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CBBCSD

Section: LAPACK routine (version 3.3.0) (1)
Updated: April 2011
Index

NAME

LAPACK-3 - computes the CS decomposition of a unitary matrix in bidiagonal-block form,

SYNOPSIS

SUBROUTINE CBBCSD(: JOBU1, JOBU2, JOBV1T, JOBV2T, TRANS, M, P, Q,THETA, PHI, U1, LDU1, U2, LDU2, V1T, LDV1T,V2T, LDV2T, B11D, B11E, B12D, B12E, B21D, B21E,B22D, B22E, RWORK, LRWORK, INFO )
: IMPLICITNONE
: CHARACTERJOBU1, JOBU2, JOBV1T, JOBV2T, TRANS
: INTEGERINFO, LDU1, LDU2, LDV1T, LDV2T, LRWORK, M, P, Q
: REALB11D( * ), B11E( * ), B12D( * ), B12E( * ),B21D( * ), B21E( * ), B22D( * ), B22E( * ),PHI( * ), THETA( * ), RWORK( * )
: COMPLEXU1( LDU1, * ), U2( LDU2, * ), V1T( LDV1T, * ),V2T( LDV2T, * )

PURPOSE

CBBCSD computes the CS decomposition of a unitary matrix inbidiagonal-block form,
     [ B11 | B12 0  0 ]

     [  0  |  0 -I  0 ]

X = [----------------]

     [ B21 | B22 0  0 ]

     [  0  |  0  0  I ]

                               [  C | -S  0  0 ]

                   [ U1 |    ] [  0 |  0 -I  0 ] [ V1 |    ]**H
                 = [---------] [---------------] [---------]   .
                   [    | U2 ] [  S |  C  0  0 ] [    | V2 ]
                               [  0 |  0  0  I ]

X is M-by-M, its top-left block is P-by-Q, and Q must be no larger
than P, M-P, or M-Q. (If Q is not the smallest index, then X must be
transposed and/or permuted. This can be done in constant time using
the TRANS and SIGNS options. See CUNCSD for details.)

The bidiagonal matrices B11, B12, B21, and B22 are represented
implicitly by angles THETA(1:Q) and PHI(1:Q-1).

The unitary matrices U1, U2, V1T, and V2T are input/output.
The input matrices are pre- or post-multiplied by the appropriate
singular vector matrices.

ARGUMENTS

JOBU1 (input) CHARACTER: = aqYaq: U1 is updated;

otherwise: U1 is not updated.
JOBU2 (input) CHARACTER: = aqYaq: U2 is updated;

otherwise: U2 is not updated.
JOBV1T (input) CHARACTER: = aqYaq: V1T is updated;

otherwise: V1T is not updated.
JOBV2T (input) CHARACTER: = aqYaq: V2T is updated;

otherwise: V2T is not updated.
TRANS (input) CHARACTER: = aqTaq: X, U1, U2, V1T, and V2T are stored in row-major
order;
otherwise: X, U1, U2, V1T, and V2T are stored in column-
major order.
M (input) INTEGER: The number of rows and columns in X, the unitary matrix in
bidiagonal-block form.
P (input) INTEGER: The number of rows in the top-left block of X. 0 <= P <= M.
Q (input) INTEGER: The number of columns in the top-left block of X.
0 <= Q <= MIN(P,M-P,M-Q).
THETA (input/output) REAL array, dimension (Q): On entry, the angles THETA(1),...,THETA(Q) that, along with
PHI(1), ...,PHI(Q-1), define the matrix in bidiagonal-block
form. On exit, the angles whose cosines and sines define the
diagonal blocks in the CS decomposition.
PHI (input/workspace) REAL array, dimension (Q-1): The angles PHI(1),...,PHI(Q-1) that, along with THETA(1),...,
THETA(Q), define the matrix in bidiagonal-block form.
U1 (input/output) COMPLEX array, dimension (LDU1,P): On entry, an LDU1-by-P matrix. On exit, U1 is postmultiplied
by the left singular vector matrix common to [ B11 ; 0 ] and
[ B12 0 0 ; 0 -I 0 0 ].
LDU1 (input) INTEGER: The leading dimension of the array U1.
U2 (input/output) COMPLEX array, dimension (LDU2,M-P): On entry, an LDU2-by-(M-P) matrix. On exit, U2 is
postmultiplied by the left singular vector matrix common to
[ B21 ; 0 ] and [ B22 0 0 ; 0 0 I ].
LDU2 (input) INTEGER: The leading dimension of the array U2.
V1T (input/output) COMPLEX array, dimension (LDV1T,Q): On entry, a LDV1T-by-Q matrix. On exit, V1T is premultiplied
by the conjugate transpose of the right singular vector
matrix common to [ B11 ; 0 ] and [ B21 ; 0 ].
LDV1T (input) INTEGER: The leading dimension of the array V1T.
V2T (input/output) COMPLEX array, dimenison (LDV2T,M-Q): On entry, a LDV2T-by-(M-Q) matrix. On exit, V2T is
premultiplied by the conjugate transpose of the right
singular vector matrix common to [ B12 0 0 ; 0 -I 0 ] and
[ B22 0 0 ; 0 0 I ].
LDV2T (input) INTEGER: The leading dimension of the array V2T.
B11D (output) REAL array, dimension (Q): When CBBCSD converges, B11D contains the cosines of THETA(1),
..., THETA(Q). If CBBCSD fails to converge, then B11D
contains the diagonal of the partially reduced top-left
block.
B11E (output) REAL array, dimension (Q-1): When CBBCSD converges, B11E contains zeros. If CBBCSD fails
to converge, then B11E contains the superdiagonal of the
partially reduced top-left block.
B12D (output) REAL array, dimension (Q): When CBBCSD converges, B12D contains the negative sines of
THETA(1), ..., THETA(Q). If CBBCSD fails to converge, then
B12D contains the diagonal of the partially reduced top-right
block.
B12E (output) REAL array, dimension (Q-1): When CBBCSD converges, B12E contains zeros. If CBBCSD fails
to converge, then B12E contains the subdiagonal of the
partially reduced top-right block.
RWORK (workspace) REAL array, dimension (MAX(1,LWORK)): On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
LRWORK (input) INTEGER: The dimension of the array RWORK. LRWORK >= MAX(1,8*Q).
If LRWORK = -1, then a workspace query is assumed; the
routine only calculates the optimal size of the RWORK array,
returns this value as the first entry of the work array, and
no error message related to LRWORK is issued by XERBLA.
INFO (output) INTEGER: = 0:  successful exit.

< 0:  if INFO = -i, the i-th argument had an illegal value.

> 0:  if CBBCSD did not converge, INFO specifies the number
of nonzero entries in PHI, and B11D, B11E, etc.,
contain the partially reduced matrix.
Reference
=========
[1] Brian D. Sutton. Computing the complete CS decomposition. Numer.
Algorithms, 50(1):33-65, 2009.

PARAMETERS

TOLMUL REAL, default = MAX(10,MIN(100,EPS**(-1/8))): TOLMUL controls the convergence criterion of the QR loop.
Angles THETA(i), PHI(i) are rounded to 0 or PI/2 when they
are within TOLMUL*EPS of either bound.

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