  A procedure for obtaining a "minimal" discretization of a partial
differential equation, preserving all of its Lie point symmetries is presented.
"Minimal" in this case means that the differential equation is replaced by a
partial difference scheme involving N difference equations, where N is the
number of independent and dependent variable. We restrict to one scalar
function of two independent variables. As examples, invariant discretizations
of the heat, Burgers and Korteweg-de Vries equations are presented. Some exact
solutions of the discrete schemes are obtained.
