# 9.8. Exercises¶

Exercise 1: In Section 9.7 we showed that the Rule 18 CA produces a fractal. Can you find other 1-D CAs that produce fractals?

Note: the Cell1D object does not wrap around from the left edge to the right, which creates artifacts at the boundaries for some rules. You might want to use Wrap1D, which is a child class of Cell1D that wraps around. It is defined in Cell1D.py in the repository for this book.

Exercise 2: In 1990 Bak, Chen and Tang proposed a cellular automaton that is an abstract model of a forest fire. Each cell is in one of three states: empty, occupied by a tree, or on fire.

The rules of the CA are:

1. An empty cell becomes occupied with probability p.

2. A cell with a tree burns if any of its neighbors is on fire.

3. A cell with a tree spontaneously burns, with probability f, even if none of its neighbors is on fire.

4. A cell with a burning tree becomes an empty cell in the next time step.

Write a program that implements this model. You might want to inherit from Cell2D. Typical values for the parameters are p=0.01 and f=0.001, but you might want to experiment with other values.

Starting from a random initial condition, run the model until it reaches a steady state where the number of trees no longer increases or decreases consistently.

In steady state, is the geometry of the forest fractal? What is its fractal dimension?