Further, we will discuss minutely this system which is a mathematical model of
the widely-known kinetic scheme bruesselator. Now we will compare the results of
its numerical solution using the functions rkfixed and Rkadapt.
The corresponding plots are shown in Fig. 3.9. They show that using a fixed step
can lead to an instable solution which can be interpreted wrongly from a physical
point of view (dashed line). The function Rkadapt, as we can see, allows us to
eliminate the mistakes of rkfixed, and reveals the true behavior of the desired
function in the given independent variable range (due to an adaptable step of
integration). In practice, Rkadapt is preferable in the solving of many direct
problems, especially in cases when the starting kinetic model is non-linear.
We also want to mention that the function Rkadapt requires the same five
arguments specified in an rkfixed body. Even though integration utilizes a
changeable step, the result will still be represented for evenly distributed points
as specified by user.
There is one more circumstance related to a variety of built-in integrators. It is
the existence of so-called stiff ODE sets. The concept of stiffness may be illustrated
by the example of the kinetic equation for a multi-step reaction: