# Error Analysis of Numerical Methods

In order to identify the essential properties of numerical methods, we define basic notions 1.

For the given ODE system

$\frac{dy}{dt}=f(t,y),\quad y(t_{0})=y_{0},$

we define $$y(t_n)$$ as the solution of IVP evaluated at $$t=t_n$$, and $$y_n$$ is a numerical approximation of $$y(t_n)$$ at the same location by a generic explicit numerical scheme (no matter explicit, implicit or multi-step scheme):

\begin{align} y_{n+1} = y_n + h \phi(t_n,y_n,h), \tag{2} \end{align}

where $$h$$ is the discretization step for $$t$$, i.e., $$h=t_{n+1}-t_n$$, and $$\phi(t_n,y_n,h)$$ is the increment function. We say that the defined numerical scheme is consistent if $$\lim_{h\to0} \phi(t,y,h) = \phi(t,y,0) = f(t,y)$$.

Then, the approximation error is defined as

$e_n = y(t_n) - y_n.$

The absolute error is defined as

$|e_n| = |y(t_n) - y_n|.$

The relative error is defined as

$r_n =\frac{|y(t_n) - y_n|}{|y(t_n)|}.$

The exact differential operator is defined as

\begin{align} L_e(y) = y' - f(t,y) = 0 \end{align}

The approximate differential operator is defined as

\begin{align} L_a(y_n) = y(t_{n+1}) - [y_n + \phi(t_n,y_n,h)]. \end{align}

Finally, the local truncation error (LTE) is defined as

\begin{align} \tau_n = \frac{1}{h} L_a(y(x_n)). \end{align}

In practice, the evaluation of the exact solution for different $$t$$ around $$t_n$$ (required by $$L_a$$) is performed using a Taylor series expansion.

Finally, we can state that a scheme is $$p$$-th order accurate by examining its LTE and observing its leading term

\begin{align} \tau_n = C h^p + H.O.T., \end{align}

where $$C$$ is a constant, independent of $$h$$, and $$H.O.T.$$ are the higher order terms of the LTE.

Example: LTE for Euler’s scheme

Consider the IVP defined by $$y' = \lambda y$$, with initial condition $$y(0)=1$$.

The approximation operator for Euler’s scheme is

\begin{align} L^{euler}_a = y(t_{n+1}) - [y_n + h \lambda y_n], \end{align}

then the LTE can be computed by

\begin{split}\begin{align} \tau_n = & \frac{1}{h}\left\{ L_a(y(t_n))\right\} = \frac{1}{h}\left\{ y(t_{n+1}) - [y(t_n) + h \lambda y(t_n)]\right\}, \\ = & \frac{1}{h}\left\{ y(t_n) + h y'(t_n) + \frac{h^2}{2} y''(t_n) + \ldots + \frac{1}{p!} h^p y^{(p)}(t_n) - y(t_n) - h \lambda y(t_n) \right\} \\ = & \frac{1}{2} h y''(t_n) + \ldots + \frac{1}{p!} h^{p-1} y^{(p)}(t_n) \\ \approx & \frac{1}{2} h y''(t_n), \end{align}\end{split}

where we assume $$y_n = y(t_n)$$.

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