NAG CL Interface
f08sqc (zhegvd)
1
Purpose
f08sqc computes all the eigenvalues and, optionally, the eigenvectors of a complex generalized Hermitian-definite eigenproblem, of the form
where
and
are Hermitian and
is also positive definite. If eigenvectors are desired, it uses a divide-and-conquer algorithm.
2
Specification
void |
f08sqc (Nag_OrderType order,
Integer itype,
Nag_JobType job,
Nag_UploType uplo,
Integer n,
Complex a[],
Integer pda,
Complex b[],
Integer pdb,
double w[],
NagError *fail) |
|
The function may be called by the names: f08sqc, nag_lapackeig_zhegvd or nag_zhegvd.
3
Description
f08sqc first performs a Cholesky factorization of the matrix
as
, when
or
, when
. The generalized problem is then reduced to a standard symmetric eigenvalue problem
which is solved for the eigenvalues and, optionally, the eigenvectors; the eigenvectors are then backtransformed to give the eigenvectors of the original problem.
For the problem
, the eigenvectors are normalized so that the matrix of eigenvectors,
, satisfies
where
is the diagonal matrix whose diagonal elements are the eigenvalues. For the problem
we correspondingly have
and for
we have
4
References
Anderson E, Bai Z, Bischof C, Blackford S, Demmel J, Dongarra J J, Du Croz J J, Greenbaum A, Hammarling S, McKenney A and Sorensen D (1999)
LAPACK Users' Guide (3rd Edition) SIAM, Philadelphia
https://www.netlib.org/lapack/lug
Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore
5
Arguments
-
1:
– 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
. 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:
or .
-
2:
– Integer
Input
-
On entry: specifies the problem type to be solved.
- .
- .
- .
Constraint:
, or .
-
3:
– Nag_JobType
Input
-
On entry: indicates whether eigenvectors are computed.
- Only eigenvalues are computed.
- Eigenvalues and eigenvectors are computed.
Constraint:
or .
-
4:
– Nag_UploType
Input
-
On entry: if
, the upper triangles of
and
are stored.
If , the lower triangles of and are stored.
Constraint:
or .
-
5:
– Integer
Input
-
On entry: , the order of the matrices and .
Constraint:
.
-
6:
– Complex
Input/Output
-
Note: the dimension,
dim, of the array
a
must be at least
.
On entry: the
by
Hermitian matrix
.
If , is stored in .
If , is stored in .
If , the upper triangular part of must be stored and the elements of the array below the diagonal are not referenced.
If , the lower triangular part of must be stored and the elements of the array above the diagonal are not referenced.
On exit: if
,
a contains the matrix
of eigenvectors. The eigenvectors are normalized as follows:
- if or , ;
- if , .
If
, the upper triangle (if
) or the lower triangle (if
) of
a, including the diagonal, is overwritten.
-
7:
– Integer
Input
-
On entry: the stride separating row or column elements (depending on the value of
order) in the array
a.
Constraint:
.
-
8:
– Complex
Input/Output
-
Note: the dimension,
dim, of the array
b
must be at least
.
On entry: the
by
Hermitian matrix
.
If , is stored in .
If , is stored in .
If , the upper triangular part of must be stored and the elements of the array below the diagonal are not referenced.
If , the lower triangular part of must be stored and the elements of the array above the diagonal are not referenced.
On exit: the triangular factor or from the Cholesky factorization or .
-
9:
– Integer
Input
-
On entry: the stride separating row or column elements (depending on the value of
order) in the array
b.
Constraint:
.
-
10:
– double
Output
-
On exit: the eigenvalues in ascending order.
-
11:
– 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 had an illegal value.
- NE_CONVERGENCE
-
The algorithm failed to converge; off-diagonal elements of an intermediate tridiagonal form did not converge to zero.
- NE_INT
-
On entry, .
Constraint: , or .
On entry, .
Constraint: .
On entry, .
Constraint: .
On entry, .
Constraint: .
- NE_INT_2
-
On entry, and .
Constraint: .
On entry, and .
Constraint: .
- 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_MAT_NOT_POS_DEF
-
If , for , then the leading minor of order of is not positive definite. The factorization of could not be completed and no eigenvalues or eigenvectors were computed.
- 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
If
is ill-conditioned with respect to inversion, then the error bounds for the computed eigenvalues and vectors may be large, although when the diagonal elements of
differ widely in magnitude the eigenvalues and eigenvectors may be less sensitive than the condition of
would suggest. See Section 4.10 of
Anderson et al. (1999) for details of the error bounds.
The example program below illustrates the computation of approximate error bounds.
8
Parallelism and Performance
f08sqc is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08sqc 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.
The total number of floating-point operations is proportional to .
The real analogue of this function is
f08scc.
10
Example
This example finds all the eigenvalues and eigenvectors of the generalized Hermitian eigenproblem
, where
and
together with an estimate of the condition number of
, and approximate error bounds for the computed eigenvalues and eigenvectors.
The example program for
f08snc illustrates solving a generalized Hermitian eigenproblem of the form
.
10.1
Program Text
10.2
Program Data
10.3
Program Results