': apply H or H**T from the Left
          = 'R': apply H or H**T from the Right
[in]TRANS
          TRANS is CHARACTER*1
          = 'N': apply H (No transpose)
          = 'C': apply H**T (Transpose)
[in]DIRECT
          DIRECT is CHARACTER*1
          Indicates how H is formed from a product of elementary
          reflectors
          = 'F': H = H(1) H(2) . . . H(k) (Forward, not supported yet)
          = 'B': H = H(k) . . . H(2) H(1) (Backward)
[in]STOREV
          STOREV is CHARACTER*1
          Indicates how the vectors which define the elementary
          reflectors are stored:
          = 'C': Columnwise                        (not supported yet)
          = 'R': Rowwise
[in]M
          M is INTEGER
          The number of rows of the matrix C.
[in]N
          N is INTEGER
          The number of columns of the matrix C.
[in]K
          K is INTEGER
          The order of the matrix T (= the number of elementary
          reflectors whose product defines the block reflector).
[in]L
          L is INTEGER
          The number of columns of the matrix V containing the
          meaningful part of the Householder reflectors.
          If SIDE = 'L', M >= L >= 0, if SIDE = 'R', N >= L >= 0.
[in]V
          V is REAL array, dimension (LDV,NV).
          If STOREV = 'C', NV = K; if STOREV = 'R', NV = L.
[in]LDV
          LDV is INTEGER
          The leading dimension of the array V.
          If STOREV = 'C', LDV >= L; if STOREV = 'R', LDV >= K.
[in]T
          T is REAL array, dimension (LDT,K)
          The triangular K-by-K matrix T in the representation of the
          block reflector.
[in]LDT
          LDT is INTEGER
          The leading dimension of the array T. LDT >= K.
[in,out]C
          C is REAL array, dimension (LDC,N)
          On entry, the M-by-N matrix C.
          On exit, C is overwritten by H*C or H**T*C or C*H or C*H**T.
[in]LDC
          LDC is INTEGER
          The leading dimension of the array C. LDC >= max(1,M).
[out]WORK
          WORK is REAL array, dimension (LDWORK,K)
[in]LDWORK
          LDWORK is INTEGER
          The leading dimension of the array WORK.
          If SIDE = 'L', LDWORK >= max(1,N);
          if SIDE = 'R', LDWORK >= max(1,M).
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Contributors:
A. Petitet, Computer Science Dept., Univ. of Tenn., Knoxville, USA
Further Details:
 

Definition at line 181 of file slarzb.f.

183 *
184 * -- LAPACK computational routine --
185 * -- LAPACK is a software package provided by Univ. of Tennessee, --
186 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
187 *
188 * .. Scalar Arguments ..
189  CHARACTER DIRECT, SIDE, STOREV, TRANS
190  INTEGER K, L, LDC, LDT, LDV, LDWORK, M, N
191 * ..
192 * .. Array Arguments ..
193  REAL C( LDC, * ), T( LDT, * ), V( LDV, * ),
194  $ WORK( LDWORK, * )
195 * ..
196 *
197 * =====================================================================
198 *
199 * .. Parameters ..
200  REAL ONE
201  parameter( one = 1.0e+0 )
202 * ..
203 * .. Local Scalars ..
204  CHARACTER TRANST
205  INTEGER I, INFO, J
206 * ..
207 * .. External Functions ..
208  LOGICAL LSAME
209  EXTERNAL lsame
210 * ..
211 * .. External Subroutines ..
212  EXTERNAL scopy, sgemm, strmm, xerbla
213 * ..
214 * .. Executable Statements ..
215 *
216 * Quick return if possible
217 *
218  IF( m.LE.0 .OR. n.LE.0 )
219  $ RETURN
220 *
221 * Check for currently supported options
222 *
223  info = 0
224  IF( .NOT.lsame( direct, 'B' ) ) THEN
225  info = -3
226  ELSE IF( .NOT.lsame( storev, 'R' ) ) THEN
227  info = -4
228  END IF
229  IF( info.NE.0 ) THEN
230  CALL xerbla( 'SLARZB', -info )
231  RETURN
232  END IF
233 *
234  IF( lsame( trans, 'N' ) ) THEN
235  transt = 'T'
236  ELSE
237  transt = 'N'
238  END IF
239 *
240  IF( lsame( side, 'L' ) ) THEN
241 *
242 * Form H * C or H**T * C
243 *
244 * W( 1:n, 1:k ) = C( 1:k, 1:n )**T
245 *
246  DO 10 j = 1, k
247  CALL scopy( n, c( j, 1 ), ldc, work( 1, j ), 1 )
248  10 CONTINUE
249 *
250 * W( 1:n, 1:k ) = W( 1:n, 1:k ) + ...
