One specifies the input to the computation by setting up an appropriate number of initial black cells. And then one determines the result of the computation by looking at how many black cells survive in the end.
Testing whether a number is even or odd is by most measures a rather simple computation. But one can also get cellular automata to do more complicated computations. And as an example the pictures below show a cellular automaton that computes the square of any number. If one starts say with 5 black squares, then after a certain number of steps the cellular automaton will produce a block of exactly 5 × 5 = 25 black squares.
At first it might seem surprising that a system with the simple underlying structure of a cellular automaton could ever be made to perform such a computation. But as we shall see later in this chapter, cellular automata can in fact perform what are in effect arbitrarily sophisticated computations. And as one example of a somewhat more sophisticated computation, the picture on the next page shows a cellular automaton that computes the successive prime numbers: 2, 3, 5, 7, 11, 13, 17, etc.
A cellular automaton that computes the square of any number. The cellular automaton effectively works by adding the original number n together n times. The underlying rule used here involves eight possible colors for each cell.