# Ngô Quốc Anh

## June 7, 2009

### Perturbation in linear systems

In this small topic, I will show several traditional results regarding to the perturbation in matrix algebra. This comes from the study of the stability of solving system of linear equations in Numerical Analysis.

Let us consider the following matrix-form linear system $Ax=b$. We split into three cases

• Perturbation on $b$

If $\delta b$ is a small perturbation on $b$, then $\delta x$ is a small perturbation on $x$ in the sense that the following system $A(x + \delta x) = b + \delta b$ holds true. This together with the fact that $Ax =b$ gives $A \delta x = \delta b$. By some simple calculation, one has

$\displaystyle\begin{gathered}\frac{{\left\| {\delta x} \right\|}}{{\left\| x \right\|}} = \frac{{\left\| {{A^{ - 1}}\left( {\delta b} \right)} \right\|}}{{\left\| x \right\|}}\leq\frac{{\left\| {{A^{ - 1}}} \right\|\left\| {\delta b} \right\|}}{{\left\| x \right\|}} \hfill \\ \qquad= \frac{{\left\| {{A^{ - 1}}} \right\|\left\| A \right\|\left\| {\delta b} \right\|}}{{\left\| A \right\|\left\| x \right\|}}\leq\frac{{\left\| {{A^{ - 1}}} \right\|\left\| A \right\|\left\| {\delta b} \right\|}}{{\left\| {Ax} \right\|}} \hfill \\ \qquad= \frac{{\left\| {{A^{ - 1}}} \right\|\left\| A \right\|\left\| {\delta b} \right\|}}{{\left\| b \right\|}}. \hfill \\ \end{gathered}$

Thus

$\displaystyle\frac{{\left\| {\delta x} \right\|}}{{\left\| x \right\|}}\leq\left\| {{A^{ - 1}}} \right\|\left\| A \right\|\frac{{\left\| {\delta b} \right\|}}{{\left\| b \right\|}}.$

• Perturbation on $A$.

If $\delta A$ is a small perturbation on $A$, then $\delta x$ is a small perturbation on $x$ in the sense that the following system $(A + \delta A)(x+\delta x) = b$ holds true. This together with the fact that $Ax =b$ gives $\left( {\delta A} \right)x + A\left( {\delta x} \right) + \left( {\delta A} \right)\left( {\delta x} \right) = 0$. By some simple calculation, one has

$\displaystyle\begin{gathered}\left\| {\delta x} \right\| = \left\| {{A^{ - 1}}\left( {\left( {\delta A} \right)\left( x \right) + \left( {\delta A} \right)\left( {\delta x} \right)} \right)} \right\| \hfill \\ \qquad\leq\left\| {{A^{ - 1}}} \right\|\left\| {\left( {\delta A} \right)\left( x \right) + \left( {\delta A} \right)\left( {\delta x} \right)} \right\| \hfill \\ \qquad\leq \left\| {{A^{ - 1}}} \right\|\left( {\left\| {\delta A} \right\|\left\| x \right\| + \left\| {\delta A} \right\|\left\| {\delta x} \right\|} \right). \hfill \\ \end{gathered}$

Thus

$\displaystyle\left\| {\delta x} \right\|\left( {1 - \left\| {{A^{ - 1}}} \right\|\left\| {\delta A} \right\|} \right) \leqslant \left\| {{A^{ - 1}}} \right\|\left\| {\delta A} \right\|\left\| x \right\|$

which implies that

$\displaystyle\frac{{\left\| {\delta x} \right\|}}{{\left\| x \right\|}} \leqslant \frac{{\left\| {{A^{ - 1}}} \right\|\left\| {\delta A} \right\|}}{{1 - \left\| {{A^{ - 1}}} \right\|\left\| {\delta A} \right\|}} = \frac{{\left\| A \right\|\left\| {{A^{ - 1}}} \right\|\frac{{\left\| {\delta A} \right\|}}{{\left\| A \right\|}}}}{{1 - \left\| A \right\|\left\| {{A^{ - 1}}} \right\|\frac{{\left\| {\delta A} \right\|}}{{\left\| A \right\|}}}}.$

• Perturbation on $A$ and $b$.

