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Chapter Contents
Chapter Introduction
NAG Toolbox

# NAG Toolbox: nag_lapack_zpptri (f07gw)

## Purpose

nag_lapack_zpptri (f07gw) computes the inverse of a complex Hermitian positive definite matrix $A$, where $A$ has been factorized by nag_lapack_zpptrf (f07gr), using packed storage.

## Syntax

[ap, info] = f07gw(uplo, n, ap)
[ap, info] = nag_lapack_zpptri(uplo, n, ap)

## Description

nag_lapack_zpptri (f07gw) is used to compute the inverse of a complex Hermitian positive definite matrix $A$, the function must be preceded by a call to nag_lapack_zpptrf (f07gr), which computes the Cholesky factorization of $A$, using packed storage.
If ${\mathbf{uplo}}=\text{'U'}$, $A={U}^{\mathrm{H}}U$ and ${A}^{-1}$ is computed by first inverting $U$ and then forming $\left({U}^{-1}\right){U}^{-\mathrm{H}}$.
If ${\mathbf{uplo}}=\text{'L'}$, $A=L{L}^{\mathrm{H}}$ and ${A}^{-1}$ is computed by first inverting $L$ and then forming ${L}^{-\mathrm{H}}\left({L}^{-1}\right)$.

## References

Du Croz J J and Higham N J (1992) Stability of methods for matrix inversion IMA J. Numer. Anal. 12 1–19

## Parameters

### Compulsory Input Parameters

1:     $\mathrm{uplo}$ – string (length ≥ 1)
Specifies how $A$ has been factorized.
${\mathbf{uplo}}=\text{'U'}$
$A={U}^{\mathrm{H}}U$, where $U$ is upper triangular.
${\mathbf{uplo}}=\text{'L'}$
$A=L{L}^{\mathrm{H}}$, where $L$ is lower triangular.
Constraint: ${\mathbf{uplo}}=\text{'U'}$ or $\text{'L'}$.
2:     $\mathrm{n}$int64int32nag_int scalar
$n$, the order of the matrix $A$.
Constraint: ${\mathbf{n}}\ge 0$.
3:     $\mathrm{ap}\left(:\right)$ – complex array
The dimension of the array ap must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{n}}×\left({\mathbf{n}}+1\right)/2\right)$
The Cholesky factor of $A$ stored in packed form, as returned by nag_lapack_zpptrf (f07gr).

None.

### Output Parameters

1:     $\mathrm{ap}\left(:\right)$ – complex array
The dimension of the array ap will be $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{n}}×\left({\mathbf{n}}+1\right)/2\right)$
The factorization stores the $n$ by $n$ matrix ${A}^{-1}$.
More precisely,
• if ${\mathbf{uplo}}=\text{'U'}$, the upper triangle of ${A}^{-1}$ must be stored with element ${A}_{ij}$ in ${\mathbf{ap}}\left(i+j\left(j-1\right)/2\right)$ for $i\le j$;
• if ${\mathbf{uplo}}=\text{'L'}$, the lower triangle of ${A}^{-1}$ must be stored with element ${A}_{ij}$ in ${\mathbf{ap}}\left(i+\left(2n-j\right)\left(j-1\right)/2\right)$ for $i\ge j$.
2:     $\mathrm{info}$int64int32nag_int scalar
${\mathbf{info}}=0$ unless the function detects an error (see Error Indicators and Warnings).

## Error Indicators and Warnings

Cases prefixed with W are classified as warnings and do not generate an error of type NAG:error_n. See nag_issue_warnings.

${\mathbf{info}}<0$
If ${\mathbf{info}}=-i$, argument $i$ had an illegal value. An explanatory message is output, and execution of the program is terminated.
W  ${\mathbf{info}}>0$
Diagonal element $_$ of the Cholesky factor is zero; the Cholesky factor is singular and the inverse of $A$ cannot be computed.

## Accuracy

The computed inverse $X$ satisfies
 $XA-I2≤cnεκ2A and AX-I2≤cnεκ2A ,$
where $c\left(n\right)$ is a modest function of $n$, $\epsilon$ is the machine precision and ${\kappa }_{2}\left(A\right)$ is the condition number of $A$ defined by
 $κ2A=A2A-12 .$

The total number of real floating-point operations is approximately $\frac{8}{3}{n}^{3}$.
The real analogue of this function is nag_lapack_dpptri (f07gj).

## Example

This example computes the inverse of the matrix $A$, where
 $A= 3.23+0.00i 1.51-1.92i 1.90+0.84i 0.42+2.50i 1.51+1.92i 3.58+0.00i -0.23+1.11i -1.18+1.37i 1.90-0.84i -0.23-1.11i 4.09+0.00i 2.33-0.14i 0.42-2.50i -1.18-1.37i 2.33+0.14i 4.29+0.00i .$
Here $A$ is Hermitian positive definite, stored in packed form, and must first be factorized by nag_lapack_zpptrf (f07gr).
```function f07gw_example

fprintf('f07gw example results\n\n');

% Symmetric A in lower packed form
uplo = 'L';
n = int64(4);
ap = [3.23 + 0i    1.51 + 1.92i    1.90 - 0.84i    0.42 - 2.50i ...
3.58 + 0i      -0.23 - 1.11i   -1.18 - 1.37i ...
4.09 + 0.00i    2.33 + 0.14i ...
4.29 + 0.00i];

[L, info] = f07gr( ...
uplo, n, ap);

% Invert
[ainv, info] = f07gw( ...
uplo, n, L);

[ifail] = x04dc( ...
uplo, 'Non-unit', n, ainv, 'Inverse');

```
```f07gw example results

Inverse
1          2          3          4
1      5.4691
0.0000

2     -1.2624     1.1024
-1.5491     0.0000

3     -2.9746     0.8989     2.1589
-0.9616    -0.5672     0.0000

4      1.1962    -0.9826    -1.3756     2.2934
2.9772    -0.2566    -1.4550     0.0000
```