Stephen Wolfram
"Firing Squad" synchronization. By choosing appropriate rules it is possible to achieve many forms of synchronization directly within cellular automata. One version posed as a problem by John Myhill in 1957 consists in setting up a rule in which all cells in a region go into a special state after exactly the same number of steps. The problem was first solved in the early 1960s; the solution using six colors and a minimal number of steps was found in 1988 by Jacques Mazoyer.

[From A New Kind of Science, page 1035.]

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