If all solutions of (28.2.1) are bounded when along the real axis, then the corresponding pair of parameters is called stable. All other pairs are unstable.
For example, positive real values of with comprise stable pairs, as do values of and that correspond to real, but noninteger, values of .
However, if , then always comprises an unstable pair. For example, as one of the solutions and tends to and the other is unbounded (compare Figure 28.13.5). Also, all nontrivial solutions of (28.2.1) are unbounded on .