1 *> \brief \b CGEBD2 reduces a general matrix to bidiagonal form using an unblocked algorithm.
3 * =========== DOCUMENTATION ===========
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21 * SUBROUTINE CGEBD2( M, N, A, LDA, D, E, TAUQ, TAUP, WORK, INFO )
23 * .. Scalar Arguments ..
24 * INTEGER INFO, LDA, M, N
26 * .. Array Arguments ..
28 * COMPLEX A( LDA, * ), TAUP( * ), TAUQ( * ), WORK( * )
37 *> CGEBD2 reduces a complex general m by n matrix A to upper or lower
38 *> real bidiagonal form B by a unitary transformation: Q**H * A * P = B.
40 *> If m >= n, B is upper bidiagonal; if m < n, B is lower bidiagonal.
49 *> The number of rows in the matrix A. M >= 0.
55 *> The number of columns in the matrix A. N >= 0.
60 *> A is COMPLEX array, dimension (LDA,N)
61 *> On entry, the m by n general matrix to be reduced.
63 *> if m >= n, the diagonal and the first superdiagonal are
64 *> overwritten with the upper bidiagonal matrix B; the
65 *> elements below the diagonal, with the array TAUQ, represent
66 *> the unitary matrix Q as a product of elementary
67 *> reflectors, and the elements above the first superdiagonal,
68 *> with the array TAUP, represent the unitary matrix P as
69 *> a product of elementary reflectors;
70 *> if m < n, the diagonal and the first subdiagonal are
71 *> overwritten with the lower bidiagonal matrix B; the
72 *> elements below the first subdiagonal, with the array TAUQ,
73 *> represent the unitary matrix Q as a product of
74 *> elementary reflectors, and the elements above the diagonal,
75 *> with the array TAUP, represent the unitary matrix P as
76 *> a product of elementary reflectors.
77 *> See Further Details.
83 *> The leading dimension of the array A. LDA >= max(1,M).
88 *> D is REAL array, dimension (min(M,N))
89 *> The diagonal elements of the bidiagonal matrix B:
95 *> E is REAL array, dimension (min(M,N)-1)
96 *> The off-diagonal elements of the bidiagonal matrix B:
97 *> if m >= n, E(i) = A(i,i+1) for i = 1,2,...,n-1;
98 *> if m < n, E(i) = A(i+1,i) for i = 1,2,...,m-1.
103 *> TAUQ is COMPLEX array dimension (min(M,N))
104 *> The scalar factors of the elementary reflectors which
105 *> represent the unitary matrix Q. See Further Details.
110 *> TAUP is COMPLEX array, dimension (min(M,N))
111 *> The scalar factors of the elementary reflectors which
112 *> represent the unitary matrix P. See Further Details.
117 *> WORK is COMPLEX array, dimension (max(M,N))
123 *> = 0: successful exit
124 *> < 0: if INFO = -i, the i-th argument had an illegal value.
130 *> \author Univ. of Tennessee
131 *> \author Univ. of California Berkeley
132 *> \author Univ. of Colorado Denver
135 *> \date September 2012
137 *> \ingroup complexGEcomputational
138 * @precisions normal c -> s d z
140 *> \par Further Details:
141 * =====================
145 *> The matrices Q and P are represented as products of elementary
150 *> Q = H(1) H(2) . . . H(n) and P = G(1) G(2) . . . G(n-1)
152 *> Each H(i) and G(i) has the form:
154 *> H(i) = I - tauq * v * v**H and G(i) = I - taup * u * u**H
156 *> where tauq and taup are complex scalars, and v and u are complex
157 *> vectors; v(1:i-1) = 0, v(i) = 1, and v(i+1:m) is stored on exit in
158 *> A(i+1:m,i); u(1:i) = 0, u(i+1) = 1, and u(i+2:n) is stored on exit in
159 *> A(i,i+2:n); tauq is stored in TAUQ(i) and taup in TAUP(i).
163 *> Q = H(1) H(2) . . . H(m-1) and P = G(1) G(2) . . . G(m)
165 *> Each H(i) and G(i) has the form:
167 *> H(i) = I - tauq * v * v**H and G(i) = I - taup * u * u**H
169 *> where tauq and taup are complex scalars, v and u are complex vectors;
170 *> v(1:i) = 0, v(i+1) = 1, and v(i+2:m) is stored on exit in A(i+2:m,i);
171 *> u(1:i-1) = 0, u(i) = 1, and u(i+1:n) is stored on exit in A(i,i+1:n);
172 *> tauq is stored in TAUQ(i) and taup in TAUP(i).
