ABSTRACT
In $7.2 the problem of wave diffraction by a gap in a breakwater, already investigated in 54.7 and 55.1.2, is examined in some detail and used to illuminate some of the main ideas of the method of matched asymptotic expansions. The problem is first solved using an informal technique that does not rely on the application of a formal matching principle. This informal method works well in the gap problem, but is known to give incorrect results in some problems. To avoid this possibility it is advisable to use a formal matching principle and this is described here and then used to solve the gap problem.
