One Dimensional $n$ary Density Classification Using Two Cellular   Automaton Rules
  Suppose each site on a one-dimensional chain with periodic boundary condition may take on any one of the states $0,1,..., n-1$, can you find out the most frequently occurring state using cellular automaton? Here, we prove that while the above density classification task cannot be resolved by a single cellular automaton, this task can be performed efficiently by applying two cellular automaton rules in succession. 
H. F. Chau, L. W. Siu and K. K. Yan