In economic theory, the following model is used to describe a possible capital investment policy. Let f(t) represent the total invested capital of a company at time t. Additional capital is invested whenever f(t) is below a certain equilibrium value E, and capital is withdrawn whenever f(t) exceeds E. The rate of investment is proportional to the difference between f(t) and E. Construct a differential equation whose solution is f(t), and sketch two or three typical solution curves.