## MB04FP

### Eigenvalues and orthogonal decomposition of a real skew-Hamiltonian/skew-Hamiltonian pencil (applying transformations on panels of columns)

[Specification] [Arguments] [Method] [References] [Comments] [Example]

Purpose

      To compute the eigenvalues of a real N-by-N skew-Hamiltonian/
      skew-Hamiltonian pencil aS - bT with

            (  A  D  )         (  B  F  )
        S = (        ) and T = (        ).                           (1)
            (  E  A' )         (  G  B' )

      Optionally, if JOB = 'T', the pencil aS - bT will be transformed
      to the structured Schur form: an orthogonal transformation matrix
      Q is computed such that

                      (  Aout  Dout  )
        J Q' J' S Q = (              ), and
                      (   0    Aout' )
                                                                     (2)
                      (  Bout  Fout  )            (  0  I  )
        J Q' J' T Q = (              ), where J = (        ),
                      (   0    Bout' )            ( -I  0  )

      Aout is upper triangular, and Bout is upper quasi-triangular. The
      notation M' denotes the transpose of the matrix M.
      Optionally, if COMPQ = 'I' or COMPQ = 'U', the orthogonal
      transformation matrix Q will be computed.

Specification

          SUBROUTINE MB04FP( JOB, COMPQ, N, A, LDA, DE, LDDE, B, LDB, FG,
         $                   LDFG, Q, LDQ, ALPHAR, ALPHAI, BETA, IWORK,
         $                   DWORK, LDWORK, INFO )
    C     .. Scalar Arguments ..
          CHARACTER          COMPQ, JOB
          INTEGER            INFO, LDA, LDB, LDDE, LDFG, LDQ, LDWORK, N
    C     .. Array Arguments ..
          DOUBLE PRECISION   A( LDA, * ), ALPHAI( * ), ALPHAR( * ),
         $                   B( LDB, * ), BETA( * ), DE( LDDE, * ),
         $                   DWORK( * ), FG( LDFG, * ), Q( LDQ, * )
          INTEGER            IWORK( * )

Arguments

Mode Parameters

      JOB     CHARACTER*1
              Specifies the computation to be performed, as follows:
              = 'E':  compute the eigenvalues only; S and T will not
                      necessarily be put into skew-Hamiltonian
                      triangular form (2);
              = 'T':  put S and T into skew-Hamiltonian triangular form
                      (2), and return the eigenvalues in ALPHAR, ALPHAI
                      and BETA.

      COMPQ   CHARACTER*1
              Specifies whether to compute the orthogonal transformation
              matrix Q as follows:
              = 'N':  Q is not computed;
              = 'I':  the array Q is initialized internally to the unit
                      matrix, and the orthogonal matrix Q is returned;
              = 'U':  the array Q contains an orthogonal matrix Q0 on
                      entry, and the product Q0*Q is returned, where Q
                      is the product of the orthogonal transformations
                      that are applied to the pencil aS - bT to reduce
                      S and T to the forms in (2), for COMPQ = 'I'.

Input/Output Parameters

      N       (input) INTEGER
              The order of the pencil aS - bT.  N >= 0, even.

      A       (input/output) DOUBLE PRECISION array, dimension
                             (LDA, N/2)
              On entry, the leading N/2-by-N/2 part of this array must
              contain the matrix A.
              On exit, if JOB = 'T', the leading N/2-by-N/2 part of this
              array contains the matrix Aout; otherwise, it contains
              meaningless elements, except for the diagonal blocks,
              which are correctly set.

      LDA     INTEGER
              The leading dimension of the array A.  LDA >= MAX(1, N/2).

