Contents

# ?tprfs

Estimates the error in the solution of a system of linear equations with a packed triangular coefficient matrix.

## Syntax

Include Files
• mkl.h
Description
The routine estimates the errors in the solution to a system of linear equations
A*X
=
B
or
A
T
*X
=
B
or
A
H
*X
=
B
with a packed triangular matrix
A
, with multiple right-hand sides. For each computed solution vector
x
, the routine computes the component-wise backward error
β
. This error is the smallest relative perturbation in elements of
A
and
b
such that
x
is the exact solution of the perturbed system:
|
δ
a
i
j
|
β
|
a
i
j
|, |
δ
b
i
|
β
|
b
i
|
such that
(
A
+
δ
A
)
x
= (
b
+
δ
b
)
.
The routine also estimates the component-wise forward error in the computed solution
||
x
-
x
e
||
/||
x
||
(here
x
e
is the exact solution).
Before calling this routine, call the solver routine
?tptrs
.
Input Parameters
matrix_layout
Specifies whether matrix storage layout is row major (
LAPACK_ROW_MAJOR
) or column major (
LAPACK_COL_MAJOR
).
uplo
Must be
'U'
or
'L'
.
Indicates whether
A
is upper or lower triangular:
If
uplo
=
'U'
, then
A
is upper triangular.
If
uplo
=
'L'
, then
A
is lower triangular.
trans
Must be
'N'
or
'T'
or
'C'
.
Indicates the form of the equations:
If
trans
=
'N'
, the system has the form
A*X
=
B
.
If
trans
=
'T'
, the system has the form
A
T
*X
=
B
.
If
trans
=
'C'
, the system has the form
A
H
*X
=
B
.
diag
Must be
'N'
or
'U'
.
If
diag
=
'N'
,
A
is not a unit triangular matrix.
If
diag
=
'U'
,
A
is unit triangular: diagonal elements of
A
are assumed to be 1 and not referenced in the array
ap
.
n
The order of the matrix
A
;
n
0.
nrhs
The number of right-hand sides;
nrhs
0.
ap
,
b
,
x
Arrays:
ap
max(1,
n
(
n
+ 1)/2)
contains the upper or lower triangular matrix
A
, as specified by
uplo
.
b
of size max(1,
ldb
*
nrhs
) for column major layout and max(1,
ldb
*
n
) for row major layout
contains the right-hand side matrix
B
.
x
of size max(1,
ldx
*
nrhs
) for column major layout and max(1,
ldx
*
n
) for row major layout
contains the solution matrix
X
.
ldb
b
;
ldb
max(1,
n
) for column major layout and
ldb
nrhs
for row major layout
.
ldx
x
;
ldb
max(1,
n
) for column major layout and
ldb
nrhs
for row major layout
.
Output Parameters
ferr
,
berr
Arrays, size at least
max(1,
nrhs
)
. Contain the component-wise forward and backward errors, respectively, for each solution vector.
Return Values
This function returns a value
info
.
If
info
= 0
, the execution is successful.
If
info
=
-i
, parameter
i
Application Notes
The bounds returned in
ferr
are not rigorous, but in practice they almost always overestimate the actual error.
A call to this routine involves, for each right-hand side, solving a number of systems of linear equations
A
*
x
=
b
; the number of systems is usually 4 or 5 and never more than 11. Each solution requires approximately
n
2
floating-point operations for real flavors or
4
n
2
for complex flavors.

#### Product and Performance Information

1

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Notice revision #20110804