1: *> \brief \b DLARFB_GETT
2: *
3: * =========== DOCUMENTATION ===========
4: *
5: * Online html documentation available at
6: * http://www.netlib.org/lapack/explore-html/
7: *
8: *> \htmlonly
9: *> Download DLARFB_GETT + dependencies
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11: *> [TGZ]</a>
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13: *> [ZIP]</a>
14: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/dlarfb_gett.f">
15: *> [TXT]</a>
16: *> \endhtmlonly
17: *
18: * Definition:
19: * ===========
20: *
21: * SUBROUTINE DLARFB_GETT( IDENT, M, N, K, T, LDT, A, LDA, B, LDB,
22: * $ WORK, LDWORK )
23: * IMPLICIT NONE
24: *
25: * .. Scalar Arguments ..
26: * CHARACTER IDENT
27: * INTEGER K, LDA, LDB, LDT, LDWORK, M, N
28: * ..
29: * .. Array Arguments ..
30: * DOUBLE PRECISION A( LDA, * ), B( LDB, * ), T( LDT, * ),
31: * $ WORK( LDWORK, * )
32: * ..
33: *
34: *> \par Purpose:
35: * =============
36: *>
37: *> \verbatim
38: *>
39: *> DLARFB_GETT applies a real Householder block reflector H from the
40: *> left to a real (K+M)-by-N "triangular-pentagonal" matrix
41: *> composed of two block matrices: an upper trapezoidal K-by-N matrix A
42: *> stored in the array A, and a rectangular M-by-(N-K) matrix B, stored
43: *> in the array B. The block reflector H is stored in a compact
44: *> WY-representation, where the elementary reflectors are in the
45: *> arrays A, B and T. See Further Details section.
46: *> \endverbatim
47: *
48: * Arguments:
49: * ==========
50: *
51: *> \param[in] IDENT
52: *> \verbatim
53: *> IDENT is CHARACTER*1
54: *> If IDENT = not 'I', or not 'i', then V1 is unit
55: *> lower-triangular and stored in the left K-by-K block of
56: *> the input matrix A,
57: *> If IDENT = 'I' or 'i', then V1 is an identity matrix and
58: *> not stored.
59: *> See Further Details section.
60: *> \endverbatim
61: *>
62: *> \param[in] M
63: *> \verbatim
64: *> M is INTEGER
65: *> The number of rows of the matrix B.
66: *> M >= 0.
67: *> \endverbatim
68: *>
69: *> \param[in] N
70: *> \verbatim
71: *> N is INTEGER
72: *> The number of columns of the matrices A and B.
73: *> N >= 0.
74: *> \endverbatim
75: *>
76: *> \param[in] K
77: *> \verbatim
78: *> K is INTEGER
79: *> The number or rows of the matrix A.
80: *> K is also order of the matrix T, i.e. the number of
81: *> elementary reflectors whose product defines the block
82: *> reflector. 0 <= K <= N.
83: *> \endverbatim
84: *>
85: *> \param[in] T
86: *> \verbatim
87: *> T is DOUBLE PRECISION array, dimension (LDT,K)
88: *> The upper-triangular K-by-K matrix T in the representation
89: *> of the block reflector.
90: *> \endverbatim
91: *>
92: *> \param[in] LDT
93: *> \verbatim
94: *> LDT is INTEGER
95: *> The leading dimension of the array T. LDT >= K.
96: *> \endverbatim
97: *>
98: *> \param[in,out] A
99: *> \verbatim
100: *> A is DOUBLE PRECISION array, dimension (LDA,N)
101: *>
102: *> On entry:
103: *> a) In the K-by-N upper-trapezoidal part A: input matrix A.
104: *> b) In the columns below the diagonal: columns of V1
105: *> (ones are not stored on the diagonal).
106: *>
107: *> On exit:
108: *> A is overwritten by rectangular K-by-N product H*A.
109: *>
110: *> See Further Details section.
111: *> \endverbatim
112: *>
113: *> \param[in] LDA
114: *> \verbatim
115: *> LDB is INTEGER
116: *> The leading dimension of the array A. LDA >= max(1,K).
