NAG CL Interface
f08psc (zhseqr)

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1 Purpose

f08psc computes all the eigenvalues and, optionally, the Schur factorization of a complex Hessenberg matrix or a complex general matrix which has been reduced to Hessenberg form.

2 Specification

#include <nag.h>
void  f08psc (Nag_OrderType order, Nag_JobType job, Nag_ComputeZType compz, Integer n, Integer ilo, Integer ihi, Complex h[], Integer pdh, Complex w[], Complex z[], Integer pdz, NagError *fail)
The function may be called by the names: f08psc, nag_lapackeig_zhseqr or nag_zhseqr.

3 Description

f08psc computes all the eigenvalues and, optionally, the Schur factorization of a complex upper Hessenberg matrix H:
H = ZTZH ,  
where T is an upper triangular matrix (the Schur form of H), and Z is the unitary matrix whose columns are the Schur vectors zi. The diagonal elements of T are the eigenvalues of H.
The function may also be used to compute the Schur factorization of a complex general matrix A which has been reduced to upper Hessenberg form H:
A = QHQH, where ​Q​ is unitary, = (QZ)T(QZ)H.  
In this case, after f08nsc has been called to reduce A to Hessenberg form, f08ntc must be called to form Q explicitly; Q is then passed to f08psc, which must be called with compz=Nag_UpdateZ.
The function can also take advantage of a previous call to f08nvc which may have balanced the original matrix before reducing it to Hessenberg form, so that the Hessenberg matrix H has the structure:
( H11 H12 H13 H22 H23 H33 )  
where H11 and H33 are upper triangular. If so, only the central diagonal block H22 (in rows and columns ilo to ihi) needs to be further reduced to Schur form (the blocks H12 and H23 are also affected). Therefore, the values of ilo and ihi can be supplied to f08psc directly. Also, f08nwc must be called after this function to permute the Schur vectors of the balanced matrix to those of the original matrix. If f08nvc has not been called however, then ilo must be set to 1 and ihi to n. Note that if the Schur factorization of A is required, f08nvc must not be called with job=Nag_Schur or Nag_DoBoth, because the balancing transformation is not unitary.
f08psc uses a multishift form of the upper Hessenberg QR algorithm, due to Bai and Demmel (1989). The Schur vectors are normalized so that zi2=1, but are determined only to within a complex factor of absolute value 1.

4 References

Bai Z and Demmel J W (1989) On a block implementation of Hessenberg multishift QR iteration Internat. J. High Speed Comput. 1 97–112
Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore

