  In this communication, some economic models given by functional mappings are
addressed. These are models for random markets where agents trade by pairs and
exchange their money in a random and conservative way. They display the
exponential wealth distribution as asymptotic equilibrium, independently of the
effectiveness of the transactions and of the limitation of the total wealth.
The entropy increases with time in these models and the existence of an
H-theorem is computationally checked. Also, it is shown that any small
perturbation of the models equations make them to lose the exponential
distribution as an equilibrium solution.