117: *> \endverbatim
118: *>
119: *> \param[in,out] B
120: *> \verbatim
121: *> B is DOUBLE PRECISION array, dimension (LDB,N)
122: *>
123: *> On entry:
124: *> a) In the M-by-(N-K) right block: input matrix B.
125: *> b) In the M-by-N left block: columns of V2.
126: *>
127: *> On exit:
128: *> B is overwritten by rectangular M-by-N product H*B.
129: *>
130: *> See Further Details section.
131: *> \endverbatim
132: *>
133: *> \param[in] LDB
134: *> \verbatim
135: *> LDB is INTEGER
136: *> The leading dimension of the array B. LDB >= max(1,M).
137: *> \endverbatim
138: *>
139: *> \param[out] WORK
140: *> \verbatim
141: *> WORK is DOUBLE PRECISION array,
142: *> dimension (LDWORK,max(K,N-K))
143: *> \endverbatim
144: *>
145: *> \param[in] LDWORK
146: *> \verbatim
147: *> LDWORK is INTEGER
148: *> The leading dimension of the array WORK. LDWORK>=max(1,K).
149: *>
150: *> \endverbatim
151: *
152: * Authors:
153: * ========
154: *
155: *> \author Univ. of Tennessee
156: *> \author Univ. of California Berkeley
157: *> \author Univ. of Colorado Denver
158: *> \author NAG Ltd.
159: *
160: *> \ingroup doubleOTHERauxiliary
161: *
162: *> \par Contributors:
163: * ==================
164: *>
165: *> \verbatim
166: *>
167: *> November 2020, Igor Kozachenko,
168: *> Computer Science Division,
169: *> University of California, Berkeley
170: *>
171: *> \endverbatim
172: *
173: *> \par Further Details:
174: * =====================
175: *>
176: *> \verbatim
177: *>
178: *> (1) Description of the Algebraic Operation.
179: *>
180: *> The matrix A is a K-by-N matrix composed of two column block
181: *> matrices, A1, which is K-by-K, and A2, which is K-by-(N-K):
182: *> A = ( A1, A2 ).
183: *> The matrix B is an M-by-N matrix composed of two column block
184: *> matrices, B1, which is M-by-K, and B2, which is M-by-(N-K):
185: *> B = ( B1, B2 ).
186: *>
187: *> Perform the operation:
188: *>
189: *> ( A_out ) := H * ( A_in ) = ( I - V * T * V**T ) * ( A_in ) =
190: *> ( B_out ) ( B_in ) ( B_in )
191: *> = ( I - ( V1 ) * T * ( V1**T, V2**T ) ) * ( A_in )
192: *> ( V2 ) ( B_in )
193: *> On input:
194: *>
195: *> a) ( A_in ) consists of two block columns:
196: *> ( B_in )
197: *>
198: *> ( A_in ) = (( A1_in ) ( A2_in )) = (( A1_in ) ( A2_in ))
199: *> ( B_in ) (( B1_in ) ( B2_in )) (( 0 ) ( B2_in )),
200: *>
201: *> where the column blocks are:
202: *>
203: *> ( A1_in ) is a K-by-K upper-triangular matrix stored in the
204: *> upper triangular part of the array A(1:K,1:K).
205: *> ( B1_in ) is an M-by-K rectangular ZERO matrix and not stored.
206: *>
207: *> ( A2_in ) is a K-by-(N-K) rectangular matrix stored
208: *> in the array A(1:K,K+1:N).
209: *> ( B2_in ) is an M-by-(N-K) rectangular matrix stored
210: *> in the array B(1:M,K+1:N).
211: *>
212: *> b) V = ( V1 )
213: *> ( V2 )
214: *>
215: *> where:
216: *> 1) if IDENT == 'I',V1 is a K-by-K identity matrix, not stored;
217: *> 2) if IDENT != 'I',V1 is a K-by-K unit lower-triangular matrix,
218: *> stored in the lower-triangular part of the array
219: *> A(1:K,1:K) (ones are not stored),
220: *> and V2 is an M-by-K rectangular stored the array B(1:M,1:K),
221: *> (because on input B1_in is a rectangular zero
222: *> matrix that is not stored and the space is
223: *> used to store V2).
