  An equation for the evolution of the distribution of wealth in a population
of economic agents making binary transactions with a constant total amount of
"money" has recently been proposed by one of us (RLR). This equation takes the
form of an iterated nonlinear map of the distribution of wealth. The
equilibrium distribution is known and takes a rather simple form. If this
distribution is such that, at some time, the higher momenta of the distribution
exist, one can find exactly their law of evolution. A seemingly simple
extension of the laws of exchange yields also explicit iteration formulae for
the higher momenta, but with a major difference with the original iteration
because high order momenta grow indefinitely. This provides a quantitative
model where the spreading of wealth, namely the difference between the rich and
the poor, tends to increase with time.
