## Fredkin's model

Edward Fredkin discovered that the parity, or

exclusive or, operation could lead to an interesting replication of patterns in cellular automata [17,18,19]; he also devised a model for reversible automata [20]. Reputedly differential equations were his model; first order equations typically have real exponential solutions which are not time reversible, whereas second order equations often have solutions composed of readily reversible pairs of complex conjugates.

Harold V. McIntosh

E-mail:mcintosh@servidor.unam.mx