NAG Library Routine Document
F07JVF (ZPTRFS)
1 Purpose
F07JVF (ZPTRFS) computes error bounds and refines the solution to a complex system of linear equations
, where
is an
by
Hermitian positive definite tridiagonal matrix and
and
are
by
matrices, using the modified Cholesky factorization returned by
F07JRF (ZPTTRF) and an initial solution returned by
F07JSF (ZPTTRS). Iterative refinement is used to reduce the backward error as much as possible.
2 Specification
SUBROUTINE F07JVF ( |
UPLO, N, NRHS, D, E, DF, EF, B, LDB, X, LDX, FERR, BERR, WORK, RWORK, INFO) |
INTEGER |
N, NRHS, LDB, LDX, INFO |
REAL (KIND=nag_wp) |
D(*), DF(*), FERR(NRHS), BERR(NRHS), RWORK(N) |
COMPLEX (KIND=nag_wp) |
E(*), EF(*), B(LDB,*), X(LDX,*), WORK(N) |
CHARACTER(1) |
UPLO |
|
The routine may be called by its
LAPACK
name zptrfs.
3 Description
F07JVF (ZPTRFS) should normally be preceded by calls to
F07JRF (ZPTTRF) and
F07JSF (ZPTTRS).
F07JRF (ZPTTRF) computes a modified Cholesky factorization of the matrix
as
where
is a unit lower bidiagonal matrix and
is a diagonal matrix, with positive diagonal elements.
F07JSF (ZPTTRS) then utilizes the factorization to compute a solution,
, to the required equations. Letting
denote a column of
, F07JVF (ZPTRFS) computes a
component-wise backward error,
, the smallest relative perturbation in each element of
and
such that
is the exact solution of a perturbed system
The routine also estimates a bound for the component-wise forward error in the computed solution defined by , where is the corresponding column of the exact solution, .
Note that the modified Cholesky factorization of
can also be expressed as
where
is unit upper bidiagonal.
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
http://www.netlib.org/lapack/lug5 Parameters
- 1: UPLO – CHARACTER(1)Input
On entry: specifies the form of the factorization as follows:
- .
- .
Constraint:
or .
- 2: N – INTEGERInput
On entry: , the order of the matrix .
Constraint:
.
- 3: NRHS – INTEGERInput
On entry: , the number of right-hand sides, i.e., the number of columns of the matrix .
Constraint:
.
- 4: D() – REAL (KIND=nag_wp) arrayInput
-
Note: the dimension of the array
D
must be at least
.
On entry: must contain the diagonal elements of the matrix of .
- 5: E() – COMPLEX (KIND=nag_wp) arrayInput
-
Note: the dimension of the array
E
must be at least
.
On entry: if
,
E must contain the
superdiagonal elements of the matrix
.
If
,
E must contain the
subdiagonal elements of the matrix
.
- 6: DF() – REAL (KIND=nag_wp) arrayInput
-
Note: the dimension of the array
DF
must be at least
.
On entry: must contain the diagonal elements of the diagonal matrix from the factorization of .
- 7: EF() – COMPLEX (KIND=nag_wp) arrayInput
-
Note: the dimension of the array
EF
must be at least
.
On entry: if
,
EF must contain the
superdiagonal elements of the unit upper bidiagonal matrix
from the
factorization of
.
If
,
EF must contain the
subdiagonal elements of the unit lower bidiagonal matrix
from the
factorization of
.
- 8: B(LDB,) – COMPLEX (KIND=nag_wp) arrayInput
-
Note: the second dimension of the array
B
must be at least
.
On entry: the by matrix of right-hand sides .
- 9: LDB – INTEGERInput
On entry: the first dimension of the array
B as declared in the (sub)program from which F07JVF (ZPTRFS) is called.
Constraint:
.
- 10: X(LDX,) – COMPLEX (KIND=nag_wp) arrayInput/Output
-
Note: the second dimension of the array
X
must be at least
.
On entry: the by initial solution matrix .
On exit: the by refined solution matrix .
- 11: LDX – INTEGERInput
On entry: the first dimension of the array
X as declared in the (sub)program from which F07JVF (ZPTRFS) is called.
Constraint:
.
- 12: FERR(NRHS) – REAL (KIND=nag_wp) arrayOutput
On exit: estimate of the forward error bound for each computed solution vector, such that
, where
is the
th column of the computed solution returned in the array
X and
is the corresponding column of the exact solution
. The estimate is almost always a slight overestimate of the true error.
- 13: BERR(NRHS) – REAL (KIND=nag_wp) arrayOutput
On exit: estimate of the component-wise relative backward error of each computed solution vector (i.e., the smallest relative change in any element of or that makes an exact solution).
- 14: WORK(N) – COMPLEX (KIND=nag_wp) arrayWorkspace
- 15: RWORK(N) – REAL (KIND=nag_wp) arrayWorkspace
- 16: INFO – INTEGEROutput
On exit:
unless the routine detects an error (see
Section 6).
6 Error Indicators and Warnings
Errors or warnings detected by the routine:
If , the th argument had an illegal value. An explanatory message is output, and execution of the program is terminated.
7 Accuracy
The computed solution for a single right-hand side,
, satisfies an equation of the form
where
and
is the
machine precision. An approximate error bound for the computed solution is given by
where
, the condition number of
with respect to the solution of the linear equations. See Section 4.4 of
Anderson et al. (1999) for further details.
Routine
F07JUF (ZPTCON) can be used to compute the condition number of
.
The total number of floating point operations required to solve the equations is proportional to . At most five steps of iterative refinement are performed, but usually only one or two steps are required.
The real analogue of this routine is
F07JHF (DPTRFS).
9 Example
This example solves the equations
where
is the Hermitian positive definite tridiagonal matrix
and
Estimates for the backward errors and forward errors are also output.
9.1 Program Text
Program Text (f07jvfe.f90)
9.2 Program Data
Program Data (f07jvfe.d)
9.3 Program Results
Program Results (f07jvfe.r)