NAG FL Interface
f08uaf (dsbgv)

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1 Purpose

f08uaf computes all the eigenvalues and, optionally, the eigenvectors of a real generalized symmetric-definite banded eigenproblem, of the form
Az=λBz ,  
where A and B are symmetric and banded, and B is also positive definite.

2 Specification

Fortran Interface
Subroutine f08uaf ( jobz, uplo, n, ka, kb, ab, ldab, bb, ldbb, w, z, ldz, work, info)
Integer, Intent (In) :: n, ka, kb, ldab, ldbb, ldz
Integer, Intent (Out) :: info
Real (Kind=nag_wp), Intent (Inout) :: ab(ldab,*), bb(ldbb,*), z(ldz,*)
Real (Kind=nag_wp), Intent (Out) :: w(n), work(3*n)
Character (1), Intent (In) :: jobz, uplo
C Header Interface
#include <nag.h>
void  f08uaf_ (const char *jobz, const char *uplo, const Integer *n, const Integer *ka, const Integer *kb, double ab[], const Integer *ldab, double bb[], const Integer *ldbb, double w[], double z[], const Integer *ldz, double work[], Integer *info, const Charlen length_jobz, const Charlen length_uplo)
The routine may be called by the names f08uaf, nagf_lapackeig_dsbgv or its LAPACK name dsbgv.

3 Description

The generalized symmetric-definite band problem
Az = λ Bz  
is first reduced to a standard band symmetric problem
Cx = λx ,  
where C is a symmetric band matrix, using Wilkinson's modification to Crawford's algorithm (see Crawford (1973) and Wilkinson (1977)). The symmetric eigenvalue problem is then solved for the eigenvalues and the eigenvectors, if required, which are then backtransformed to the eigenvectors of the original problem.
The eigenvectors are normalized so that the matrix of eigenvectors, Z, satisfies
ZT A Z = Λ   and   ZT B Z = I ,  
where Λ is the diagonal matrix whose diagonal elements are the eigenvalues.

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
Crawford C R (1973) Reduction of a band-symmetric generalized eigenvalue problem Comm. ACM 16 41–44
Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore
Wilkinson J H (1977) Some recent advances in numerical linear algebra The State of the Art in Numerical Analysis (ed D A H Jacobs) Academic Press

5 Arguments

1: jobz Character(1) Input
On entry: indicates whether eigenvectors are computed.
jobz='N'
Only eigenvalues are computed.
jobz='V'
Eigenvalues and eigenvectors are computed.
Constraint: jobz='N' or 'V'.
2: uplo Character(1) Input
On entry: if uplo='U', the upper triangles of A and B are stored.
If uplo='L', the lower triangles of A and B are stored.
Constraint: uplo='U' or 'L'.
3: n Integer Input
On entry: n, the order of the matrices A and B.
Constraint: n0.
4: ka Integer Input
On entry: if uplo='U', the number of superdiagonals, ka, of the matrix A.
If uplo='L', the number of subdiagonals, ka, of the matrix A.
Constraint: ka0.
5: kb Integer Input
On entry: if uplo='U', the number of superdiagonals, kb, of the matrix B.
If uplo='L', the number of subdiagonals, kb, of the matrix B.
Constraint: kakb0.
6: ab(ldab,*) Real (Kind=nag_wp) array Input/Output
Note: the second dimension of the array ab must be at least max(1,n).
On entry: the upper or lower triangle of the n×n symmetric band matrix A.
The matrix is stored in rows 1 to ka+1, more precisely,
  • if uplo='U', the elements of the upper triangle of A within the band must be stored with element Aij in ab(ka+1+i-j,j)​ for ​max(1,j-ka)ij;
  • if uplo='L', the elements of the lower triangle of A within the band must be stored with element Aij in ab(1+i-j,j)​ for ​jimin(n,j+ka).
On exit: the contents of ab are overwritten.
7: ldab Integer Input
On entry: the first dimension of the array ab as declared in the (sub)program from which f08uaf is called.
Constraint: ldabka+1.
8: bb(ldbb,*) Real (Kind=nag_wp) array Input/Output
Note: the second dimension of the array bb must be at least max(1,n).
On entry: the upper or lower triangle of the n×n symmetric band matrix B.
The matrix is stored in rows 1 to kb+1, more precisely,
  • if uplo='U', the elements of the upper triangle of B within the band must be stored with element Bij in bb(kb+1+i-j,j)​ for ​max(1,j-kb)ij;
  • if uplo='L', the elements of the lower triangle of B within the band must be stored with element Bij in bb(1+i-j,j)​ for ​jimin(n,j+kb).
On exit: the factor S from the split Cholesky factorization B=STS, as returned by f08uff.
9: ldbb Integer Input
On entry: the first dimension of the array bb as declared in the (sub)program from which f08uaf is called.
Constraint: ldbbkb+1.
10: w(n) Real (Kind=nag_wp) array Output
On exit: the eigenvalues in ascending order.
11: z(ldz,*) Real (Kind=nag_wp) array Output
Note: the second dimension of the array z must be at least max(1,n) if jobz='V', and at least 1 otherwise.
On exit: if jobz='V', z contains the matrix Z of eigenvectors, with the ith column of Z holding the eigenvector associated with w(i). The eigenvectors are normalized so that ZTBZ=I.
If jobz='N', z is not referenced.
12: ldz Integer Input
On entry: the first dimension of the array z as declared in the (sub)program from which f08uaf is called.
Constraints:
  • if jobz='V', ldz max(1,n) ;
  • otherwise ldz1.
13: work(3×n) Real (Kind=nag_wp) array Workspace
14: info Integer Output
On exit: info=0 unless the routine detects an error (see Section 6).

6 Error Indicators and Warnings

info<0
If info=-i, argument i had an illegal value. An explanatory message is output, and execution of the program is terminated.
info=1,,n
The algorithm failed to converge; value off-diagonal elements of an intermediate tridiagonal form did not converge to zero.
info>n
If info=n+value, for 1valuen, f08uff returned info=value: B is not positive definite. The factorization of B could not be completed and no eigenvalues or eigenvectors were computed.

7 Accuracy

If B 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 B differ widely in magnitude the eigenvalues and eigenvectors may be less sensitive than the condition of B would suggest. See Section 4.10 of Anderson et al. (1999) for details of the error bounds.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
f08uaf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08uaf 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 routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

The total number of floating-point operations is proportional to n3 if jobz='V' and, assuming that nka , is approximately proportional to n2 ka otherwise.
The complex analogue of this routine is f08unf.

10 Example

This example finds all the eigenvalues of the generalized band symmetric eigenproblem Az = λ Bz , where
A = ( 0.24 0.39 0.42 0.00 0.39 -0.11 0.79 0.63 0.42 0.79 -0.25 0.48 0.00 0.63 0.48 -0.03 )   and   B = ( 2.07 0.95 0.00 0.00 0.95 1.69 -0.29 0.00 0.00 -0.29 0.65 -0.33 0.00 0.00 -0.33 1.17 ) .  

10.1 Program Text

Program Text (f08uafe.f90)

10.2 Program Data

Program Data (f08uafe.d)

10.3 Program Results

Program Results (f08uafe.r)