      DE      (input/output) DOUBLE PRECISION array, dimension
                             (LDDE, N/2+1)
              On entry, the leading N/2-by-N/2 strictly lower triangular
              part of this array must contain the strictly lower
              triangular part of the skew-symmetric matrix E, and the
              N/2-by-N/2 strictly upper triangular part of the submatrix
              in the columns 2 to N/2+1 of this array must contain the
              strictly upper triangular part of the skew-symmetric
              matrix D.
              The entries on the diagonal and the first superdiagonal of
              this array are not referenced, but are assumed to be zero.
              On exit, if JOB = 'T', the leading N/2-by-N/2 strictly
              upper triangular part of the submatrix in the columns
              2 to N/2+1 of this array contains the strictly upper
              triangular part of the skew-symmetric matrix Dout.
              If JOB = 'E', the leading N/2-by-N/2 strictly upper
              triangular part of the submatrix in the columns 2 to N/2+1
              of this array contains the strictly upper triangular part
              of the skew-symmetric matrix D just before the application
              of the QZ algorithm. The remaining entries are
              meaningless.

      LDDE    INTEGER
              The leading dimension of the array DE.
              LDDE >= MAX(1, N/2).

      B       (input/output) DOUBLE PRECISION array, dimension
                             (LDB, N/2)
              On entry, the leading N/2-by-N/2 part of this array must
              contain the matrix B.
              On exit, if JOB = 'T', the leading N/2-by-N/2 part of this
              array contains the matrix Bout; otherwise, it contains
              meaningless elements, except for the diagonal 1-by-1 and
              2-by-2 blocks, which are correctly set.

      LDB     INTEGER
              The leading dimension of the array B.  LDB >= MAX(1, N/2).

      FG      (input/output) DOUBLE PRECISION array, dimension
                             (LDFG, N/2+1)
              On entry, the leading N/2-by-N/2 strictly lower triangular
              part of this array must contain the strictly lower
              triangular part of the skew-symmetric matrix G, and the
              N/2-by-N/2 strictly upper triangular part of the submatrix
              in the columns 2 to N/2+1 of this array must contain the
              strictly upper triangular part of the skew-symmetric
              matrix F.
              The entries on the diagonal and the first superdiagonal of
              this array are not referenced, but are assumed to be zero.
              On exit, if JOB = 'T', the leading N/2-by-N/2 strictly
              upper triangular part of the submatrix in the columns
              2 to N/2+1 of this array contains the strictly upper
              triangular part of the skew-symmetric matrix Fout.
              If JOB = 'E', the leading N/2-by-N/2 strictly upper
              triangular part of the submatrix in the columns 2 to N/2+1
              of this array contains the strictly upper triangular part
              of the skew-symmetric matrix F just before the application
              of the QZ algorithm. The remaining entries are
              meaningless.

      LDFG    INTEGER
              The leading dimension of the array FG.
              LDFG >= MAX(1, N/2).

      Q       (input/output) DOUBLE PRECISION array, dimension (LDQ, N)
              On entry, if COMPQ = 'U', then the leading N-by-N part of
              this array must contain a given matrix Q0, and on exit,
              the leading N-by-N part of this array contains the product
              of the input matrix Q0 and the transformation matrix Q
              used to transform the matrices S and T.
              On exit, if COMPQ = 'I', then the leading N-by-N part of
              this array contains the orthogonal transformation matrix
              Q.
              If COMPQ = 'N', this array is not referenced.

      LDQ     INTEGER
              The leading dimension of the array Q.
              LDQ >= 1,         if COMPQ = 'N';
              LDQ >= MAX(1, N), if COMPQ = 'I' or COMPQ = 'U'.

      ALPHAR  (output) DOUBLE PRECISION array, dimension (N/2)
              The real parts of each scalar alpha defining an eigenvalue
              of the pencil aS - bT.

      ALPHAI  (output) DOUBLE PRECISION array, dimension (N/2)
              The imaginary parts of each scalar alpha defining an
              eigenvalue of the pencil aS - bT.
              If ALPHAI(j) is zero, then the j-th eigenvalue is real; if
              positive, then the j-th and (j+1)-st eigenvalues are a
              complex conjugate pair.

