CUNCSD2BY1 computes the CS decomposition of an M-by-Q matrix X with


 orthonormal columns that has been partitioned into a 2-by-1 block


 structure:


                                [  I1 0  0 ]


                                [  0  C  0 ]


          [ X11 ]   [ U1 |    ] [  0  0  0 ]


      X = [-----] = [---------] [----------] V1**T .


          [ X21 ]   [    | U2 ] [  0  0  0 ]


                                [  0  S  0 ]


                                [  0  0  I2]


 X11 is P-by-Q. The unitary matrices U1, U2, and V1 are P-by-P,


 (M-P)-by-(M-P), and Q-by-Q, respectively. C and S are R-by-R


 nonnegative diagonal matrices satisfying C^2 + S^2 = I, in which


 R = MIN(P,M-P,Q,M-Q). I1 is a K1-by-K1 identity matrix and I2 is a


 K2-by-K2 identity matrix, where K1 = MAX(Q+P-M,0), K2 = MAX(Q-P,0).

JOBU1

          JOBU1 is CHARACTER


          = 'Y':      U1 is computed;


          otherwise:  U1 is not computed.

JOBU2

          JOBU2 is CHARACTER


          = 'Y':      U2 is computed;


          otherwise:  U2 is not computed.

JOBV1T

          JOBV1T is CHARACTER


          = 'Y':      V1T is computed;


          otherwise:  V1T is not computed.

M

          M is INTEGER


          The number of rows in X.

P

          P is INTEGER


          The number of rows in X11. 0 <= P <= M.

Q

          Q is INTEGER


          The number of columns in X11 and X21. 0 <= Q <= M.

X11

          X11 is COMPLEX array, dimension (LDX11,Q)


          On entry, part of the unitary matrix whose CSD is desired.

LDX11

          LDX11 is INTEGER


          The leading dimension of X11. LDX11 >= MAX(1,P).

X21

          X21 is COMPLEX array, dimension (LDX21,Q)


          On entry, part of the unitary matrix whose CSD is desired.

LDX21

          LDX21 is INTEGER


          The leading dimension of X21. LDX21 >= MAX(1,M-P).

THETA

          THETA is REAL array, dimension (R), in which R =


          MIN(P,M-P,Q,M-Q).


          C = DIAG( COS(THETA(1)), ... , COS(THETA(R)) ) and


          S = DIAG( SIN(THETA(1)), ... , SIN(THETA(R)) ).

U1

          U1 is COMPLEX array, dimension (P)


          If JOBU1 = 'Y', U1 contains the P-by-P unitary matrix U1.

LDU1

          LDU1 is INTEGER


          The leading dimension of U1. If JOBU1 = 'Y', LDU1 >=


          MAX(1,P).

U2

          U2 is COMPLEX array, dimension (M-P)


          If JOBU2 = 'Y', U2 contains the (M-P)-by-(M-P) unitary


          matrix U2.

LDU2

          LDU2 is INTEGER


          The leading dimension of U2. If JOBU2 = 'Y', LDU2 >=


          MAX(1,M-P).

V1T

          V1T is COMPLEX array, dimension (Q)


          If JOBV1T = 'Y', V1T contains the Q-by-Q matrix unitary


          matrix V1**T.

LDV1T

          LDV1T is INTEGER


          The leading dimension of V1T. If JOBV1T = 'Y', LDV1T >=


          MAX(1,Q).

WORK

          WORK is COMPLEX array, dimension (MAX(1,LWORK))


          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

LWORK

          LWORK is INTEGER


          The dimension of the array WORK.


          If LWORK = -1, then a workspace query is assumed; the routine


          only calculates the optimal size of the WORK array, returns


          this value as the first entry of the work array, and no error


          message related to LWORK is issued by XERBLA.

RWORK

          RWORK is REAL array, dimension (MAX(1,LRWORK))


          On exit, if INFO = 0, RWORK(1) returns the optimal LRWORK.


          If INFO > 0 on exit, RWORK(2:R) contains the values PHI(1),


          ..., PHI(R-1) that, together with THETA(1), ..., THETA(R),


          define the matrix in intermediate bidiagonal-block form


          remaining after nonconvergence. INFO specifies the number


          of nonzero PHI's.

LRWORK

          LRWORK is INTEGER


          The dimension of the array RWORK.


          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.

IWORK

          IWORK is INTEGER array, dimension (M-MIN(P,M-P,Q,M-Q))

INFO

          INFO is INTEGER


          = 0:  successful exit.


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


          > 0:  CBBCSD did not converge. See the description of WORK


                above for details.

[1] Brian D. Sutton. Computing the complete CS decomposition. Numer. Algorithms, 50(1):33-65, 2009.

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

June 2016