251 * C( m-l+1:m, 1:n )**T * V( 1:k, 1:l )**T
252 *
253  IF( l.GT.0 )
254  $ CALL sgemm( 'Transpose', 'Transpose', n, k, l, one,
255  $ c( m-l+1, 1 ), ldc, v, ldv, one, work, ldwork )
256 *
257 * W( 1:n, 1:k ) = W( 1:n, 1:k ) * T**T or W( 1:m, 1:k ) * T
258 *
259  CALL strmm( 'Right', 'Lower', transt, 'Non-unit', n, k, one, t,
260  $ ldt, work, ldwork )
261 *
262 * C( 1:k, 1:n ) = C( 1:k, 1:n ) - W( 1:n, 1:k )**T
263 *
264  DO 30 j = 1, n
265  DO 20 i = 1, k
266  c( i, j ) = c( i, j ) - work( j, i )
267  20 CONTINUE
268  30 CONTINUE
269 *
270 * C( m-l+1:m, 1:n ) = C( m-l+1:m, 1:n ) - ...
271 * V( 1:k, 1:l )**T * W( 1:n, 1:k )**T
272 *
273  IF( l.GT.0 )
274  $ CALL sgemm( 'Transpose', 'Transpose', l, n, k, -one, v, ldv,
275  $ work, ldwork, one, c( m-l+1, 1 ), ldc )
276 *
277  ELSE IF( lsame( side, 'R' ) ) THEN
278 *
279 * Form C * H or C * H**T
280 *
281 * W( 1:m, 1:k ) = C( 1:m, 1:k )
282 *
283  DO 40 j = 1, k
284  CALL scopy( m, c( 1, j ), 1, work( 1, j ), 1 )
285  40 CONTINUE
286 *
287 * W( 1:m, 1:k ) = W( 1:m, 1:k ) + ...
288 * C( 1:m, n-l+1:n ) * V( 1:k, 1:l )**T
289 *
290  IF( l.GT.0 )
291  $ CALL sgemm( 'No transpose', 'Transpose', m, k, l, one,
292  $ c( 1, n-l+1 ), ldc, v, ldv, one, work, ldwork )
293 *
294 * W( 1:m, 1:k ) = W( 1:m, 1:k ) * T or W( 1:m, 1:k ) * T**T
295 *
296  CALL strmm( 'Right', 'Lower', trans, 'Non-unit', m, k, one, t,
297  $ ldt, work, ldwork )
298 *
299 * C( 1:m, 1:k ) = C( 1:m, 1:k ) - W( 1:m, 1:k )
300 *
301  DO 60 j = 1, k
302  DO 50 i = 1, m
303  c( i, j ) = c( i, j ) - work( i, j )
304  50 CONTINUE
305  60 CONTINUE
306 *
307 * C( 1:m, n-l+1:n ) = C( 1:m, n-l+1:n ) - ...
308 * W( 1:m, 1:k ) * V( 1:k, 1:l )
309 *
310  IF( l.GT.0 )
311  $ CALL sgemm( 'No transpose', 'No transpose', m, l, k, -one,
312  $ work, ldwork, v, ldv, one, c( 1, n-l+1 ), ldc )
313 *
314  END IF
315 *
316  RETURN
317 *
318 * End of SLARZB
319 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine scopy(N, SX, INCX, SY, INCY)
SCOPY
Definition: scopy.f:82
subroutine strmm(SIDE, UPLO, TRANSA, DIAG, M, N, ALPHA, A, LDA, B, LDB)
STRMM
Definition: strmm.f:177
subroutine sgemm(TRANSA, TRANSB, M, N, K, ALPHA, A, LDA, B, LDB, BETA, C, LDC)
SGEMM
Definition: sgemm.f:187
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