5 Arguments

1: order Nag_OrderType Input
On entry: the order argument specifies the two-dimensional storage scheme being used, i.e., row-major ordering or column-major ordering. C language defined storage is specified by order=Nag_RowMajor. See Section 3.1.3 in the Introduction to the NAG Library CL Interface for a more detailed explanation of the use of this argument.
Constraint: order=Nag_RowMajor or Nag_ColMajor.
2: job Nag_JobType Input
On entry: indicates whether eigenvalues only or the Schur form T is required.
job=Nag_EigVals
Eigenvalues only are required.
job=Nag_Schur
The Schur form T is required.
Constraint: job=Nag_EigVals or Nag_Schur.
3: compz Nag_ComputeZType Input
On entry: indicates whether the Schur vectors are to be computed.
compz=Nag_NotZ
No Schur vectors are computed (and the array z is not referenced).
compz=Nag_UpdateZ
The Schur vectors of A are computed (and the array z must contain the matrix Q on entry).
compz=Nag_InitZ
The Schur vectors of H are computed (and the array z is initialized by the function).
Constraint: compz=Nag_NotZ, Nag_UpdateZ or Nag_InitZ.
4: n Integer Input
On entry: n, the order of the matrix H.
Constraint: n0.
5: ilo Integer Input
6: ihi Integer Input
On entry: if the matrix A has been balanced by f08nvc, ilo and ihi must contain the values returned by that function. Otherwise, ilo must be set to 1 and ihi to n.
Constraint: ilo1 and min(ilo,n) ihin .
7: h[dim] Complex Input/Output
Note: the dimension, dim, of the array h must be at least max(1,pdh×n).
The (i,j)th element of the matrix H is stored in
  • h[(j-1)×pdh+i-1] when order=Nag_ColMajor;
  • h[(i-1)×pdh+j-1] when order=Nag_RowMajor.
On entry: the n×n upper Hessenberg matrix H, as returned by f08nsc.
On exit: if job=Nag_EigVals, the array contains no useful information.
If job=Nag_Schur, h is overwritten by the upper triangular matrix T from the Schur decomposition (the Schur form) unless fail.code= NE_CONVERGENCE.
8: pdh Integer Input
On entry: the stride separating row or column elements (depending on the value of order) in the array h.
Constraint: pdhmax(1,n).
9: w[dim] Complex Output
Note: the dimension, dim, of the array w must be at least max(1,n).
On exit: the computed eigenvalues, unless fail.code= NE_CONVERGENCE (in which case see Section 6). The eigenvalues are stored in the same order as on the diagonal of the Schur form T (if computed).
10: z[dim] Complex Input/Output
Note: the dimension, dim, of the array z must be at least
  • max(1,pdz×n) when compz=Nag_UpdateZ or Nag_InitZ;
  • 1 when compz=Nag_NotZ.
The (i,j)th element of the matrix Z is stored in
  • z[(j-1)×pdz+i-1] when order=Nag_ColMajor;
  • z[(i-1)×pdz+j-1] when order=Nag_RowMajor.
On entry: if compz=Nag_UpdateZ, z must contain the unitary matrix Q from the reduction to Hessenberg form.
If compz=Nag_InitZ, z need not be set.
On exit: if compz=Nag_UpdateZ or Nag_InitZ, z contains the unitary matrix of the required Schur vectors, unless fail.code= NE_CONVERGENCE.
If compz=Nag_NotZ, z is not referenced.
11: pdz Integer Input
On entry: the stride separating row or column elements (depending on the value of order) in the array z.
Constraints:
  • if compz=Nag_UpdateZ or Nag_InitZ, pdz max(1,n) ;
  • if compz=Nag_NotZ, pdz1.
12: fail NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
NE_BAD_PARAM
On entry, argument value had an illegal value.
NE_CONVERGENCE
The algorithm has failed to find all the eigenvalues after a total of 30(ihi-ilo+1) iterations.
NE_ENUM_INT_2
On entry, compz=value, pdz=value and n=value.
Constraint: if compz=Nag_UpdateZ or Nag_InitZ, pdz max(1,n) ;
if compz=Nag_NotZ, pdz1.
NE_INT
On entry, n=value.
Constraint: n0.
On entry, pdh=value.
Constraint: pdh>0.
On entry, pdz=value.
Constraint: pdz>0.
NE_INT_2
On entry, pdh=value and n=value.
Constraint: pdhmax(1,n).
NE_INT_3
On entry, n=value, ilo=value and ihi=value.
Constraint: ilo1 and min(ilo,n) ihin .
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.

7 Accuracy

The computed Schur factorization is the exact factorization of a nearby matrix (H+E), where
E2 = O(ε) H2 ,  
and ε is the machine precision.
If λi is an exact eigenvalue, and λ~i is the corresponding computed value, then
|λ~i-λi| c (n) ε H2 si ,  
where c(n) is a modestly increasing function of n, and si is the reciprocal condition number of λi. The condition numbers si may be computed by calling f08qyc.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
f08psc is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08psc makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

The total number of real floating-point operations depends on how rapidly the algorithm converges, but is typically about:
The real analogue of this function is f08pec.

10 Example

This example computes all the eigenvalues and the Schur factorization of the upper Hessenberg matrix H, where
H = ( -3.9700-5.0400i -1.1318-2.5693i -4.6027-0.1426i -1.4249+1.7330i -5.4797+0.0000i 1.8585-1.5502i 4.4145-0.7638i -0.4805-1.1976i 0.0000+0.0000i 6.2673+0.0000i -0.4504-0.0290i -1.3467+1.6579i 0.0000+0.0000i 0.0000+0.0000i -3.5000+0.0000i 2.5619-3.3708i ) .  
See also f08ntc, which illustrates the use of this function to compute the Schur factorization of a general matrix.

10.1 Program Text

Program Text (f08psce.c)

10.2 Program Data

Program Data (f08psce.d)

10.3 Program Results

Program Results (f08psce.r)