A * differential equation* is an equation involving an unknown function
and one or more of its derivatives. The equation is an * ordinary
differential equation* (ODE) if the unknown function depends on only
one independent variable. Some examples of ODEs follow:

In (1--5)
**t** is the independent variable;
the dependent variables are , and **p**, respectively.

In what follows we will frequently use the notation to represent , to represent , to represent , and, in general, to represent .