  The control of chaotic systems implies inducing an unpredictable system to
follow a desired trajectory using the smallest "force". In low-dimensional
continuous systems, one method is that of reconstructing the tangent space, so
that the control may be concentrated on the most expanding directions. This
scheme is hard to follow for high-dimensional (extended) systems. This is
particularly true for extended systems that exhibit stable chaos, that is,
systems which are not chaotic in the usual sense, but are unpredictable for
finite perturbations. Prototypical of this class are cellular automata, aka
completely discrete dynamical systems. Although usual indicators of chaoticity
such as the maximum Lyapunov exponent may be defined for such systems, we show
that the usual approach may lead to counter intuitive results, and that it is
possible to exploit the characteristics of the system in order to reduce the
distance between two replicas with less control.
