# 9 Nonlinear Systems and Phenomena

# 9.1 Stability and the Phase Plane

A wide variety of natural phenomena are modeled by two-dimensional first-order systems of the form

in which the independent variable `t` does not appear explicitly. We usually think of the dependent variables `x` and `y` as position variables in the `xy`-plane and of `t` as a time variable. We will see that the absence of `t` on the right-hand sides in (1) makes the system easier to analyze and its solutions easier to visualize. Using the terminology of Section 2.2, such a system of differential equations in which the derivative values are independent (or “autonomous”) ...

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