224: *>
225: *> c) T is a K-by-K upper-triangular matrix stored
226: *> in the array T(1:K,1:K).
227: *>
228: *> On output:
229: *>
230: *> a) ( A_out ) consists of two block columns:
231: *> ( B_out )
232: *>
233: *> ( A_out ) = (( A1_out ) ( A2_out ))
234: *> ( B_out ) (( B1_out ) ( B2_out )),
235: *>
236: *> where the column blocks are:
237: *>
238: *> ( A1_out ) is a K-by-K square matrix, or a K-by-K
239: *> upper-triangular matrix, if V1 is an
240: *> identity matrix. AiOut is stored in
241: *> the array A(1:K,1:K).
242: *> ( B1_out ) is an M-by-K rectangular matrix stored
243: *> in the array B(1:M,K:N).
244: *>
245: *> ( A2_out ) is a K-by-(N-K) rectangular matrix stored
246: *> in the array A(1:K,K+1:N).
247: *> ( B2_out ) is an M-by-(N-K) rectangular matrix stored
248: *> in the array B(1:M,K+1:N).
249: *>
250: *>
251: *> The operation above can be represented as the same operation
252: *> on each block column:
253: *>
254: *> ( A1_out ) := H * ( A1_in ) = ( I - V * T * V**T ) * ( A1_in )
255: *> ( B1_out ) ( 0 ) ( 0 )
256: *>
257: *> ( A2_out ) := H * ( A2_in ) = ( I - V * T * V**T ) * ( A2_in )
258: *> ( B2_out ) ( B2_in ) ( B2_in )
259: *>
260: *> If IDENT != 'I':
261: *>
262: *> The computation for column block 1:
263: *>
264: *> A1_out: = A1_in - V1*T*(V1**T)*A1_in
265: *>
266: *> B1_out: = - V2*T*(V1**T)*A1_in
267: *>
268: *> The computation for column block 2, which exists if N > K:
269: *>
270: *> A2_out: = A2_in - V1*T*( (V1**T)*A2_in + (V2**T)*B2_in )
271: *>
272: *> B2_out: = B2_in - V2*T*( (V1**T)*A2_in + (V2**T)*B2_in )
273: *>
274: *> If IDENT == 'I':
275: *>
276: *> The operation for column block 1:
277: *>
278: *> A1_out: = A1_in - V1*T**A1_in
279: *>
280: *> B1_out: = - V2*T**A1_in
281: *>
282: *> The computation for column block 2, which exists if N > K:
283: *>
284: *> A2_out: = A2_in - T*( A2_in + (V2**T)*B2_in )
285: *>
286: *> B2_out: = B2_in - V2*T*( A2_in + (V2**T)*B2_in )
287: *>
288: *> (2) Description of the Algorithmic Computation.
289: *>
290: *> In the first step, we compute column block 2, i.e. A2 and B2.
291: *> Here, we need to use the K-by-(N-K) rectangular workspace
292: *> matrix W2 that is of the same size as the matrix A2.
293: *> W2 is stored in the array WORK(1:K,1:(N-K)).
294: *>
295: *> In the second step, we compute column block 1, i.e. A1 and B1.
296: *> Here, we need to use the K-by-K square workspace matrix W1
297: *> that is of the same size as the as the matrix A1.
298: *> W1 is stored in the array WORK(1:K,1:K).
299: *>
300: *> NOTE: Hence, in this routine, we need the workspace array WORK
301: *> only of size WORK(1:K,1:max(K,N-K)) so it can hold both W2 from
302: *> the first step and W1 from the second step.
303: *>
304: *> Case (A), when V1 is unit lower-triangular, i.e. IDENT != 'I',
305: *> more computations than in the Case (B).
