Contents

# ?spcon

Estimates the reciprocal of the condition number of a packed symmetric matrix.

## Syntax

Include Files
• mkl.h
Description
The routine estimates the reciprocal of the condition number of a packed symmetric matrix
A
:
κ
1
(
A
) = ||
A
||
1
||
A
-1
||
1
(since
A
is symmetric,
κ
(
A
) =
κ
1
(
A
)
).
An estimate is obtained for
||
A
-1
||
, and the reciprocal of the condition number is computed as
rcond
= 1 / (||
A
|| ||
A
-1
||)
.
Before calling this routine:
• compute
anorm
(either ||
A
||
1
= max
j
Σ
i
|
a
i
j
| or ||
A
||
= max
i
Σ
j
|
a
i
j
|)
• call
?sptrf
to compute the factorization of
A
.
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 how the input matrix
A
has been factored:
If
uplo
=
'U'
, the array
ap
stores the packed upper triangular factor
U
of the factorization
A
=
U*D*U
T
.
If
uplo
=
'L'
, the array
ap
stores the packed lower triangular factor
L
of the factorization
A
=
L*D*L
T
.
n
The order of matrix
A
;
n
0
.
ap
The array
ap
contains the packed factored matrix
A
, as returned by
?sptrf
. The dimension of
ap
must be at least max(1,
n
(
n
+1)/2).
ipiv
Array, size at least
max(1,
n
)
.
The array
ipiv
, as returned by
?sptrf
.
anorm
The norm of the
original
matrix
A
(see
Description
)
.
Output Parameters
rcond
An estimate of the reciprocal of the condition number. The routine sets
rcond
= 0 if the estimate underflows; in this case the matrix is singular (to working precision). However, anytime
rcond
is small compared to 1.0, for the working precision, the matrix may be poorly conditioned or even singular.
Return Values
This function returns a value
info
.
If
info
= 0
, the execution is successful.
If
info
=
-i
, parameter
i
Application Notes
The computed
rcond
is never less than
r
(the reciprocal of the true condition number) and in practice is nearly always less than 10
r
. A call to this routine involves solving a number of systems of linear equations
A
*
x
=
b
; the number is usually 4 or 5 and never more than 11. Each solution requires approximately
2
n
2
floating-point operations for real flavors and
8
n
2
for complex flavors.

#### Product and Performance Information

1

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Notice revision #20110804