Provided by: liblapack-doc_3.3.1-1_all

**NAME**

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

**SYNOPSIS**

SUBROUTINE ZBBCSD( 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 ) IMPLICIT NONE CHARACTER JOBU1, JOBU2, JOBV1T, JOBV2T, TRANS INTEGER INFO, LDU1, LDU2, LDV1T, LDV2T, LRWORK, M, P, Q DOUBLE PRECISION B11D( * ), B11E( * ), B12D( * ), B12E( * ), B21D( * ), B21E( * ), B22D( * ), B22E( * ), PHI( * ), THETA( * ), RWORK( * ) COMPLEX*16 U1( LDU1, * ), U2( LDU2, * ), V1T( LDV1T, * ), V2T( LDV2T, * )

**PURPOSE**

ZBBCSD computes the CS decomposition of a unitary matrix in bidiagonal-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 ZUNCSD 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 = 'Y': U1 is updated; otherwise: U1 is not updated. JOBU2 (input) CHARACTER = 'Y': U2 is updated; otherwise: U2 is not updated. JOBV1T (input) CHARACTER = 'Y': V1T is updated; otherwise: V1T is not updated. JOBV2T (input) CHARACTER = 'Y': V2T is updated; otherwise: V2T is not updated. TRANS (input) CHARACTER = 'T': 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) DOUBLE PRECISION 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) DOUBLE PRECISION 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*16 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*16 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*16 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*16 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) DOUBLE PRECISION array, dimension (Q) When ZBBCSD converges, B11D contains the cosines of THETA(1), ..., THETA(Q). If ZBBCSD fails to converge, then B11D contains the diagonal of the partially reduced top-left block. B11E (output) DOUBLE PRECISION array, dimension (Q-1) When ZBBCSD converges, B11E contains zeros. If ZBBCSD fails to converge, then B11E contains the superdiagonal of the partially reduced top-left block. B12D (output) DOUBLE PRECISION array, dimension (Q) When ZBBCSD converges, B12D contains the negative sines of THETA(1), ..., THETA(Q). If ZBBCSD fails to converge, then B12D contains the diagonal of the partially reduced top-right block. B12E (output) DOUBLE PRECISION array, dimension (Q-1) When ZBBCSD converges, B12E contains zeros. If ZBBCSD fails to converge, then B12E contains the subdiagonal of the partially reduced top-right block. RWORK (workspace) DOUBLE PRECISION 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 ZBBCSD 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 DOUBLE PRECISION, 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. LAPACK routine (version 3.3.0) April 2011 ZBBCSD(3lapack)