Developer Reference for Intel® oneAPI Math Kernel Library for C
?hbevx
Computes selected eigenvalues and, optionally, eigenvectors of a Hermitian band matrix.
Syntax
lapack_int LAPACKE_chbevx ( intmatrix_layout , charjobz , charrange , charuplo , lapack_intn , lapack_intkd , lapack_complex_float*ab , lapack_intldab , lapack_complex_float*q , lapack_intldq , floatvl , floatvu , lapack_intil , lapack_intiu , floatabstol , lapack_int*m , float*w , lapack_complex_float*z , lapack_intldz , lapack_int*ifail );
lapack_int LAPACKE_zhbevx ( intmatrix_layout , charjobz , charrange , charuplo , lapack_intn , lapack_intkd , lapack_complex_double*ab , lapack_intldab , lapack_complex_double*q , lapack_intldq , doublevl , doublevu , lapack_intil , lapack_intiu , doubleabstol , lapack_int*m , double*w , lapack_complex_double*z , lapack_intldz , lapack_int*ifail );
Include Files
mkl.h
Description
chbevx zhbevx hbevx
The routine computes selected eigenvalues and, optionally, eigenvectors of a complex Hermitian band matrix A . Eigenvalues and eigenvectors can be selected by specifying either a range of values or a range of indices for the desired eigenvalues.
Input Parameters
- matrix_layout
-
Specifies whether matrix storage layout is row major ( LAPACK_ROW_MAJOR ) or column major ( LAPACK_COL_MAJOR ).
jobz
Must be ‘N’ or ‘V’ .
If job = 'N' , then only eigenvalues are computed. If job = 'V' , then eigenvalues and eigenvectors are computed.
range
Must be ‘A’ or ‘V’ or ‘I’ .
If range = 'A' , the routine computes all eigenvalues. If range = 'V' , the routine computes eigenvalues w(i)w[i] in the half-open interval: vl<w(i)w[i]≤vu . If range = 'I' , the routine computes eigenvalues with indices il to iu .
uplo
Must be ‘U’ or ‘L’ .
If uplo = 'U' , ab stores the upper triangular part of A . If uplo = 'L' , ab stores the lower triangular part of A .
n
The order of the matrix A ( n≥ 0 ).
kd
The number of super- or sub-diagonals in A
( kd≥ 0 ).
- ab , work
-
COMPLEX for chbevx DOUBLE COMPLEX for zhbevx . Arrays:
ab (size at least max(1, ldab * n ) for column major layout and at least max(1, ldab *( kd + 1)) for row major layout) is an array containing either upper or lower triangular part of the Hermitian matrix A (as specified by uplo ) in band storage format.
The second dimension of ab must be at least max(1, n ). work (*) is a workspace array. The dimension of work must be at least max(1, n ).
ldab
The leading dimension of ab ; must be at least kd +1 for column major layout and n for row major layout .
- vl , vu
-
REAL for chbevx DOUBLE PRECISION for zhbevx . If range = 'V' , the lower and upper bounds of the interval to be searched for eigenvalues. Constraint: vl< vu . If range = 'A' or ‘I’ , vl and vu are not referenced.
il , iu
If range = 'I' , the indices in ascending order of the smallest and largest eigenvalues to be returned. Constraint: 1 ≤il≤iu≤n , if n > 0 ; il=1 and iu=0 if n = 0 . If range = 'A' or ‘V’ , il and iu are not referenced.
- abstol
-
REAL for chbevx DOUBLE PRECISION for zhbevx . The absolute error tolerance to which each eigenvalue is required. See Application notes for details on error tolerance.
ldq , ldz
The leading dimensions of the output arrays q and z , respectively.
Constraints: ldq≥ 1, ldz≥ 1 ;
If jobz = 'V' , then ldq≥ max (1, n ) and ldz≥ max (1, n ) for column major layout and ldz ≥ max(1, m ) for row major layout .
- rwork
-
REAL for chbevx DOUBLE PRECISION for zhbevx Workspace array, size at least max(1, 7n) .
iwork
Workspace array, size at least max(1, 5n) .
Output Parameters
- q
-
COMPLEX for chbevx DOUBLE COMPLEX for zhbevx .
Array, size max(1, ldz * n ) .
If jobz = 'V' , the n -by- n unitary matrix is used in the reduction to tridiagonal form. If jobz = 'N' , the array q is not referenced.
m
The total number of eigenvalues found,
0 ≤m≤n . If range = 'A' , m = n , if range = 'I' , m = iu-il+1 , and if range = 'V' , the exact value of m is not known in advance..
- w
-
REAL for chbevx DOUBLE PRECISION for zhbevx Array, size at least max(1, n ). The first m elements contain the selected eigenvalues of the matrix A in ascending order.
- z
-
COMPLEX for chbevx DOUBLE COMPLEX for zhbevx .
Array z (size at least max(1, ldz * m ) for column major layout and max(1, ldz * n ) for row major layout) .
The second dimension of z must be at least max(1, m ). If jobz = 'V' , then if info = 0 , the first m columns of z contain the orthonormal eigenvectors of the matrix A corresponding to the selected eigenvalues, with the i -th column of z holding the eigenvector associated with w(i)w[i - 1] . If an eigenvector fails to converge, then that column of z contains the latest approximation to the eigenvector, and the index of the eigenvector is returned in ifail . If jobz = 'N' , then z is not referenced. Note: you must ensure that at least max(1, m ) columns are supplied in the array z ; if range = 'V' , the exact value of m is not known in advance and an upper bound must be used.
- ab
-
On exit, this array is overwritten by the values generated during the reduction to tridiagonal form. If uplo = 'U' , the first superdiagonal and the diagonal of the tridiagonal matrix T are returned in rows kd and kd +1 of ab , and if uplo = 'L' , the diagonal and first subdiagonal of T are returned in the first two rows of ab .
ifail
Array, size at least max(1, n ). If jobz = 'V' , then if info = 0 , the first m elements of ifail are zero; if info > 0 , the ifail contains the indices of the eigenvectors that failed to converge. If jobz = 'N' , then ifail is not referenced.
Return Values
This function returns a value info .
If info=0 , the execution is successful.
If info = -i , the i -th parameter had an illegal value.
If info = i , then i eigenvectors failed to converge; their indices are stored in the array ifail .
LAPACK 95 Interface Notes
There exist FORTRAN 77 and FORTRAN 95 interfaces for this routine. See the Intel® oneMKL Fortran Developer Reference for details.
Application Notes
An approximate eigenvalue is accepted as converged when it is determined to lie in an interval [a,b] of width less than or equal to \(\text{abstol} + \varepsilon \max(|a|,|b|)\) , where \(\varepsilon\) is the machine precision.
If abstol is less than or equal to zero, then \(\varepsilon ||T||_{1}\) will be used in its place, where \(T\) is the tridiagonal matrix obtained by reducing \(A\) to tridiagonal form. Eigenvalues will be computed most accurately when abstol is set to twice the underflow threshold 2* ?lamch (‘S’), not zero.
If this routine returns with info > 0 , indicating that some eigenvectors did not converge, try setting abstol to 2* ?lamch (‘S’).