306: *>
307: *> if( IDENT != 'I' ) then
308: *> if ( N > K ) then
309: *> (First Step - column block 2)
310: *> col2_(1) W2: = A2
311: *> col2_(2) W2: = (V1**T) * W2 = (unit_lower_tr_of_(A1)**T) * W2
312: *> col2_(3) W2: = W2 + (V2**T) * B2 = W2 + (B1**T) * B2
313: *> col2_(4) W2: = T * W2
314: *> col2_(5) B2: = B2 - V2 * W2 = B2 - B1 * W2
315: *> col2_(6) W2: = V1 * W2 = unit_lower_tr_of_(A1) * W2
316: *> col2_(7) A2: = A2 - W2
317: *> else
318: *> (Second Step - column block 1)
319: *> col1_(1) W1: = A1
320: *> col1_(2) W1: = (V1**T) * W1 = (unit_lower_tr_of_(A1)**T) * W1
321: *> col1_(3) W1: = T * W1
322: *> col1_(4) B1: = - V2 * W1 = - B1 * W1
323: *> col1_(5) square W1: = V1 * W1 = unit_lower_tr_of_(A1) * W1
324: *> col1_(6) square A1: = A1 - W1
325: *> end if
326: *> end if
327: *>
328: *> Case (B), when V1 is an identity matrix, i.e. IDENT == 'I',
329: *> less computations than in the Case (A)
330: *>
331: *> if( IDENT == 'I' ) then
332: *> if ( N > K ) then
333: *> (First Step - column block 2)
334: *> col2_(1) W2: = A2
335: *> col2_(3) W2: = W2 + (V2**T) * B2 = W2 + (B1**T) * B2
336: *> col2_(4) W2: = T * W2
337: *> col2_(5) B2: = B2 - V2 * W2 = B2 - B1 * W2
338: *> col2_(7) A2: = A2 - W2
339: *> else
340: *> (Second Step - column block 1)
341: *> col1_(1) W1: = A1
342: *> col1_(3) W1: = T * W1
343: *> col1_(4) B1: = - V2 * W1 = - B1 * W1
344: *> col1_(6) upper-triangular_of_(A1): = A1 - W1
345: *> end if
346: *> end if
347: *>
348: *> Combine these cases (A) and (B) together, this is the resulting
349: *> algorithm:
350: *>
351: *> if ( N > K ) then
352: *>
353: *> (First Step - column block 2)
354: *>
355: *> col2_(1) W2: = A2
356: *> if( IDENT != 'I' ) then
357: *> col2_(2) W2: = (V1**T) * W2
358: *> = (unit_lower_tr_of_(A1)**T) * W2
359: *> end if
360: *> col2_(3) W2: = W2 + (V2**T) * B2 = W2 + (B1**T) * B2]
361: *> col2_(4) W2: = T * W2
362: *> col2_(5) B2: = B2 - V2 * W2 = B2 - B1 * W2
363: *> if( IDENT != 'I' ) then
364: *> col2_(6) W2: = V1 * W2 = unit_lower_tr_of_(A1) * W2
365: *> end if
366: *> col2_(7) A2: = A2 - W2
367: *>
368: *> else
369: *>
370: *> (Second Step - column block 1)
371: *>
372: *> col1_(1) W1: = A1
373: *> if( IDENT != 'I' ) then
374: *> col1_(2) W1: = (V1**T) * W1
375: *> = (unit_lower_tr_of_(A1)**T) * W1
376: *> end if
377: *> col1_(3) W1: = T * W1
378: *> col1_(4) B1: = - V2 * W1 = - B1 * W1
379: *> if( IDENT != 'I' ) then
380: *> col1_(5) square W1: = V1 * W1 = unit_lower_tr_of_(A1) * W1
381: *> col1_(6_a) below_diag_of_(A1): = - below_diag_of_(W1)
382: *> end if
383: *> col1_(6_b) up_tr_of_(A1): = up_tr_of_(A1) - up_tr_of_(W1)
384: *>
385: *> end if
386: *>
387: *> \endverbatim
388: *>
389: * =====================================================================
390: SUBROUTINE DLARFB_GETT( IDENT, M, N, K, T, LDT, A, LDA, B, LDB,
391: $ WORK, LDWORK )
392: IMPLICIT NONE
393: *
394: * -- LAPACK auxiliary routine --
395: * -- LAPACK is a software package provided by Univ. of Tennessee, --
396: * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
397: *
398: * .. Scalar Arguments ..
