1*> \brief \b DORG2R generates all or part of the orthogonal matrix Q from a QR factorization determined by sgeqrf (unblocked algorithm).
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 DORG2R( M, N, K, A, LDA, TAU, WORK, INFO )
22*
23*       .. Scalar Arguments ..
24*       INTEGER            INFO, K, LDA, M, N
25*       ..
26*       .. Array Arguments ..
27*       DOUBLE PRECISION   A( LDA, * ), TAU( * ), WORK( * )
28*       ..
29*
30*
31*> \par Purpose:
32*  =============
33*>
34*> \verbatim
35*>
36*> DORG2R generates an m by n real matrix Q with orthonormal columns,
37*> which is defined as the first n columns of a product of k elementary
38*> reflectors of order m
39*>
40*>       Q  =  H(1) H(2) . . . H(k)
41*>
42*> as returned by DGEQRF.
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 DOUBLE PRECISION array, dimension (LDA,N)
70*>          On entry, the i-th column must contain the vector which
71*>          defines the elementary reflector H(i), for i = 1,2,...,k, as
72*>          returned by DGEQRF in the first 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 DOUBLE PRECISION array, dimension (K)
86*>          TAU(i) must contain the scalar factor of the elementary
87*>          reflector H(i), as returned by DGEQRF.
88*> \endverbatim
89*>
90*> \param[out] WORK
91*> \verbatim
92*>          WORK is DOUBLE PRECISION array, dimension (N)
93*> \endverbatim
94*>
95*> \param[out] INFO
96*> \verbatim
97*>          INFO is INTEGER
98*>          = 0: successful exit
99*>          < 0: if INFO = -i, the i-th argument has an illegal value
100*> \endverbatim
101*
102*  Authors:
103*  ========
104*
105*> \author Univ. of Tennessee
106*> \author Univ. of California Berkeley
107*> \author Univ. of Colorado Denver
108*> \author NAG Ltd.
109*
110*> \ingroup doubleOTHERcomputational
111*
112*  =====================================================================
113      SUBROUTINE DORG2R( M, N, K, A, LDA, TAU, WORK, INFO )
114*
115*  -- LAPACK computational routine --
116*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
117*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
118*
119*     .. Scalar Arguments ..
120      INTEGER            INFO, K, LDA, M, N
121*     ..
122*     .. Array Arguments ..
123      DOUBLE PRECISION   A( LDA, * ), TAU( * ), WORK( * )
124*     ..
125*
126*  =====================================================================
127*
128*     .. Parameters ..
129      DOUBLE PRECISION   ONE, ZERO
130      PARAMETER          ( ONE = 1.0D+0, ZERO = 0.0D+0 )
131*     ..
132*     .. Local Scalars ..
133      INTEGER            I, J, L
134*     ..
135*     .. External Subroutines ..
136      EXTERNAL           DLARF, DSCAL, XERBLA
137*     ..
138*     .. Intrinsic Functions ..
139      INTRINSIC          MAX
140*     ..
141*     .. Executable Statements ..
142*
143*     Test the input arguments
144*
145      INFO = 0
146      IF( M.LT.0 ) THEN
147         INFO = -1
148      ELSE IF( N.LT.0 .OR. N.GT.M ) THEN
149         INFO = -2
150      ELSE IF( K.LT.0 .OR. K.GT.N ) THEN
151         INFO = -3
152      ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
153         INFO = -5
154      END IF
155      IF( INFO.NE.0 ) THEN
156         CALL XERBLA( 'DORG2R', -INFO )
157         RETURN
158      END IF
159*
160*     Quick return if possible
161*
162      IF( N.LE.0 )
163     $   RETURN
164*
165*     Initialise columns k+1:n to columns of the unit matrix
166*
167      DO 20 J = K + 1, N
168         DO 10 L = 1, M
169            A( L, J ) = ZERO
170   10    CONTINUE
171         A( J, J ) = ONE
172   20 CONTINUE
173*
174      DO 40 I = K, 1, -1
175*
176*        Apply H(i) to A(i:m,i:n) from the left
177*
178         IF( I.LT.N ) THEN
179            A( I, I ) = ONE
180            CALL DLARF( 'Left', M-I+1, N-I, A( I, I ), 1, TAU( I ),
181     $                  A( I, I+1 ), LDA, WORK )
182         END IF
183         IF( I.LT.M )
184     $      CALL DSCAL( M-I, -TAU( I ), A( I+1, I ), 1 )
185         A( I, I ) = ONE - TAU( I )
186*
187*        Set A(1:i-1,i) to zero
188*
189         DO 30 L = 1, I - 1
190            A( L, I ) = ZERO
191   30    CONTINUE
192   40 CONTINUE
193      RETURN
194*
195*     End of DORG2R
196*
197      END
198