174 *> The contents of A on exit are illustrated by the following examples:
176 *> m = 6 and n = 5 (m > n): m = 5 and n = 6 (m < n):
178 *> ( d e u1 u1 u1 ) ( d u1 u1 u1 u1 u1 )
179 *> ( v1 d e u2 u2 ) ( e d u2 u2 u2 u2 )
180 *> ( v1 v2 d e u3 ) ( v1 e d u3 u3 u3 )
181 *> ( v1 v2 v3 d e ) ( v1 v2 e d u4 u4 )
182 *> ( v1 v2 v3 v4 d ) ( v1 v2 v3 e d u5 )
183 *> ( v1 v2 v3 v4 v5 )
185 *> where d and e denote diagonal and off-diagonal elements of B, vi
186 *> denotes an element of the vector defining H(i), and ui an element of
187 *> the vector defining G(i).
190 * =====================================================================
191 SUBROUTINE CGEBD2( M, N, A, LDA, D, E, TAUQ, TAUP, WORK, INFO )
193 * -- LAPACK computational routine (version 3.4.2) --
194 * -- LAPACK is a software package provided by Univ. of Tennessee, --
195 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
198 * .. Scalar Arguments ..
199 INTEGER INFO, LDA, M, N
201 * .. Array Arguments ..
203 COMPLEX A( LDA, * ), TAUP( * ), TAUQ( * ), WORK( * )
206 * =====================================================================
210 PARAMETER ( ZERO = ( 0.0E+0, 0.0E+0 ),
211 $ ONE = ( 1.0E+0, 0.0E+0 ) )
213 * .. Local Scalars ..
217 * .. External Subroutines ..
218 EXTERNAL CLACGV, CLARF, CLARFG, XERBLA
220 * .. Intrinsic Functions ..
221 INTRINSIC CONJG, MAX, MIN
223 * .. Executable Statements ..
225 * Test the input parameters
230 ELSE IF( N.LT.0 ) THEN
232 ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
236 CALL XERBLA( 'CGEBD2', -INFO )
242 * Reduce to upper bidiagonal form
246 * Generate elementary reflector H(i) to annihilate A(i+1:m,i)
249 CALL CLARFG( M-I+1, ALPHA, A( MIN( I+1, M ), I ), 1,
254 * Apply H(i)**H to A(i:m,i+1:n) from the left
257 $ CALL CLARF( 'Left', M-I+1, N-I, A( I, I ), 1,
258 $ CONJG( TAUQ( I ) ), A( I, I+1 ), LDA, WORK )
263 * Generate elementary reflector G(i) to annihilate
266 CALL CLACGV( N-I, A( I, I+1 ), LDA )
268 CALL CLARFG( N-I, ALPHA, A( I, MIN( I+2, N ) ),
273 * Apply G(i) to A(i+1:m,i+1:n) from the right
275 CALL CLARF( 'Right', M-I, N-I, A( I, I+1 ), LDA,
276 $ TAUP( I ), A( I+1, I+1 ), LDA, WORK )
277 CALL CLACGV( N-I, A( I, I+1 ), LDA )
285 * Reduce to lower bidiagonal form
289 * Generate elementary reflector G(i) to annihilate A(i,i+1:n)
291 CALL CLACGV( N-I+1, A( I, I ), LDA )
293 CALL CLARFG( N-I+1, ALPHA, A( I, MIN( I+1, N ) ), LDA,
298 * Apply G(i) to A(i+1:m,i:n) from the right
301 $ CALL CLARF( 'Right', M-I, N-I+1, A( I, I ), LDA,
302 $ TAUP( I ), A( I+1, I ), LDA, WORK )
303 CALL CLACGV( N-I+1, A( I, I ), LDA )
308 * Generate elementary reflector H(i) to annihilate
312 CALL CLARFG( M-I, ALPHA, A( MIN( I+2, M ), I ), 1,
317 * Apply H(i)**H to A(i+1:m,i+1:n) from the left
319 CALL CLARF( 'Left', M-I, N-I, A( I+1, I ), 1,
320 $ CONJG( TAUQ( I ) ), A( I+1, I+1 ), LDA,