NAG FL Interface
f08hqf (zhbevd)

1 Purpose

f08hqf computes all the eigenvalues and, optionally, all the eigenvectors of a complex Hermitian band matrix. If the eigenvectors are requested, then it uses a divide-and-conquer algorithm to compute eigenvalues and eigenvectors. However, if only eigenvalues are required, then it uses the Pal–Walker–Kahan variant of the QL or QR algorithm.

2 Specification

Fortran Interface
Subroutine f08hqf ( job, uplo, n, kd, ab, ldab, w, z, ldz, work, lwork, rwork, lrwork, iwork, liwork, info)
Integer, Intent (In) :: n, kd, ldab, ldz, lwork, lrwork, liwork
Integer, Intent (Out) :: iwork(max(1,liwork)), info
Real (Kind=nag_wp), Intent (Inout) :: w(*)
Real (Kind=nag_wp), Intent (Out) :: rwork(max(1,lrwork))
Complex (Kind=nag_wp), Intent (Inout) :: ab(ldab,*), z(ldz,*)
Complex (Kind=nag_wp), Intent (Out) :: work(max(1,lwork))
Character (1), Intent (In) :: job, uplo
C Header Interface
#include <nag.h>
void  f08hqf_ (const char *job, const char *uplo, const Integer *n, const Integer *kd, Complex ab[], const Integer *ldab, double w[], Complex z[], const Integer *ldz, Complex work[], const Integer *lwork, double rwork[], const Integer *lrwork, Integer iwork[], const Integer *liwork, Integer *info, const Charlen length_job, const Charlen length_uplo)
The routine may be called by the names f08hqf, nagf_lapackeig_zhbevd or its LAPACK name zhbevd.

3 Description

f08hqf computes all the eigenvalues and, optionally, all the eigenvectors of a complex Hermitian band matrix A. In other words, it can compute the spectral factorization of A as
A=ZΛZH,  
where Λ is a real diagonal matrix whose diagonal elements are the eigenvalues λi, and Z is the (complex) unitary matrix whose columns are the eigenvectors zi. Thus
Azi=λizi,  i=1,2,,n.  

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: job Character(1) Input
On entry: indicates whether eigenvectors are computed.
job='N'
Only eigenvalues are computed.
job='V'
Eigenvalues and eigenvectors are computed.
Constraint: job='N' or 'V'.
2: uplo Character(1) Input
On entry: indicates whether the upper or lower triangular part of A is stored.
uplo='U'
The upper triangular part of A is stored.
uplo='L'
The lower triangular part of A is stored.
Constraint: uplo='U' or 'L'.
3: n Integer Input
On entry: n, the order of the matrix A.
Constraint: n0.
4: kd Integer Input
On entry: if uplo='U', the number of superdiagonals, kd, of the matrix A.
If uplo='L', the number of subdiagonals, kd, of the matrix A.
Constraint: kd0.
5: abldab* Complex (Kind=nag_wp) array Input/Output
Note: the second dimension of the array ab must be at least max1,n.
On entry: the upper or lower triangle of the n by n Hermitian band matrix A.
The matrix is stored in rows 1 to kd+1, more precisely,
  • if uplo='U', the elements of the upper triangle of A within the band must be stored with element Aij in abkd+1+i-jj​ for ​max1,j-kdij;
  • if uplo='L', the elements of the lower triangle of A within the band must be stored with element Aij in ab1+i-jj​ for ​jiminn,j+kd.
On exit: ab is overwritten by values generated during the reduction to tridiagonal form.
The first superdiagonal or subdiagonal and the diagonal of the tridiagonal matrix T are returned in ab using the same storage format as described above.
6: ldab Integer Input
On entry: the first dimension of the array ab as declared in the (sub)program from which f08hqf is called.
Constraint: ldabkd+1.
7: w* Real (Kind=nag_wp) array Output
Note: the dimension of the array w must be at least max1,n.
On exit: the eigenvalues of the matrix A in ascending order.
8: zldz* Complex (Kind=nag_wp) array Output
Note: the second dimension of the array z must be at least max1,n if job='V' and at least 1 if job='N'.
On exit: if job='V', z is overwritten by the unitary matrix Z which contains the eigenvectors of A. The ith column of Z contains the eigenvector which corresponds to the eigenvalue wi.
If job='N', z is not referenced.
9: ldz Integer Input
On entry: the first dimension of the array z as declared in the (sub)program from which f08hqf is called.
Constraints:
  • if job='V', ldz max1,n ;
  • if job='N', ldz1.
10: workmax1,lwork Complex (Kind=nag_wp) array Workspace
On exit: if info=0, the real part of work1 contains the required minimal size of lwork.
11: lwork Integer Input
On entry: the dimension of the array work as declared in the (sub)program from which f08hqf is called.
If lwork=-1, a workspace query is assumed; the routine only calculates the minimum dimension of the work array, returns this value as the first entry of the work array, and no error message related to lwork is issued.
Constraints:
  • if n1, lwork1 or lwork=-1;
  • if job='N' and n>1, lworkn or lwork=-1;
  • if job='V' and n>1, lwork2×n2 or lwork=-1.
12: rworkmax1,lrwork Real (Kind=nag_wp) array Workspace
On exit: if info=0, rwork1 contains the required minimal size of lrwork.
13: lrwork Integer Input
On entry: the dimension of the array rwork as declared in the (sub)program from which f08hqf is called.
If lrwork=-1, a workspace query is assumed; the routine only calculates the minimum dimension of the rwork array, returns this value as the first entry of the rwork array, and no error message related to lrwork is issued.
Constraints:
  • if n1, lrwork1 or lrwork=-1;
  • if job='N' and n>1, lrworkn or lrwork=-1;
  • if job='V' and n>1, lrwork2×n2+5×n+1 or lrwork=-1.
14: iworkmax1,liwork Integer array Workspace
On exit: if info=0, iwork1 contains the required minimal size of liwork.
15: liwork Integer Input
On entry: the dimension of the array iwork as declared in the (sub)program from which f08hqf is called.
If liwork=-1, a workspace query is assumed; the routine only calculates the minimum dimension of the iwork array, returns this value as the first entry of the iwork array, and no error message related to liwork is issued.
Constraints:
  • if job='N' or n1, liwork1 or liwork=-1;
  • if job='V' and n>1, liwork5×n+3 or liwork=-1.
16: 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>0
If info=value and job='N', the algorithm failed to converge; value elements of an intermediate tridiagonal form did not converge to zero; if info=value and job='V', then the algorithm failed to compute an eigenvalue while working on the submatrix lying in rows and column value/n+1 through value mod n+1.

7 Accuracy

The computed eigenvalues and eigenvectors are exact for a nearby matrix A+E, where
E2 = Oε A2 ,  
and ε is the machine precision. See Section 4.7 of Anderson et al. (1999) for further details.

8 Parallelism and Performance

f08hqf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08hqf 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 real analogue of this routine is f08hcf.

10 Example

This example computes all the eigenvalues and eigenvectors of the Hermitian band matrix A, where
A = 1+0i 2-1i 3-1i 0+0i 0+0i 2+1i 2+0i 3-2i 4-2i 0+0i 3+1i 3+2i 3+0i 4-3i 5-3i 0+0i 4+2i 4+3i 4+0i 5-4i 0+0i 0+0i 5+3i 5+4i 5+0i .  

10.1 Program Text

Program Text (f08hqfe.f90)

10.2 Program Data

Program Data (f08hqfe.d)

10.3 Program Results

Program Results (f08hqfe.r)