## MB3LZP

### Eigenvalues and right deflating subspace of a complex skew-Hamiltonian/Hamiltonian pencil (applying transformations on panels of columns)

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

Purpose

```  To compute the eigenvalues of a complex N-by-N skew-Hamiltonian/
Hamiltonian pencil aS - bH, with

(  A  D  )         (  B  F  )
S = (        ) and H = (        ).                           (1)
(  E  A' )         (  G -B' )

The structured Schur form of the embedded real skew-Hamiltonian/
skew-Hamiltonian pencil aB_S - bB_T, defined as

(  Re(A)  -Im(A)  |  Re(D)  -Im(D)  )
(                 |                 )
(  Im(A)   Re(A)  |  Im(D)   Re(D)  )
(                 |                 )
B_S = (-----------------+-----------------) , and
(                 |                 )
(  Re(E)  -Im(E)  |  Re(A')  Im(A') )
(                 |                 )
(  Im(E)   Re(E)  | -Im(A')  Re(A') )
(2)
( -Im(B)  -Re(B)  | -Im(F)  -Re(F)  )
(                 |                 )
(  Re(B)  -Im(B)  |  Re(F)  -Im(F)  )
(                 |                 )
B_T = (-----------------+-----------------) ,  T = i*H,
(                 |                 )
( -Im(G)  -Re(G)  | -Im(B')  Re(B') )
(                 |                 )
(  Re(G)  -Im(G)  | -Re(B') -Im(B') )

is determined and used to compute the eigenvalues. The notation M'
denotes the conjugate transpose of the matrix M. Optionally,
if COMPQ = 'C', an orthonormal basis of the right deflating
subspace of the pencil aS - bH, corresponding to the eigenvalues
with strictly negative real part, is computed. Namely, after
transforming aB_S - bB_H by unitary matrices, we have

( BA  BD  )              ( BB  BF  )
B_Sout = (         ) and B_Hout = (         ),               (3)
(  0  BA' )              (  0 -BB' )

and the eigenvalues with strictly negative real part of the
complex pencil aB_Sout - bB_Hout are moved to the top. The
embedding doubles the multiplicities of the eigenvalues of the
pencil aS - bH.

```
Specification
```      SUBROUTINE MB3LZP( COMPQ, ORTH, N, A, LDA, DE, LDDE, B, LDB, FG,
\$                   LDFG, NEIG, Q, LDQ, ALPHAR, ALPHAI, BETA,
\$                   IWORK, DWORK, LDWORK, ZWORK, LZWORK, BWORK,
\$                   INFO )
C     .. Scalar Arguments ..
CHARACTER          COMPQ, ORTH
INTEGER            INFO, LDA, LDB, LDDE, LDFG, LDQ, LDWORK,
\$                   LZWORK, N, NEIG
C     .. Array Arguments ..
LOGICAL            BWORK( * )
INTEGER            IWORK( * )
DOUBLE PRECISION   ALPHAI( * ), ALPHAR( * ), BETA( * ), DWORK( * )
COMPLEX*16         A( LDA, * ), B( LDB, * ), DE( LDDE, * ),
\$                   FG( LDFG, * ), Q( LDQ, * ), ZWORK( * )

```
Arguments

Mode Parameters

```  COMPQ   CHARACTER*1
Specifies whether to compute the deflating subspace
corresponding to the eigenvalues of aS - bH with strictly
negative real part.
= 'N': do not compute the deflating subspace; compute the
eigenvalues only;
= 'C': compute the deflating subspace and store it in the

ORTH    CHARACTER*1
If COMPQ = 'C', specifies the technique for computing an
orthonormal basis of the deflating subspace, as follows:
= 'P':  QR factorization with column pivoting;
= 'S':  singular value decomposition.
If COMPQ = 'N', the ORTH value is not used.

