LAPACK  3.10.1
LAPACK: Linear Algebra PACKage

◆ stplqt2()

subroutine stplqt2 ( integer  M,
integer  N,
integer  L,
real, dimension( lda, * )  A,
integer  LDA,
real, dimension( ldb, * )  B,
integer  LDB,
real, dimension( ldt, * )  T,
integer  LDT,
integer  INFO 
)

STPLQT2 computes a LQ factorization of a real or complex "triangular-pentagonal" matrix, which is composed of a triangular block and a pentagonal block, using the compact WY representation for Q.

Download STPLQT2 + dependencies [TGZ] [ZIP] [TXT]

Purpose:
 STPLQT2 computes a LQ a factorization of a real "triangular-pentagonal"
 matrix C, which is composed of a triangular block A and pentagonal block B,
 using the compact WY representation for Q.
Parameters
[in]M
          M is INTEGER
          The total number of rows of the matrix B.
          M >= 0.
[in]N
          N is INTEGER
          The number of columns of the matrix B, and the order of
          the triangular matrix A.
          N >= 0.
[in]L
          L is INTEGER
          The number of rows of the lower trapezoidal part of B.
          MIN(M,N) >= L >= 0.  See Further Details.
[in,out]A
          A is REAL array, dimension (LDA,M)
          On entry, the lower triangular M-by-M matrix A.
          On exit, the elements on and below the diagonal of the array
          contain the lower triangular matrix L.
[in]LDA
          LDA is INTEGER
          The leading dimension of the array A.  LDA >= max(1,M).
[in,out]B
          B is REAL array, dimension (LDB,N)
          On entry, the pentagonal M-by-N matrix B.  The first N-L columns
          are rectangular, and the last L columns are lower trapezoidal.
          On exit, B contains the pentagonal matrix V.  See Further Details.
[in]LDB
          LDB is INTEGER
          The leading dimension of the array B.  LDB >= max(1,M).
[out]T
          T is REAL array, dimension (LDT,M)
          The N-by-N upper triangular factor T of the block reflector.
          See Further Details.
[in]LDT
          LDT is INTEGER
          The leading dimension of the array T.  LDT >= max(1,M)
[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.
Further Details:
  The input matrix C is a M-by-(M+N) matrix

               C = [ A ][ B ]


  where A is an lower triangular M-by-M matrix, and B is M-by-N pentagonal
  matrix consisting of a M-by-(N-L) rectangular matrix B1 left of a M-by-L
  upper trapezoidal matrix B2:

               B = [ B1 ][ B2 ]
                   [ B1 ]  <-     M-by-(N-L) rectangular
                   [ B2 ]  <-     M-by-L lower trapezoidal.

  The lower trapezoidal matrix B2 consists of the first L columns of a
  N-by-N lower triangular matrix, where 0 <= L <= MIN(M,N).  If L=0,
  B is rectangular M-by-N; if M=L=N, B is lower triangular.

  The matrix W stores the elementary reflectors H(i) in the i-th row
  above the diagonal (of A) in the M-by-(M+N) input matrix C

               C = [ A ][ B ]
                   [ A ]  <- lower triangular M-by-M
                   [ B ]  <- M-by-N pentagonal

  so that W can be represented as

               W = [ I ][ V ]
                   [ I ]  <- identity, M-by-M
                   [ V ]  <- M-by-N, same form as B.

  Thus, all of information needed for W is contained on exit in B, which
  we call V above.  Note that V has the same form as B; that is,

               W = [ V1 ][ V2 ]
                   [ V1 ] <-     M-by-(N-L) rectangular
                   [ V2 ] <-     M-by-L lower trapezoidal.

  The rows of V represent the vectors which define the H(i)'s.
  The (M+N)-by-(M+N) block reflector H is then given by

               H = I - W**T * T * W

  where W^H is the conjugate transpose of W and T is the upper triangular
  factor of the block reflector.

Definition at line 176 of file stplqt2.f.

