Let us return to Fig. 44. Recall, that this figure shows the onset of
chaos, via a cascade of period-doubling bifurcations, as the quality-factor
is gradually increased. Figure 53 is essentially a continuation of Fig. 44
which shows the full extent of the chaotic region (in - space). It can be
seen that the chaotic region ends abruptly when exceeds a critical
value, which is about . Beyond this critical value, the time-asymptotic motion appears to
revert to period-1 motion (*i.e.*, the solid black region collapses to a single curve).
It can also be seen that the chaotic region contains many narrow windows in which
chaos reverts to periodic motion (*i.e.*, the solid black region collapses
to curves, where is the period of the motion) for a short interval in . The four widest
windows are indicated on the figure.

Figure 54 is a blow-up of the period-3 window shown in Fig. 53. It can be seen that the window appears ``out of the blue'' as is gradually increased. However, it can also be seen that, as is further increased, the window breaks down, and eventually disappears, due to the action of a cascade of period-doubling bifurcations. The same basic mechanism operates here as in the original period-doubling cascade, discussed in Sect. 4.9, except that now the orbits are of period , instead of . Note that all of the other periodic windows seen in Fig. 53 break down in an analogous manner, as is increased.

We now understand how periodic windows break down. But, how do they appear in the
first place? Figures 55-57 show details of the pendulum's time-asymptotic
motion calculated *just before* the appearance of the period-3 window (shown in Fig. 54), *just at* the appearance of the window, and *just after* the appearance of the window,
respectively. It can be seen, from Fig. 55, that just before the appearance of the
window the attractor is chaotic (*i.e.*, its Poincaré section consists of a line,
rather than a discrete set of points), and the time-asymptotic motion of the pendulum
consists of intervals of period-3 motion interspersed with intervals of chaotic motion.
Figure 56 shows that just at the appearance of the window the attractor
loses much of its chaotic nature (*i.e.*, its Poincaré section breaks up into a series
of points), and the chaotic intervals become shorter and much less frequent. Finally,
Fig. 57 shows that just after the appearance of the window the attractor
collapses to a period-3 attractor, and the chaotic intervals cease altogether.
All of the other periodic windows seen in Fig. 53 appear in an analogous manner to that
just described.

According to the above discussion,
the typical time-asymptotic motion seen just prior to the appearance of a period- window
consists of intervals of period- motion interspersed with intervals of chaotic
motion. This type of behaviour is called *intermittency*, and is observed in a
wide variety of non-linear systems. As we move away from the window, in parameter
space, the intervals of periodic motion become gradually shorter and more infrequent.
Eventually, they cease altogether. Likewise, as we move towards the window, the
intervals of periodic motion become gradually longer and more frequent. Eventually,
the whole motion becomes periodic.

In 1973, Metropolis and co-workers investigated a class of simple mathematical models which all
exhibit a transition to chaos, via a cascade of period-doubling bifurcations, as
some control parameter is increased.^{32} They were able to demonstrate
that, for these maps, the order in which stable periodic orbits occur as is
increased is fixed.
That is, *stable periodic attractors always occur in the same sequence* as
is varied. This sequence is
called the universal or *U-sequence*. It is possible to make a fairly
convincing argument that any physical system which exhibits a transition to chaos
via a sequence of period-doubling bifurcations should also exhibit the U-sequence
of stable periodic attractors.
Up to period-6, the U-sequence is

The beginning of this sequence is familiar: periods 1, 2, are the first stages of the period-doubling cascade. (The later period-doublings give rise to periods greater than 6, and so are omitted here). The next periods, correspond to the first three periodic windows shown in Fig. 53. Period is the first component of the period-doubling cascade which breaks up the period-3 window. The next period, 5, corresponds to the last periodic window shown in Fig. 53. The remaining periods, 6, 4, 6, 5, 6, correspond to tiny periodic windows, which, in practice, are virtually impossible to observe. It follows that our driven pendulum system exhibits the U-sequence of stable periodic orbits fairly convincingly. This sequence has also been observed experimentally in other, quite different, dynamical systems.