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# NAG Toolbox: nag_lapack_dsycon (f07mg)

## Purpose

nag_lapack_dsycon (f07mg) estimates the condition number of a real symmetric indefinite matrix A$A$, where A$A$ has been factorized by nag_lapack_dsytrf (f07md).

## Syntax

[rcond, info] = f07mg(uplo, a, ipiv, anorm, 'n', n)
[rcond, info] = nag_lapack_dsycon(uplo, a, ipiv, anorm, 'n', n)

## Description

nag_lapack_dsycon (f07mg) estimates the condition number (in the 1$1$-norm) of a real symmetric indefinite matrix A$A$:
 κ1(A) = ‖A‖1‖A − 1‖1 . $κ1(A)=‖A‖1‖A-1‖1 .$
Since A$A$ is symmetric, κ1(A) = κ(A) = AA1${\kappa }_{1}\left(A\right)={\kappa }_{\infty }\left(A\right)={‖A‖}_{\infty }{‖{A}^{-1}‖}_{\infty }$.
Because κ1(A)${\kappa }_{1}\left(A\right)$ is infinite if A$A$ is singular, the function actually returns an estimate of the reciprocal of κ1(A)${\kappa }_{1}\left(A\right)$.

## References

Higham N J (1988) FORTRAN codes for estimating the one-norm of a real or complex matrix, with applications to condition estimation ACM Trans. Math. Software 14 381–396

## Parameters

### Compulsory Input Parameters

1:     uplo – string (length ≥ 1)
Specifies how A$A$ has been factorized.
uplo = 'U'${\mathbf{uplo}}=\text{'U'}$
A = PUDUTPT$A=PUD{U}^{\mathrm{T}}{P}^{\mathrm{T}}$, where U$U$ is upper triangular.
uplo = 'L'${\mathbf{uplo}}=\text{'L'}$
A = PLDLTPT$A=PLD{L}^{\mathrm{T}}{P}^{\mathrm{T}}$, where L$L$ is lower triangular.
Constraint: uplo = 'U'${\mathbf{uplo}}=\text{'U'}$ or 'L'$\text{'L'}$.
2:     a(lda, : $:$) – double array
The first dimension of the array a must be at least max (1,n)$\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{n}}\right)$
The second dimension of the array must be at least max (1,n)$\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{n}}\right)$
Details of the factorization of A$A$, as returned by nag_lapack_dsytrf (f07md).
3:     ipiv( : $:$) – int64int32nag_int array
Note: the dimension of the array ipiv must be at least max (1,n)$\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{n}}\right)$.
Details of the interchanges and the block structure of D$D$, as returned by nag_lapack_dsytrf (f07md).
4:     anorm – double scalar
The 1$1$-norm of the original matrix A$A$, which may be computed by calling nag_blas_dlansy (f06rc) with its parameter norm = '1'${\mathbf{norm}}=\text{'1'}$. anorm must be computed either before calling nag_lapack_dsytrf (f07md) or else from a copy of the original matrix A$A$.
Constraint: anorm0.0${\mathbf{anorm}}\ge 0.0$.

### Optional Input Parameters

1:     n – int64int32nag_int scalar
Default: The first dimension of the array a The second dimension of the arrays a, ipiv.
n$n$, the order of the matrix A$A$.
Constraint: n0${\mathbf{n}}\ge 0$.

lda work iwork

### Output Parameters

1:     rcond – double scalar
An estimate of the reciprocal of the condition number of A$A$. rcond is set to zero if exact singularity is detected or the estimate underflows. If rcond is less than machine precision, A$A$ is singular to working precision.
2:     info – int64int32nag_int scalar
info = 0${\mathbf{info}}=0$ unless the function detects an error (see Section [Error Indicators and Warnings]).

## Error Indicators and Warnings

info = i${\mathbf{info}}=-i$
If info = i${\mathbf{info}}=-i$, parameter i$i$ had an illegal value on entry. The parameters are numbered as follows:
1: uplo, 2: n, 3: a, 4: lda, 5: ipiv, 6: anorm, 7: rcond, 8: work, 9: iwork, 10: info.
It is possible that info refers to a parameter that is omitted from the MATLAB interface. This usually indicates that an error in one of the other input parameters has caused an incorrect value to be inferred.

## Accuracy

The computed estimate rcond is never less than the true value ρ$\rho$, and in practice is nearly always less than 10ρ$10\rho$, although examples can be constructed where rcond is much larger.

## Further Comments

A call to nag_lapack_dsycon (f07mg) involves solving a number of systems of linear equations of the form Ax = b$Ax=b$; the number is usually 4$4$ or 5$5$ and never more than 11$11$. Each solution involves approximately 2n2$2{n}^{2}$ floating point operations but takes considerably longer than a call to nag_lapack_dsytrs (f07me) with one right-hand side, because extra care is taken to avoid overflow when A$A$ is approximately singular.
The complex analogues of this function are nag_lapack_zhecon (f07mu) for Hermitian matrices and nag_lapack_zsycon (f07nu) for symmetric matrices.

## Example

```function nag_lapack_dsycon_example
uplo = 'L';
a = [2.07, 0, 0, 0;
4.2, 1.15, 0, 0;
0.2230413840558341, 0.8115010321439103, -2.59067708640519, 0;
0.6536583767489105, -0.5959697237786296, 0.3030846795506181, 0.4073851981348882];
ipiv = [int64(-3);-3;3;4];
anorm = 11.29;
[rcond, info] = nag_lapack_dsycon(uplo, a, ipiv, anorm)
```
```

rcond =

0.0132

info =

0

```
```function f07mg_example
uplo = 'L';
a = [2.07, 0, 0, 0;
4.2, 1.15, 0, 0;
0.2230413840558341, 0.8115010321439103, -2.59067708640519, 0;
0.6536583767489105, -0.5959697237786296, 0.3030846795506181, 0.4073851981348882];
ipiv = [int64(-3);-3;3;4];
anorm = 11.29;
[rcond, info] = f07mg(uplo, a, ipiv, anorm)
```
```

rcond =

0.0132

info =

0

```

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