ABSTRACT
The relative ease of treating linear problems frequently leads one to introduce approximations that produce a linear problem. There is something of an art in the formulation of mathematical models since the model is useless if it is intractable and equally useless if it fails to describe the salient features of the physical system. One recommended procedure would be to formulate a detailed and relatively precise model into which one can subsequently introduce approximations of a mathematical nature. It is usually possible to use the approximate solution thereby obtained to assess the validity of the approximations that have been made. Thus, the contemplation of an approximate solution is an important part of analysis.
