Page 285: Structures in Class 4 Systems
Persistent structures with repetition periods
All the persistent structures with repetition periods up to 15 steps in the code 20 cellular automaton. The structures shown were found by a systematic method similar to the one used to find all sequences that satisfy the constraints on page 268.

So what about other class 4 cellular automata—like the ones I showed at the beginning of this section? Do they also end up having complicated sets of possible persistent structures?

From Stephen Wolfram: A New Kind of Science [citation]