  We present a method for computing probability of occurence of 1s in a
configuration obtained by iteration of a probabilistic cellular automata (PCA),
starting from a random initial configuration. If the PCA is sufficiently
simple, one can construct a set of words (or blocks of symbols) which is
complete, meaning that probabilities of occurence of words from this set can be
expressed as linear combinations of probabilities of occurence of these words
at the previous time step. One can then setup and solve a recursion for block
probabilities. We demonstrate an example of such PCA, which can be viewed as a
simple model of diffusion of information or spread of rumors. Expressions for
the density of ones are obtained for this rule using the proposed method.