399: CHARACTER IDENT
400: INTEGER K, LDA, LDB, LDT, LDWORK, M, N
401: * ..
402: * .. Array Arguments ..
403: DOUBLE PRECISION A( LDA, * ), B( LDB, * ), T( LDT, * ),
404: $ WORK( LDWORK, * )
405: * ..
406: *
407: * =====================================================================
408: *
409: * .. Parameters ..
410: DOUBLE PRECISION ONE, ZERO
411: PARAMETER ( ONE = 1.0D+0, ZERO = 0.0D+0 )
412: * ..
413: * .. Local Scalars ..
414: LOGICAL LNOTIDENT
415: INTEGER I, J
416: * ..
417: * .. EXTERNAL FUNCTIONS ..
418: LOGICAL LSAME
419: EXTERNAL LSAME
420: * ..
421: * .. External Subroutines ..
422: EXTERNAL DCOPY, DGEMM, DTRMM
423: * ..
424: * .. Executable Statements ..
425: *
426: * Quick return if possible
427: *
428: IF( M.LT.0 .OR. N.LE.0 .OR. K.EQ.0 .OR. K.GT.N )
429: $ RETURN
430: *
431: LNOTIDENT = .NOT.LSAME( IDENT, 'I' )
432: *
433: * ------------------------------------------------------------------
434: *
435: * First Step. Computation of the Column Block 2:
436: *
437: * ( A2 ) := H * ( A2 )
438: * ( B2 ) ( B2 )
439: *
440: * ------------------------------------------------------------------
441: *
442: IF( N.GT.K ) THEN
443: *
444: * col2_(1) Compute W2: = A2. Therefore, copy A2 = A(1:K, K+1:N)
445: * into W2=WORK(1:K, 1:N-K) column-by-column.
446: *
447: DO J = 1, N-K
448: CALL DCOPY( K, A( 1, K+J ), 1, WORK( 1, J ), 1 )
449: END DO
450:
451: IF( LNOTIDENT ) THEN
452: *
453: * col2_(2) Compute W2: = (V1**T) * W2 = (A1**T) * W2,
454: * V1 is not an identy matrix, but unit lower-triangular
455: * V1 stored in A1 (diagonal ones are not stored).
456: *
457: *
458: CALL DTRMM( 'L', 'L', 'T', 'U', K, N-K, ONE, A, LDA,
459: $ WORK, LDWORK )
460: END IF
461: *
462: * col2_(3) Compute W2: = W2 + (V2**T) * B2 = W2 + (B1**T) * B2
463: * V2 stored in B1.
464: *
465: IF( M.GT.0 ) THEN
466: CALL DGEMM( 'T', 'N', K, N-K, M, ONE, B, LDB,
467: $ B( 1, K+1 ), LDB, ONE, WORK, LDWORK )
468: END IF
469: *
470: * col2_(4) Compute W2: = T * W2,
471: * T is upper-triangular.
472: *
473: CALL DTRMM( 'L', 'U', 'N', 'N', K, N-K, ONE, T, LDT,
474: $ WORK, LDWORK )
475: *
476: * col2_(5) Compute B2: = B2 - V2 * W2 = B2 - B1 * W2,
477: * V2 stored in B1.
478: *
479: IF( M.GT.0 ) THEN
480: CALL DGEMM( 'N', 'N', M, N-K, K, -ONE, B, LDB,
481: $ WORK, LDWORK, ONE, B( 1, K+1 ), LDB )
482: END IF
483: *
484: IF( LNOTIDENT ) THEN
485: *
486: * col2_(6) Compute W2: = V1 * W2 = A1 * W2,
487: * V1 is not an identity matrix, but unit lower-triangular,
488: * V1 stored in A1 (diagonal ones are not stored).
