NAME

       cunbdb2.f -

SYNOPSIS

   Functions/Subroutines
       subroutine cunbdb2 (M, P, Q, X11, LDX11, X21, LDX21, THETA, PHI, TAUP1, TAUP2, TAUQ1,
           WORK, LWORK, INFO)
           CUNBDB2

Function/Subroutine Documentation

   subroutine cunbdb2 (integer M, integer P, integer Q, complex, dimension(ldx11,*) X11, integer
       LDX11, complex, dimension(ldx21,*) X21, integer LDX21, real, dimension(*) THETA, real,
       dimension(*) PHI, complex, dimension(*) TAUP1, complex, dimension(*) TAUP2, complex,
       dimension(*) TAUQ1, complex, dimension(*) WORK, integer LWORK, integer INFO)
       CUNBDB2

Purpose:

        CUNBDB2 simultaneously bidiagonalizes the blocks of a tall and skinny
        matrix X with orthonomal columns:

                                   [ B11 ]
             [ X11 ]   [ P1 |    ] [  0  ]
             [-----] = [---------] [-----] Q1**T .
             [ X21 ]   [    | P2 ] [ B21 ]
                                   [  0  ]

        X11 is P-by-Q, and X21 is (M-P)-by-Q. P must be no larger than M-P,
        Q, or M-Q. Routines CUNBDB1, CUNBDB3, and CUNBDB4 handle cases in
        which P is not the minimum dimension.

        The unitary matrices P1, P2, and Q1 are P-by-P, (M-P)-by-(M-P),
        and (M-Q)-by-(M-Q), respectively. They are represented implicitly by
        Householder vectors.

        B11 and B12 are P-by-P bidiagonal matrices represented implicitly by
        angles THETA, PHI..fi

       Parameters:
           M

                     M is INTEGER
                      The number of rows X11 plus the number of rows in X21.

           P

                     P is INTEGER
                      The number of rows in X11. 0 <= P <= min(M-P,Q,M-Q).

           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, the top block of the matrix X to be reduced. On
                      exit, the columns of tril(X11) specify reflectors for P1 and
                      the rows of triu(X11,1) specify reflectors for Q1.

           LDX11

                     LDX11 is INTEGER
                      The leading dimension of X11. LDX11 >= P.

           X21

                     X21 is COMPLEX array, dimension (LDX21,Q)
                      On entry, the bottom block of the matrix X to be reduced. On
                      exit, the columns of tril(X21) specify reflectors for P2.

           LDX21

                     LDX21 is INTEGER
                      The leading dimension of X21. LDX21 >= M-P.

           THETA

                     THETA is REAL array, dimension (Q)
                      The entries of the bidiagonal blocks B11, B21 are defined by
                      THETA and PHI. See Further Details.

           PHI

                     PHI is REAL array, dimension (Q-1)
                      The entries of the bidiagonal blocks B11, B21 are defined by
                      THETA and PHI. See Further Details.

           TAUP1

                     TAUP1 is COMPLEX array, dimension (P)
                      The scalar factors of the elementary reflectors that define
                      P1.

           TAUP2

                     TAUP2 is COMPLEX array, dimension (M-P)
                      The scalar factors of the elementary reflectors that define
                      P2.

           TAUQ1

                     TAUQ1 is COMPLEX array, dimension (Q)
                      The scalar factors of the elementary reflectors that define
                      Q1.

           WORK

                     WORK is COMPLEX array, dimension (LWORK)

           LWORK

                     LWORK is INTEGER
                      The dimension of the array WORK. LWORK >= M-Q.

                      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.

           INFO

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

       Author:
           Univ. of Tennessee

           Univ. of California Berkeley

           Univ. of Colorado Denver

           NAG Ltd.

       Date:
           July 2012

       Further Details:

             The upper-bidiagonal blocks B11, B21 are represented implicitly by
             angles THETA(1), ..., THETA(Q) and PHI(1), ..., PHI(Q-1). Every entry
             in each bidiagonal band is a product of a sine or cosine of a THETA
             with a sine or cosine of a PHI. See [1] or CUNCSD for details.

             P1, P2, and Q1 are represented as products of elementary reflectors.
             See CUNCSD2BY1 for details on generating P1, P2, and Q1 using CUNGQR
             and CUNGLQ.

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

Author

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