CPTEQR (3)  Linux Man Pages
NAME
cpteqr.f 
SYNOPSIS
Functions/Subroutines
subroutine cpteqr (COMPZ, N, D, E, Z, LDZ, WORK, INFO)
CPTEQR
Function/Subroutine Documentation
subroutine cpteqr (characterCOMPZ, integerN, real, dimension( * )D, real, dimension( * )E, complex, dimension( ldz, * )Z, integerLDZ, real, dimension( * )WORK, integerINFO)
CPTEQR
Purpose:

CPTEQR computes all eigenvalues and, optionally, eigenvectors of a symmetric positive definite tridiagonal matrix by first factoring the matrix using SPTTRF and then calling CBDSQR to compute the singular values of the bidiagonal factor. This routine computes the eigenvalues of the positive definite tridiagonal matrix to high relative accuracy. This means that if the eigenvalues range over many orders of magnitude in size, then the small eigenvalues and corresponding eigenvectors will be computed more accurately than, for example, with the standard QR method. The eigenvectors of a full or band positive definite Hermitian matrix can also be found if CHETRD, CHPTRD, or CHBTRD has been used to reduce this matrix to tridiagonal form. (The reduction to tridiagonal form, however, may preclude the possibility of obtaining high relative accuracy in the small eigenvalues of the original matrix, if these eigenvalues range over many orders of magnitude.)
Parameters:

COMPZ
COMPZ is CHARACTER*1 = 'N': Compute eigenvalues only. = 'V': Compute eigenvectors of original Hermitian matrix also. Array Z contains the unitary matrix used to reduce the original matrix to tridiagonal form. = 'I': Compute eigenvectors of tridiagonal matrix also.
NN is INTEGER The order of the matrix. N >= 0.
DD is REAL array, dimension (N) On entry, the n diagonal elements of the tridiagonal matrix. On normal exit, D contains the eigenvalues, in descending order.
EE is REAL array, dimension (N1) On entry, the (n1) subdiagonal elements of the tridiagonal matrix. On exit, E has been destroyed.
ZZ is COMPLEX array, dimension (LDZ, N) On entry, if COMPZ = 'V', the unitary matrix used in the reduction to tridiagonal form. On exit, if COMPZ = 'V', the orthonormal eigenvectors of the original Hermitian matrix; if COMPZ = 'I', the orthonormal eigenvectors of the tridiagonal matrix. If INFO > 0 on exit, Z contains the eigenvectors associated with only the stored eigenvalues. If COMPZ = 'N', then Z is not referenced.
LDZLDZ is INTEGER The leading dimension of the array Z. LDZ >= 1, and if COMPZ = 'V' or 'I', LDZ >= max(1,N).
WORKWORK is REAL array, dimension (4*N)
INFOINFO is INTEGER = 0: successful exit. < 0: if INFO = i, the ith argument had an illegal value. > 0: if INFO = i, and i is: <= N the Cholesky factorization of the matrix could not be performed because the ith principal minor was not positive definite. > N the SVD algorithm failed to converge; if INFO = N+i, i offdiagonal elements of the bidiagonal factor did not converge to zero.
Author:

Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
 September 2012
Definition at line 146 of file cpteqr.f.
Author
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