?hegvd
Computes all eigenvalues and, optionally, eigenvectors of a complex generalized Hermitian positivedefinite eigenproblem using a divide and conquer method.
Syntax

call chegvd(itype, jobz, uplo, n, a, lda, b, ldb, w, work, lwork, rwork, lrwork, iwork, liwork, info)
call zhegvd(itype, jobz, uplo, n, a, lda, b, ldb, w, work, lwork, rwork, lrwork, iwork, liwork, info)

call hegvd(a, b, w [,itype] [,jobz] [,uplo] [,info])
Description
The routine computes all the eigenvalues, and optionally, the eigenvectors of a complex generalized Hermitian positivedefinite eigenproblem, of the form
A*x = λ*B*x, A*B*x = λ*x
, or B*A*x = λ*x
.
Here A and B are assumed to be Hermitian and B is also positive definite.
It uses a divide and conquer algorithm.
Input Parameters
 itype

INTEGER. Must be 1 or 2 or 3. Specifies the problem type to be solved:
if
itype = 1
, the problem type isA*x = lambda*B*x
;if
itype = 2
, the problem type isA*B*x = lambda*x
;if
itype = 3
, the problem type isB*A*x = lambda*x
.  jobz

CHARACTER*1. Must be 'N' or 'V'.
If
jobz = 'N'
, then compute eigenvalues only.If
jobz = 'V'
, then compute eigenvalues and eigenvectors.  uplo

CHARACTER*1. Must be 'U' or 'L'.
If
uplo = 'U'
, arrays a and b store the upper triangles of A and B;If
uplo = 'L'
, arrays a and b store the lower triangles of A and B.  n

INTEGER. The order of the matrices A and B (
n ≥ 0
).  a, b, work

COMPLEX for chegvd
DOUBLE COMPLEX for zhegvd.
Arrays:
a(lda,*) contains the upper or lower triangle of the Hermitian matrix A, as specified by uplo.
The second dimension of a must be at least max(1, n).
b(ldb,*) contains the upper or lower triangle of the Hermitian positive definite matrix B, as specified by uplo.
The second dimension of b must be at least max(1, n).
work is a workspace array, its dimension
max(1, lwork)
.  lda

INTEGER. The leading dimension of a; at least max(1, n).
 ldb

INTEGER. The leading dimension of b; at least max(1, n).
 lwork

INTEGER.
The dimension of the array work.
Constraints:
If
n ≤ 1
,lwork ≥ 1
;If
jobz = 'N'
andn>1
,lwork ≥ n
+1;If
jobz = 'V'
andn>1
,lwork ≥ n^{2}+2n
.If
lwork = 1
, then a workspace query is assumed; the routine only calculates the optimal size of the work, rwork and iwork arrays, returns these values as the first entries of the work, rwork and iwork arrays, and no error message related to lwork or lrwork or liwork is issued by xerbla. See Application Notes for details.  rwork

REAL for chegvd
DOUBLE PRECISION for zhegvd.
Workspace array, size
max(1, lrwork)
.  lrwork

INTEGER.
The dimension of the array rwork.
Constraints:
If
n ≤ 1
,lrwork ≥ 1
;If
jobz = 'N'
andn>1
,lrwork ≥ n
;If
jobz = 'V'
andn>1
,lrwork ≥ 2n^{2}+5n+1
.If
lrwork = 1
, then a workspace query is assumed; the routine only calculates the optimal size of the work, rwork and iwork arrays, returns these values as the first entries of the work, rwork and iwork arrays, and no error message related to lwork or lrwork or liwork is issued by xerbla. See Application Notes for details.  iwork

INTEGER.
Workspace array, size
max(1, liwork)
.  liwork

INTEGER.
The dimension of the array iwork.
Constraints:
If
n ≤ 1
,liwork ≥ 1
;If
jobz = 'N'
andn>1
,liwork ≥ 1
;If
jobz = 'V'
andn>1
,liwork ≥ 5n+3
.If
liwork = 1
, then a workspace query is assumed; the routine only calculates the optimal size of the work, rwork and iwork arrays, returns these values as the first entries of the work, rwork and iwork arrays, and no error message related to lwork or lrwork or liwork is issued by xerbla. See Application Notes for details.
Output Parameters
 a

On exit, if
jobz = 'V'
, then ifinfo = 0
, a contains the matrix Z of eigenvectors. The eigenvectors are normalized as follows:if
itype = 1
or 2,Z^{H}* B*Z = I
;if
itype = 3
,Z^{H}*inv(B)*Z = I
;If
jobz = 'N'
, then on exit the upper triangle (ifuplo = 'U'
) or the lower triangle (ifuplo = 'L'
) of A, including the diagonal, is destroyed.  b

On exit, if
info ≤ n
, the part of b containing the matrix is overwritten by the triangular factor U or L from the Cholesky factorizationB = U^{H}*U
orB = L*L^{H}
.  w

REAL for chegvd
DOUBLE PRECISION for zhegvd.
Array, size at least max(1, n).
If
info = 0
, contains the eigenvalues in ascending order.  work
(1)

On exit, if
info = 0
, then work(1)
returns the required minimal size of lwork.  rwork
(1)

On exit, if
info = 0
, then rwork(1)
returns the required minimal size of lrwork.  iwork
(1)

On exit, if
info = 0
, then iwork(1)
returns the required minimal size of liwork.  info

INTEGER.
If
info = 0
, the execution is successful.If
info = i
, the ith argument had an illegal value.If
info = i
, andjobz = 'N'
, then the algorithm failed to converge; i offdiagonal elements of an intermediate tridiagonal form did not converge to zero;if
info = i
, andjobz = 'V'
, then the algorithm failed to compute an eigenvalue while working on the submatrix lying in rows and columnsinfo/(n+1)
throughmod(info, n+1)
.If
info = n + i
, for1 ≤ i ≤ n
, then the leading minor of order i of B is not positivedefinite. The factorization of B could not be completed and no eigenvalues or eigenvectors were computed.
LAPACK 95 Interface Notes
Routines in Fortran 95 interface have fewer arguments in the calling sequence than their FORTRAN 77 counterparts. For general conventions applied to skip redundant or restorable arguments, see LAPACK 95 Interface Conventions.
Specific details for the routine hegvd interface are the following:
 a

Holds the matrix A of size (n, n).
 b

Holds the matrix B of size (n, n).
 w

Holds the vector of length n.
 itype

Must be 1, 2, or 3. The default value is 1.
 jobz

Must be 'N' or 'V'. The default value is 'N'.
 uplo

Must be 'U' or 'L'. The default value is 'U'.
Application Notes
If you are in doubt how much workspace to supply, use a generous value of lwork (liwork or lrwork) for the first run or set lwork = 1
(liwork = 1
, lrwork = 1
).
If you choose the first option and set any of admissible lwork (liwork or lrwork) sizes, which is no less than the minimal value described, the routine completes the task, though probably not so fast as with a recommended workspace, and provides the recommended workspace in the first element of the corresponding array (work, iwork, rwork) on exit. Use this value (work(1)
, iwork(1)
, rwork(1)
) for subsequent runs.
If you set lwork = 1
(liwork = 1
, lrwork = 1
), the routine returns immediately and provides the recommended workspace in the first element of the corresponding array (work, iwork, rwork). This operation is called a workspace query.
Note that if you set lwork (liwork, lrwork) to less than the minimal required value and not 1, the routine returns immediately with an error exit and does not provide any information on the recommended workspace.