Owing to the complexity of the PDEs involved in some mathematical mod­ els, it is not always possible to find an exact solution to an initial/boundary value problem. The next best thing in such situations is to compute an ap­ proximate solution instead. This is the idea behind the method of asymp­ totic expansion, which is applicable to problems th a t depend on a small positive param eter and relies on the expansion of the solution in a series of powers of the parameter. If the series converges, then the technique is called a perturbation method; when the series diverges but is asymptotic (in a sense th a t will be explained below), we have an asymptotic method.