Provided by: liblapack-doc_3.3.1-1_all #### NAME

```       LAPACK-3  -  ZLAQR4 compute the eigenvalues of a Hessenberg matrix H  and, optionally, the
matrices T and Z from the Schur decomposition   H  =  Z  T  Z**H,  where  T  is  an  upper
triangular matrix (the  Schur form), and Z is the unitary matrix of Schur vectors

```

#### SYNOPSIS

```       SUBROUTINE ZLAQR4( WANTT,  WANTZ, N, ILO, IHI, H, LDH, W, ILOZ, IHIZ, Z, LDZ, WORK, LWORK,
INFO )

INTEGER        IHI, IHIZ, ILO, ILOZ, INFO, LDH, LDZ, LWORK, N

LOGICAL        WANTT, WANTZ

COMPLEX*16     H( LDH, * ), W( * ), WORK( * ), Z( LDZ, * )

```

#### PURPOSE

```          ZLAQR4 computes the eigenvalues of a Hessenberg matrix H
and, optionally, the matrices T and Z from the Schur decomposition
H = Z T Z**H, where T is an upper triangular matrix (the
Schur form), and Z is the unitary matrix of Schur vectors.
Optionally Z may be postmultiplied into an input unitary
matrix Q so that this routine can give the Schur factorization
of a matrix A which has been reduced to the Hessenberg form H
by the unitary matrix Q:  A = Q*H*Q**H = (QZ)*H*(QZ)**H.

```

#### ARGUMENTS

```        WANTT   (input) LOGICAL
= .TRUE. : the full Schur form T is required;
= .FALSE.: only eigenvalues are required.

WANTZ   (input) LOGICAL
= .TRUE. : the matrix of Schur vectors Z is required;
= .FALSE.: Schur vectors are not required.

N     (input) INTEGER
The order of the matrix H.  N .GE. 0.

ILO   (input) INTEGER
IHI   (input) INTEGER
It is assumed that H is already upper triangular in rows
and columns 1:ILO-1 and IHI+1:N and, if ILO.GT.1,
H(ILO,ILO-1) is zero. ILO and IHI are normally set by a
previous call to ZGEBAL, and then passed to ZGEHRD when the
matrix output by ZGEBAL is reduced to Hessenberg form.
Otherwise, ILO and IHI should be set to 1 and N,
respectively.  If N.GT.0, then 1.LE.ILO.LE.IHI.LE.N.
If N = 0, then ILO = 1 and IHI = 0.

H     (input/output) COMPLEX*16 array, dimension (LDH,N)
On entry, the upper Hessenberg matrix H.
On exit, if INFO = 0 and WANTT is .TRUE., then H
contains the upper triangular matrix T from the Schur
decomposition (the Schur form). If INFO = 0 and WANT is
.FALSE., then the contents of H are unspecified on exit.
(The output value of H when INFO.GT.0 is given under the
description of INFO below.)
This subroutine may explicitly set H(i,j) = 0 for i.GT.j and
j = 1, 2, ... ILO-1 or j = IHI+1, IHI+2, ... N.

LDH   (input) INTEGER
The leading dimension of the array H. LDH .GE. max(1,N).

W        (output) COMPLEX*16 array, dimension (N)
The computed eigenvalues of H(ILO:IHI,ILO:IHI) are stored
in W(ILO:IHI). If WANTT is .TRUE., then the eigenvalues are
stored in the same order as on the diagonal of the Schur
form returned in H, with W(i) = H(i,i).

Z     (input/output) COMPLEX*16 array, dimension (LDZ,IHI)
If WANTZ is .FALSE., then Z is not referenced.
If WANTZ is .TRUE., then Z(ILO:IHI,ILOZ:IHIZ) is
replaced by Z(ILO:IHI,ILOZ:IHIZ)*U where U is the
orthogonal Schur factor of H(ILO:IHI,ILO:IHI).
(The output value of Z when INFO.GT.0 is given under
the description of INFO below.)

LDZ   (input) INTEGER
The leading dimension of the array Z.  if WANTZ is .TRUE.
then LDZ.GE.MAX(1,IHIZ).  Otherwize, LDZ.GE.1.

WORK  (workspace/output) COMPLEX*16 array, dimension LWORK
On exit, if LWORK = -1, WORK(1) returns an estimate of
the optimal value for LWORK.
LWORK (input) INTEGER
The dimension of the array WORK.  LWORK .GE. max(1,N)
is sufficient, but LWORK typically as large as 6*N may
be required for optimal performance.  A workspace query
to determine the optimal workspace size is recommended.
If LWORK = -1, then ZLAQR4 does a workspace query.
In this case, ZLAQR4 checks the input parameters and
estimates the optimal workspace size for the given
values of N, ILO and IHI.  The estimate is returned
in WORK(1).  No error message related to LWORK is
issued by XERBLA.  Neither H nor Z are accessed.

INFO  (output) INTEGER
=  0:  successful exit
.GT. 0:  if INFO = i, ZLAQR4 failed to compute all of
the eigenvalues.  Elements 1:ilo-1 and i+1:n of WR
and WI contain those eigenvalues which have been
successfully computed.  (Failures are rare.)
If INFO .GT. 0 and WANT is .FALSE., then on exit,
the remaining unconverged eigenvalues are the eigen-
values of the upper Hessenberg matrix rows and
columns ILO through INFO of the final, output
value of H.
If INFO .GT. 0 and WANTT is .TRUE., then on exit

(*)  (initial value of H)*U  = U*(final value of H)
where U is a unitary matrix.  The final
value of  H is upper Hessenberg and triangular in
rows and columns INFO+1 through IHI.
If INFO .GT. 0 and WANTZ is .TRUE., then on exit
(final value of Z(ILO:IHI,ILOZ:IHIZ)
=  (initial value of Z(ILO:IHI,ILOZ:IHIZ)*U
where U is the unitary matrix in (*) (regard-
less of the value of WANTT.)
If INFO .GT. 0 and WANTZ is .FALSE., then Z is not
accessed.

LAPACK auxiliary routine (version 3.2)     April 2011                            ZLAQR4(3lapack)
```