If $\delta A$ is a small perturbation on $A$ and $\delta b$ is a small perturbation on $b$, then $\delta x$ is a small perturbation on $x$ in the sense that the following system $(A + \delta A)(x+\delta x) = b + \delta b$ holds true. This together with the fact that $Ax =b$ gives $A\left( {\delta x} \right) + \left( {\delta A} \right)x + \left( {\delta A} \right)\left( {\delta x} \right) = \delta b$. By some simple calculation, one has

$\displaystyle\begin{gathered}\left\| {\delta x} \right\| = \left\| {{A^{ - 1}}\left( {\delta b - \left( {\delta A} \right)\left( x \right) - \left( {\delta A} \right)\left( {\delta x} \right)} \right)} \right\| \hfill \\ \qquad\leqslant \left\| {{A^{ - 1}}} \right\|\left\| {\delta b - \left( {\delta A} \right)\left( x \right) - \left( {\delta A} \right)\left( {\delta x} \right)} \right\| \hfill \\ \qquad\leqslant \left\| {{A^{ - 1}}} \right\|\left( {\left\| {\delta b} \right\| + \left\| {\delta A} \right\|\left\| x \right\| + \left\| {\delta A} \right\|\left\| {\delta x} \right\|} \right) \hfill \\ \end{gathered}$

which implies that

$\displaystyle\left\|{\delta x}\right\|\left({1-\left\|{{A^{-1}}}\right\|\left\|{\delta A}\right\|}\right)\leqslant\left\|{{A^{-1}}}\right\|\left({\left\|{\delta b}\right\|+\left\|{\delta A}\right\|\left\| x\right\|}\right).$

Thus,

$\displaystyle\begin{gathered}\frac{{\left\| {\delta x} \right\|}}{{\left\| x \right\|}} \leqslant \frac{{\left\| {{A^{ - 1}}} \right\|\left( {\frac{{\left\| {\delta b} \right\|}}{{\left\| x \right\|}} + \left\| {\delta A} \right\|} \right)}}{{1 - \left\| {{A^{ - 1}}} \right\|\left\| {\delta A} \right\|}} \hfill \\ \qquad= \frac{{\left\| A \right\|\left\| {{A^{ - 1}}} \right\|\left( {\frac{{\left\| {\delta b} \right\|}}{{\left\| A \right\|\left\| x \right\|}} + \frac{{\left\| {\delta A} \right\|}}{{\left\| A \right\|}}} \right)}}{{1 - \left\| A \right\|\left\| {{A^{ - 1}}} \right\|\frac{{\left\| {\delta A} \right\|}}{{\left\| A \right\|}}}} \hfill \\ \qquad\leqslant \frac{{\left\| A \right\|\left\| {{A^{ - 1}}} \right\|\left( {\frac{{\left\| {\delta b} \right\|}}{{\left\| {Ax} \right\|}} + \frac{{\left\| {\delta A} \right\|}}{{\left\| A \right\|}}} \right)}}{{1 - \left\| A \right\|\left\| {{A^{ - 1}}} \right\|\frac{{\left\| {\delta A} \right\|}}{{\left\| A \right\|}}}} \hfill \\ \qquad\leqslant \frac{{\left\| A \right\|\left\| {{A^{ - 1}}} \right\|\left( {\frac{{\left\| {\delta b} \right\|}}{{\left\| b \right\|}} + \frac{{\left\| {\delta A} \right\|}}{{\left\| A \right\|}}} \right)}}{{1 - \left\| A \right\|\left\| {{A^{ - 1}}} \right\|\frac{{\left\| {\delta A} \right\|}}{{\left\| A \right\|}}}}. \hfill \\ \end{gathered}$

In the literature, the expression $\|A^{-1}\|\|A\|$ is called the condition number of the matrix $A$, denoted by $K(A)$. It is easy to see that $K(A) \geq 1$. If $K(A)$ is small then the system $Ax=b$ is called to be well-posed. Alternatively, in the case when $1 \ll K(A)$, the system $Ax=b$ is ill-posed.