  The pair approximation is a simple, low-order method to incorporate effects
of local spatial structure in epidemiological models. However, since for K
state variables in a model there are K(K+1)/2 equations in the pair
approximation, generating these equations, although straightforward, can become
tedious. In this paper we describe two programs written in Perl to simplify
this process - one to construct the equations, and the other to generate
Matlab/Octave functions to numerically integrate the equations using a
positivity-preserving method. A third utility program is also included which
generates a Maple file that can be used within the Maple symbolic manipulation
program to simplify algebraically some of the terms in the generated file
describing the equations, as well as to check that the usual combination of
equations sum up to zero, which is expected in cases where the total population
sizes are conserved.
