The next chapter on differential and difference equations of order greater than one requires the use of complex numbers. Only a little knowledge of complex numbers is required for this purpose but it is essential that this small amount of information be properly understood. The relevant material is given in §13.1–§13.5 inclusive.
The application of complex numbers to the solution of differential equations is just one of a vast number of their uses. Some applications in statistics, for example, are given at the end of the chapter. More important, however, than all their many uses in applied work is the light an understanding ...