489: *
490: CALL DTRMM( 'L', 'L', 'N', 'U', K, N-K, ONE, A, LDA,
491: $ WORK, LDWORK )
492: END IF
493: *
494: * col2_(7) Compute A2: = A2 - W2 =
495: * = A(1:K, K+1:N-K) - WORK(1:K, 1:N-K),
496: * column-by-column.
497: *
498: DO J = 1, N-K
499: DO I = 1, K
500: A( I, K+J ) = A( I, K+J ) - WORK( I, J )
501: END DO
502: END DO
503: *
504: END IF
505: *
506: * ------------------------------------------------------------------
507: *
508: * Second Step. Computation of the Column Block 1:
509: *
510: * ( A1 ) := H * ( A1 )
511: * ( B1 ) ( 0 )
512: *
513: * ------------------------------------------------------------------
514: *
515: * col1_(1) Compute W1: = A1. Copy the upper-triangular
516: * A1 = A(1:K, 1:K) into the upper-triangular
517: * W1 = WORK(1:K, 1:K) column-by-column.
518: *
519: DO J = 1, K
520: CALL DCOPY( J, A( 1, J ), 1, WORK( 1, J ), 1 )
521: END DO
522: *
523: * Set the subdiagonal elements of W1 to zero column-by-column.
524: *
525: DO J = 1, K - 1
526: DO I = J + 1, K
527: WORK( I, J ) = ZERO
528: END DO
529: END DO
530: *
531: IF( LNOTIDENT ) THEN
532: *
533: * col1_(2) Compute W1: = (V1**T) * W1 = (A1**T) * W1,
534: * V1 is not an identity matrix, but unit lower-triangular
535: * V1 stored in A1 (diagonal ones are not stored),
536: * W1 is upper-triangular with zeroes below the diagonal.
537: *
538: CALL DTRMM( 'L', 'L', 'T', 'U', K, K, ONE, A, LDA,
539: $ WORK, LDWORK )
540: END IF
541: *
542: * col1_(3) Compute W1: = T * W1,
543: * T is upper-triangular,
544: * W1 is upper-triangular with zeroes below the diagonal.
545: *
546: CALL DTRMM( 'L', 'U', 'N', 'N', K, K, ONE, T, LDT,
547: $ WORK, LDWORK )
548: *
549: * col1_(4) Compute B1: = - V2 * W1 = - B1 * W1,
550: * V2 = B1, W1 is upper-triangular with zeroes below the diagonal.
551: *
552: IF( M.GT.0 ) THEN
553: CALL DTRMM( 'R', 'U', 'N', 'N', M, K, -ONE, WORK, LDWORK,
554: $ B, LDB )
555: END IF
556: *
557: IF( LNOTIDENT ) THEN
558: *
559: * col1_(5) Compute W1: = V1 * W1 = A1 * W1,
560: * V1 is not an identity matrix, but unit lower-triangular
561: * V1 stored in A1 (diagonal ones are not stored),
562: * W1 is upper-triangular on input with zeroes below the diagonal,
563: * and square on output.
564: *
565: CALL DTRMM( 'L', 'L', 'N', 'U', K, K, ONE, A, LDA,
566: $ WORK, LDWORK )
567: *
568: * col1_(6) Compute A1: = A1 - W1 = A(1:K, 1:K) - WORK(1:K, 1:K)
569: * column-by-column. A1 is upper-triangular on input.
570: * If IDENT, A1 is square on output, and W1 is square,
571: * if NOT IDENT, A1 is upper-triangular on output,
572: * W1 is upper-triangular.
573: *
574: * col1_(6)_a Compute elements of A1 below the diagonal.
575: *
576: DO J = 1, K - 1
577: DO I = J + 1, K
578: A( I, J ) = - WORK( I, J )
579: END DO
580: END DO
581: *
582: END IF
583: *
584: * col1_(6)_b Compute elements of A1 on and above the diagonal.
585: *
586: DO J = 1, K
587: DO I = 1, J
588: A( I, J ) = A( I, J ) - WORK( I, J )
589: END DO
590: END DO
591: *
592: RETURN
593: *
594: * End of DLARFB_GETT
595: *
596: END
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