 exponential stability can be inferred if
      we can find a negatively-sloped line passing through the equilibrium point
      which separates the graph of $f(x)$ from the horizontal axis. The existence of this separating line
      implies that the nonlinear system will converge at least as fast as the
      linear system represented by the straight line.  I will graphically
      illustrate unstable fixed points with open circles and stable fixed points
      (i.s.L.) with filled circles. </p>

      <p