This article offers a new method of solution for linear difference equations with Rational Expectations. We provide a description of the complete set of solutions which is shown to depend on arbitrary martingales. We thus avoid the use of "differences of martingales" as introduced by Gourieroux, Broze, Szafarz, while describing the general solution as a dynamic equation with lower order. At the same time we provide a simple algorithm, based on polynomial divisions, to calculate some basic solutions, namely all ARMA solutions.

The article develops new approximations to the distribution function of a general econometric estimator or test statistic from a linear model. The method relies heavily on the general arguments for the validity of Edgeworth approximations, and produces formulae of a similar type, except that, instead of using the cumulative normal distribution, an Imhof distribution is used derived from the exact distribution of a linear combination of the sample data second moments. On theoretical grounds this might be expected to give better approximations for some estimators. In practice comparisons were made between computed approximations of both types for several simple cases.