177 *
178 * -- LAPACK computational routine --
179 * -- LAPACK is a software package provided by Univ. of Tennessee, --
180 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
181 *
182 * .. Scalar Arguments ..
183  INTEGER INFO, LDA, LDB, LDT, N, M, L
184 * ..
185 * .. Array Arguments ..
186  REAL A( LDA, * ), B( LDB, * ), T( LDT, * )
187 * ..
188 *
189 * =====================================================================
190 *
191 * .. Parameters ..
192  REAL ONE, ZERO
193  parameter( one = 1.0, zero = 0.0 )
194 * ..
195 * .. Local Scalars ..
196  INTEGER I, J, P, MP, NP
197  REAL ALPHA
198 * ..
199 * .. External Subroutines ..
200  EXTERNAL slarfg, sgemv, sger, strmv, xerbla
201 * ..
202 * .. Intrinsic Functions ..
203  INTRINSIC max, min
204 * ..
205 * .. Executable Statements ..
206 *
207 * Test the input arguments
208 *
209  info = 0
210  IF( m.LT.0 ) THEN
211  info = -1
212  ELSE IF( n.LT.0 ) THEN
213  info = -2
214  ELSE IF( l.LT.0 .OR. l.GT.min(m,n) ) THEN
215  info = -3
216  ELSE IF( lda.LT.max( 1, m ) ) THEN
217  info = -5
218  ELSE IF( ldb.LT.max( 1, m ) ) THEN
219  info = -7
220  ELSE IF( ldt.LT.max( 1, m ) ) THEN
221  info = -9
222  END IF
223  IF( info.NE.0 ) THEN
224  CALL xerbla( 'STPLQT2', -info )
225  RETURN
226  END IF
227 *
228 * Quick return if possible
229 *
230  IF( n.EQ.0 .OR. m.EQ.0 ) RETURN
231 *
232  DO i = 1, m
233 *
234 * Generate elementary reflector H(I) to annihilate B(I,:)
235 *
236  p = n-l+min( l, i )
237  CALL slarfg( p+1, a( i, i ), b( i, 1 ), ldb, t( 1, i ) )
238  IF( i.LT.m ) THEN
239 *
240 * W(M-I:1) := C(I+1:M,I:N) * C(I,I:N) [use W = T(M,:)]
241 *
242  DO j = 1, m-i
243  t( m, j ) = (a( i+j, i ))
244  END DO
245  CALL sgemv( 'N', m-i, p, one, b( i+1, 1 ), ldb,
246  $ b( i, 1 ), ldb, one, t( m, 1 ), ldt )
247 *
248 * C(I+1:M,I:N) = C(I+1:M,I:N) + alpha * C(I,I:N)*W(M-1:1)^H
249 *
250  alpha = -(t( 1, i ))
251  DO j = 1, m-i
252  a( i+j, i ) = a( i+j, i ) + alpha*(t( m, j ))
253  END DO
254  CALL sger( m-i, p, alpha, t( m, 1 ), ldt,
255  $ b( i, 1 ), ldb, b( i+1, 1 ), ldb )
256  END IF
257  END DO
258 *
259  DO i = 2, m
260 *
261 * T(I,1:I-1) := C(I:I-1,1:N) * (alpha * C(I,I:N)^H)
262 *
263  alpha = -t( 1, i )
264 
265  DO j = 1, i-1
266  t( i, j ) = zero
267  END DO
268  p = min( i-1, l )
269  np = min( n-l+1, n )
270  mp = min( p+1, m )
271 *
272 * Triangular part of B2
273 *
274  DO j = 1, p
275  t( i, j ) = alpha*b( i, n-l+j )
276  END DO
277  CALL strmv( 'L', 'N', 'N', p, b( 1, np ), ldb,
278  $ t( i, 1 ), ldt )
279 *
280 * Rectangular part of B2
281 *
282  CALL sgemv( 'N', i-1-p, l, alpha, b( mp, np ), ldb,
283  $ b( i, np ), ldb, zero, t( i,mp ), ldt )
284 *
285 * B1
286 *
287  CALL sgemv( 'N', i-1, n-l, alpha, b, ldb, b( i, 1 ), ldb,
288  $ one, t( i, 1 ), ldt )
289 *
290 * T(1:I-1,I) := T(1:I-1,1:I-1) * T(I,1:I-1)
291 *
292  CALL strmv( 'L', 'T', 'N', i-1, t, ldt, t( i, 1 ), ldt )
293 *
294 * T(I,I) = tau(I)
295 *
296  t( i, i ) = t( 1, i )
297  t( 1, i ) = zero
298  END DO
299  DO i=1,m
300  DO j= i+1,m
301  t(i,j)=t(j,i)
302  t(j,i)= zero
303  END DO
304  END DO
305 
306 *
307 * End of STPLQT2
308 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
subroutine slarfg(N, ALPHA, X, INCX, TAU)
SLARFG generates an elementary reflector (Householder matrix).
Definition: slarfg.f:106
subroutine strmv(UPLO, TRANS, DIAG, N, A, LDA, X, INCX)
STRMV
Definition: strmv.f:147
subroutine sger(M, N, ALPHA, X, INCX, Y, INCY, A, LDA)
SGER
Definition: sger.f:130
subroutine sgemv(TRANS, M, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
SGEMV
Definition: sgemv.f:156
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