The past cells affecting the state of a cell at time t in a 2nd-order cellular automatonElementary CA rule 18 (left) and its second-order counterpart rule 18R (right). Time runs downwards. Note the up/down asymmetric triangles in the nonreversible rule.
A second-order cellular automaton is a type of reversible cellular automaton (CA) invented by Edward Fredkin[1][2] where the state of a cell at time t depends not only on its neighborhood at time t − 1, but also on its state at time t − 2.[3]
^Margolus, N. (1984), "Physics-like models of computation", Physica D, 10: 81–95, doi:10.1016/0167-2789(84)90252-5. Reprinted in Wolfram, Stephen, ed. (1986), Theory and Applications of Cellular Automata, Advanced series on complex systems, vol. 1, World Scientific, pp. 232–246.
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