dstech.f - Man Page
TESTING/EIG/dstech.f
Synopsis
Functions/Subroutines
subroutine dstech (n, a, b, eig, tol, work, info)
DSTECH
Function/Subroutine Documentation
subroutine dstech (integer n, double precision, dimension( * ) a, double precision, dimension( * ) b, double precision, dimension( * ) eig, double precision tol, double precision, dimension( * ) work, integer info)
DSTECH
Purpose:
Let T be the tridiagonal matrix with diagonal entries A(1) ,..., A(N) and offdiagonal entries B(1) ,..., B(N-1)). DSTECH checks to see if EIG(1) ,..., EIG(N) are indeed accurate eigenvalues of T. It does this by expanding each EIG(I) into an interval [SVD(I) - EPS, SVD(I) + EPS], merging overlapping intervals if any, and using Sturm sequences to count and verify whether each resulting interval has the correct number of eigenvalues (using DSTECT). Here EPS = TOL*MAZHEPS*MAXEIG, where MACHEPS is the machine precision and MAXEIG is the absolute value of the largest eigenvalue. If each interval contains the correct number of eigenvalues, INFO = 0 is returned, otherwise INFO is the index of the first eigenvalue in the first bad interval.
- Parameters
N
N is INTEGER The dimension of the tridiagonal matrix T.
A
A is DOUBLE PRECISION array, dimension (N) The diagonal entries of the tridiagonal matrix T.
B
B is DOUBLE PRECISION array, dimension (N-1) The offdiagonal entries of the tridiagonal matrix T.
EIG
EIG is DOUBLE PRECISION array, dimension (N) The purported eigenvalues to be checked.
TOL
TOL is DOUBLE PRECISION Error tolerance for checking, a multiple of the machine precision.
WORK
WORK is DOUBLE PRECISION array, dimension (N)
INFO
INFO is INTEGER 0 if the eigenvalues are all correct (to within 1 +- TOL*MAZHEPS*MAXEIG) >0 if the interval containing the INFO-th eigenvalue contains the incorrect number of eigenvalues.
- Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Definition at line 100 of file dstech.f.
Author
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Referenced By
The man page dstech(3) is an alias of dstech.f(3).
Tue Nov 28 2023 12:08:42 Version 3.12.0 LAPACK