# What next?

This eBook is only the very beginning of the story. We have explored the world of differential equations modeling the rate of change of some physical, biological or chemical quantities, e.g., the densities of two interacting populations, the concentration of a chemical, the angle and angular velocity of a pendulum, etc. In any case, in each model we have a finite number of variables (i.e., a finite-dimensional phase space) evolving in continuous time. Here we point out a few other directions, any of which might be the subject of one or several eBooks.