1*> \brief \b SORGQL
2*
3*  =========== DOCUMENTATION ===========
4*
5* Online html documentation available at
6*            http://www.netlib.org/lapack/explore-html/
7*
8*> \htmlonly
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15*> [TXT]</a>
16*> \endhtmlonly
17*
18*  Definition:
19*  ===========
20*
21*       SUBROUTINE SORGQL( M, N, K, A, LDA, TAU, WORK, LWORK, INFO )
22*
23*       .. Scalar Arguments ..
24*       INTEGER            INFO, K, LDA, LWORK, M, N
25*       ..
26*       .. Array Arguments ..
27*       REAL               A( LDA, * ), TAU( * ), WORK( * )
28*       ..
29*
30*
31*> \par Purpose:
32*  =============
33*>
34*> \verbatim
35*>
36*> SORGQL generates an M-by-N real matrix Q with orthonormal columns,
37*> which is defined as the last N columns of a product of K elementary
38*> reflectors of order M
39*>
40*>       Q  =  H(k) . . . H(2) H(1)
41*>
42*> as returned by SGEQLF.
43*> \endverbatim
44*
45*  Arguments:
46*  ==========
47*
48*> \param[in] M
49*> \verbatim
50*>          M is INTEGER
51*>          The number of rows of the matrix Q. M >= 0.
52*> \endverbatim
53*>
54*> \param[in] N
55*> \verbatim
56*>          N is INTEGER
57*>          The number of columns of the matrix Q. M >= N >= 0.
58*> \endverbatim
59*>
60*> \param[in] K
61*> \verbatim
62*>          K is INTEGER
63*>          The number of elementary reflectors whose product defines the
64*>          matrix Q. N >= K >= 0.
65*> \endverbatim
66*>
67*> \param[in,out] A
68*> \verbatim
69*>          A is REAL array, dimension (LDA,N)
70*>          On entry, the (n-k+i)-th column must contain the vector which
71*>          defines the elementary reflector H(i), for i = 1,2,...,k, as
72*>          returned by SGEQLF in the last k columns of its array
73*>          argument A.
74*>          On exit, the M-by-N matrix Q.
75*> \endverbatim
76*>
77*> \param[in] LDA
78*> \verbatim
79*>          LDA is INTEGER
80*>          The first dimension of the array A. LDA >= max(1,M).
81*> \endverbatim
82*>
83*> \param[in] TAU
84*> \verbatim
85*>          TAU is REAL array, dimension (K)
86*>          TAU(i) must contain the scalar factor of the elementary
87*>          reflector H(i), as returned by SGEQLF.
88*> \endverbatim
89*>
90*> \param[out] WORK
91*> \verbatim
92*>          WORK is REAL array, dimension (MAX(1,LWORK))
93*>          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
94*> \endverbatim
95*>
96*> \param[in] LWORK
97*> \verbatim
98*>          LWORK is INTEGER
99*>          The dimension of the array WORK. LWORK >= max(1,N).
100*>          For optimum performance LWORK >= N*NB, where NB is the
101*>          optimal blocksize.
102*>
103*>          If LWORK = -1, then a workspace query is assumed; the routine
104*>          only calculates the optimal size of the WORK array, returns
105*>          this value as the first entry of the WORK array, and no error
106*>          message related to LWORK is issued by XERBLA.
107*> \endverbatim
108*>
109*> \param[out] INFO
110*> \verbatim
111*>          INFO is INTEGER
112*>          = 0:  successful exit
113*>          < 0:  if INFO = -i, the i-th argument has an illegal value
114*> \endverbatim
115*
116*  Authors:
117*  ========
118*
119*> \author Univ. of Tennessee
120*> \author Univ. of California Berkeley
121*> \author Univ. of Colorado Denver
122*> \author NAG Ltd.
123*
124*> \ingroup realOTHERcomputational
125*
126*  =====================================================================
127      SUBROUTINE SORGQL( M, N, K, A, LDA, TAU, WORK, LWORK, INFO )
128*
129*  -- LAPACK computational routine --
130*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
131*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
132*
133*     .. Scalar Arguments ..
134      INTEGER            INFO, K, LDA, LWORK, M, N
135*     ..
136*     .. Array Arguments ..
137      REAL               A( LDA, * ), TAU( * ), WORK( * )
138*     ..
139*
140*  =====================================================================
141*
142*     .. Parameters ..
143      REAL               ZERO
144      PARAMETER          ( ZERO = 0.0E+0 )
145*     ..
146*     .. Local Scalars ..
147      LOGICAL            LQUERY
148      INTEGER            I, IB, IINFO, IWS, J, KK, L, LDWORK, LWKOPT,
149     $                   NB, NBMIN, NX
150*     ..
151*     .. External Subroutines ..
152      EXTERNAL           SLARFB, SLARFT, SORG2L, XERBLA
153*     ..
154*     .. Intrinsic Functions ..
155      INTRINSIC          MAX, MIN
156*     ..
157*     .. External Functions ..
158      INTEGER            ILAENV
159      EXTERNAL           ILAENV
160*     ..
161*     .. Executable Statements ..
162*
163*     Test the input arguments
164*
165      INFO = 0
166      LQUERY = ( LWORK.EQ.-1 )
167      IF( M.LT.0 ) THEN
168         INFO = -1
169      ELSE IF( N.LT.0 .OR. N.GT.M ) THEN
170         INFO = -2
171      ELSE IF( K.LT.0 .OR. K.GT.N ) THEN
172         INFO = -3
173      ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
174         INFO = -5
175      END IF
176*
177      IF( INFO.EQ.0 ) THEN
178         IF( N.EQ.0 ) THEN
179            LWKOPT = 1
180         ELSE
181            NB = ILAENV( 1, 'SORGQL', ' ', M, N, K, -1 )
182            LWKOPT = N*NB
183         END IF
184         WORK( 1 ) = LWKOPT
185*
186         IF( LWORK.LT.MAX( 1, N ) .AND. .NOT.LQUERY ) THEN
187            INFO = -8
188         END IF
189      END IF
190*
191      IF( INFO.NE.0 ) THEN
192         CALL XERBLA( 'SORGQL', -INFO )
193         RETURN
194      ELSE IF( LQUERY ) THEN
195         RETURN
196      END IF
197*
198*     Quick return if possible
199*
200      IF( N.LE.0 ) THEN
201         RETURN
202      END IF
203*
204      NBMIN = 2
205      NX = 0
206      IWS = N
207      IF( NB.GT.1 .AND. NB.LT.K ) THEN
208*
209*        Determine when to cross over from blocked to unblocked code.
210*
211         NX = MAX( 0, ILAENV( 3, 'SORGQL', ' ', M, N, K, -1 ) )
212         IF( NX.LT.K ) THEN
213*
214*           Determine if workspace is large enough for blocked code.
215*
216            LDWORK = N
217            IWS = LDWORK*NB
218            IF( LWORK.LT.IWS ) THEN
219*
220*              Not enough workspace to use optimal NB:  reduce NB and
221*              determine the minimum value of NB.
222*
223               NB = LWORK / LDWORK
224               NBMIN = MAX( 2, ILAENV( 2, 'SORGQL', ' ', M, N, K, -1 ) )
225            END IF
226         END IF
227      END IF
228*
229      IF( NB.GE.NBMIN .AND. NB.LT.K .AND. NX.LT.K ) THEN
230*
231*        Use blocked code after the first block.
232*        The last kk columns are handled by the block method.
233*
234         KK = MIN( K, ( ( K-NX+NB-1 ) / NB )*NB )
235*
236*        Set A(m-kk+1:m,1:n-kk) to zero.
237*
238         DO 20 J = 1, N - KK
239            DO 10 I = M - KK + 1, M
240               A( I, J ) = ZERO
241   10       CONTINUE
242   20    CONTINUE
243      ELSE
244         KK = 0
245      END IF
246*
247*     Use unblocked code for the first or only block.
248*
249      CALL SORG2L( M-KK, N-KK, K-KK, A, LDA, TAU, WORK, IINFO )
250*
251      IF( KK.GT.0 ) THEN
252*
253*        Use blocked code
254*
255         DO 50 I = K - KK + 1, K, NB
256            IB = MIN( NB, K-I+1 )
257            IF( N-K+I.GT.1 ) THEN
258*
259*              Form the triangular factor of the block reflector
260*              H = H(i+ib-1) . . . H(i+1) H(i)
261*
262               CALL SLARFT( 'Backward', 'Columnwise', M-K+I+IB-1, IB,
263     $                      A( 1, N-K+I ), LDA, TAU( I ), WORK, LDWORK )
264*
265*              Apply H to A(1:m-k+i+ib-1,1:n-k+i-1) from the left
266*
267               CALL SLARFB( 'Left', 'No transpose', 'Backward',
268     $                      'Columnwise', M-K+I+IB-1, N-K+I-1, IB,
269     $                      A( 1, N-K+I ), LDA, WORK, LDWORK, A, LDA,
270     $                      WORK( IB+1 ), LDWORK )
271            END IF
272*
273*           Apply H to rows 1:m-k+i+ib-1 of current block
274*
275            CALL SORG2L( M-K+I+IB-1, IB, IB, A( 1, N-K+I ), LDA,
276     $                   TAU( I ), WORK, IINFO )
277*
278*           Set rows m-k+i+ib:m of current block to zero
279*
280            DO 40 J = N - K + I, N - K + I + IB - 1
281               DO 30 L = M - K + I + IB, M
282                  A( L, J ) = ZERO
283   30          CONTINUE
284   40       CONTINUE
285   50    CONTINUE
286      END IF
287*
288      WORK( 1 ) = IWS
289      RETURN
290*
291*     End of SORGQL
292*
293      END
294