```
Input/Output Parameters
```  N       (input) INTEGER
The order of the pencil aS - bH.  N >= 0, even.

A       (input/output) COMPLEX*16 array, dimension (LDA, N)
On entry, the leading N/2-by-N/2 part of this array must
contain the matrix A.
On exit, if COMPQ = 'C', the leading N-by-N part of this
array contains the upper triangular matrix BA in (3) (see
also METHOD). The strictly lower triangular part is not
zeroed; it is preserved in the leading N/2-by-N/2 part.
If COMPQ = 'N', this array is unchanged on exit.

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

DE      (input/output) COMPLEX*16 array, dimension (LDDE, N)
On entry, the leading N/2-by-N/2 lower triangular part of
this array must contain the lower triangular part of the
skew-Hermitian matrix E, and the N/2-by-N/2 upper
triangular part of the submatrix in the columns 2 to N/2+1
of this array must contain the upper triangular part of
the skew-Hermitian matrix D.
On exit, if COMPQ = 'C', the leading N-by-N part of this
array contains the skew-Hermitian matrix BD in (3) (see
also METHOD). The strictly lower triangular part of the
input matrix is preserved.
If COMPQ = 'N', this array is unchanged on exit.

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

B       (input/output) COMPLEX*16 array, dimension (LDB, N)
On entry, the leading N/2-by-N/2 part of this array must
contain the matrix B.
On exit, if COMPQ = 'C', the leading N-by-N part of this
array contains the upper triangular matrix BB in (3) (see
also METHOD). The strictly lower triangular part is not
zeroed; the elements below the first subdiagonal of the
input matrix are preserved.
If COMPQ = 'N', this array is unchanged on exit.

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

FG      (input/output) COMPLEX*16 array, dimension (LDFG, N)
On entry, the leading N/2-by-N/2 lower triangular part of
this array must contain the lower triangular part of the
Hermitian matrix G, and the N/2-by-N/2 upper triangular
part of the submatrix in the columns 2 to N/2+1 of this
array must contain the upper triangular part of the
Hermitian matrix F.
On exit, if COMPQ = 'C', the leading N-by-N part of this
METHOD). The strictly lower triangular part of the input
matrix is preserved. The diagonal elements might have tiny
imaginary parts.
If COMPQ = 'N', this array is unchanged on exit.

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

NEIG    (output) INTEGER
If COMPQ = 'C', the number of eigenvalues in aS - bH with
strictly negative real part.

Q       (output) COMPLEX*16 array, dimension (LDQ, 2*N)
On exit, if COMPQ = 'C', the leading N-by-NEIG part of
this array contains an orthonormal basis of the right
deflating subspace corresponding to the eigenvalues of the
pencil aS - bH with strictly negative real part.
The remaining entries are meaningless.
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, 2*N), if COMPQ = 'C'.

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

ALPHAI  (output) DOUBLE PRECISION array, dimension (N)
The imaginary parts of each scalar alpha defining an
eigenvalue of the pencil aS - bH.
If ALPHAI(j) is zero, then the j-th eigenvalue is real.

BETA    (output) DOUBLE PRECISION array, dimension (N)
The scalars beta that define the eigenvalues of the pencil
aS - bH.
Together, the quantities alpha = (ALPHAR(j),ALPHAI(j)) and
beta = BETA(j) represent the j-th eigenvalue of the pencil
aS - bH, in the form lambda = alpha/beta. Since lambda may
overflow, the ratios should not, in general, be computed.

```
Workspace
```  IWORK   INTEGER array, dimension (N+1)

DWORK   DOUBLE PRECISION array, dimension (LDWORK)
On exit, if INFO = 0, DWORK(1) returns the optimal LDWORK.
On exit, if INFO = -20, DWORK(1) returns the minimum value
of LDWORK.

LDWORK  INTEGER
The dimension of the array DWORK.
LDWORK >= MAX( 4*N*N + 2*N + MAX(3,N) ), if COMPQ = 'N';
LDWORK >= MAX( 1, 11*N*N + 2*N ),        if COMPQ = 'C'.
For good performance LDWORK should be generally 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.

ZWORK   COMPLEX*16 array, dimension (LZWORK)
On exit, if INFO = 0, ZWORK(1) returns the optimal LZWORK.
On exit, if INFO = -22, ZWORK(1) returns the minimum
value of LZWORK.

LZWORK  INTEGER
The dimension of the array ZWORK.
LZWORK >= 1,       if COMPQ = 'N';
LZWORK >= 8*N + 4, if COMPQ = 'C'.
For good performance LZWORK should be generally larger.

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

BWORK   LOGICAL array, dimension (LBWORK)
LBWORK >= 0, if COMPQ = 'N';
LBWORK >= N, if COMPQ = 'C'.

```
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 SLICOT Library routine
MB04FP (QZ iteration did not converge or computation
of the shifts failed);
= 2: QZ iteration failed in the LAPACK routine ZHGEQZ when
trying to triangularize the 2-by-2 blocks;
= 3: the singular value decomposition failed in the LAPACK
routine ZGESVD (for ORTH = 'S');
= 4: warning: the pencil is numerically singular.

```
Method
```  First, T = i*H is set. Then, the embeddings, B_S and B_T, of the
matrices S and T, are determined and, subsequently, the SLICOT
Library routine MB04FP is applied to compute the structured Schur
form, i.e., the factorizations

~                     (  S11  S12  )
B_S = J Q' J' B_S Q = (            ) and
(   0   S11' )

~                     (  T11  T12  )           (  0  I  )
B_T = J Q' J' B_T Q = (            ), with J = (        ),
(   0   T11' )           ( -I  0  )

where Q is real orthogonal, S11 is upper triangular, and T11 is
upper quasi-triangular.

Second, the SLICOT Library routine MB3JZP is applied, to compute a
~
unitary matrix Q, such that

~    ~
~     ~   ~   (  S11  S12  )
J Q' J' B_S Q = (       ~    ) =: B_Sout,
(   0   S11' )

~        ~    ~   (  H11  H12  )
J Q' J'(-i*B_T) Q = (            ) =: B_Hout,
(   0  -H11' )
~                                               ~       ~
with S11, H11 upper triangular, and such that Spec_-(B_S, -i*B_T)
is contained in the spectrum of the 2*NEIG-by-2*NEIG leading
~
principal subpencil aS11 - bH11.

Finally, the right deflating subspace is computed.

```
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 )
complex floating point operations.

```
```  This routine does not perform any scaling of the matrices. Scaling
might sometimes be useful, and it should be done externally.
For large values of N, the routine applies the transformations 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
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Program Data
```  None
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Program Results
```  None
```