      BETA    (output) DOUBLE PRECISION array, dimension (N/2)
              The scalars beta that define the eigenvalues of the pencil
              aS - bT.
              Together, the quantities alpha = (ALPHAR(j),ALPHAI(j)) and
              beta = BETA(j) represent the j-th eigenvalue of the pencil
              aS - bT, in the form lambda = alpha/beta. Since lambda may
              overflow, the ratios should not, in general, be computed.
              Due to the skew-Hamiltonian/skew-Hamiltonian structure of
              the pencil, every eigenvalue occurs twice and thus it has
              only to be saved once in ALPHAR, ALPHAI and BETA.

Workspace

      IWORK   INTEGER array, dimension (N/2+1)
              On exit, IWORK(1) contains the number of (pairs of)
              possibly inaccurate eigenvalues, q <= N/2, and the
              nonzero absolute values in IWORK(2), ..., IWORK(N/2+1) are
              indices of the possibly inaccurate eigenvalues, as well as
              of the corresponding 1-by-1 or 2-by-2 diagonal blocks in
              the arrays A and B on exit. The 2-by-2 blocks correspond
              to negative values in IWORK. One negative value is stored
              for each such eigenvalue pair. Its modulus indicates the
              starting index of a 2-by-2 block.

      DWORK   DOUBLE PRECISION array, dimension (LDWORK)
              On exit, if INFO = 0, DWORK(1) returns the optimal LDWORK;
              DWORK(2) and DWORK(3) contain the Frobenius norms of the
              matrices S and T on entry. These norms are used in the
              tests to decide that some eigenvalues are considered as
              unreliable.
              On exit, if INFO = -19, DWORK(1) returns the minimum
              value of LDWORK.

      LDWORK  INTEGER
              The dimension of the array DWORK.
              LDWORK >= MAX(3,N/2,2*N-6),  if JOB = 'E' and COMPQ = 'N';
              LDWORK >= MAX(3,N**2/4+N/2), if JOB = 'T' and COMPQ = 'N';
              LDWORK >= MAX(1,3*N**2/4),   if               COMPQ<> 'N'.
              For good performance LDWORK should generally be larger.

              If LDWORK = -1, then a workspace query is assumed;
              the routine only calculates the optimal size of the
              DWORK array, returns this value as the first entry of
              the DWORK array, and no error message related to LDWORK
              is issued by XERBLA.

Error Indicator

      INFO    INTEGER
              = 0: succesful exit;
              < 0: if INFO = -i, the i-th argument had an illegal value;
              = 1: QZ iteration failed in the LAPACK Library routine
                   DHGEQZ. (QZ iteration did not converge or computation
                   of the shifts failed.)
              = 2: warning: the pencil is numerically singular.

Method

      The algorithm uses Givens rotations and Householder reflections to
      annihilate elements in S and T such that S is in skew-Hamiltonian
      triangular form and T is in skew-Hamiltonian Hessenberg form:

          (  A1  D1  )      (  B1  F1  )
      S = (          ), T = (          ),
          (   0  A1' )      (   0  B1' )

      where A1 is upper triangular and B1 is upper Hessenberg.
      Subsequently, the QZ algorithm is applied to the pencil aA1 - bB1
      to determine orthogonal matrices Q1 and Q2 such that
      Q2' A1 Q1 is upper triangular and Q2' B1 Q1 is upper quasi-
      triangular.
      See also page 40 in [1] for more details.

References

      [1] Benner, P., Byers, R., Mehrmann, V. and Xu, H.
          Numerical Computation of Deflating Subspaces of Embedded
          Hamiltonian Pencils.
          Tech. Rep. SFB393/99-15, Technical University Chemnitz,
          Germany, June 1999.

Numerical Aspects

                                                                3
      The algorithm is numerically backward stable and needs O(N )
      real floating point operations.

Further Comments

      For large values of N, the routine applies the transformations
      for reducing T on panels of columns. The user may specify in INFO
      the desired number of columns. If on entry INFO <= 0, then the
      routine estimates a suitable value of this number.

Example

Program Text

      None

Program Data

      None

Program Results

      None

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