Gerhard Keller
| Globally coupled piecewise expanding maps with bistable
behaviour
Among the simplest globally coupled systems of chaotic maps are
systems of piecewise monotone interval maps coupled by their mean
field. Around 1990, Kaneko studied globally coupled logistic maps. He
observed (numerically) that the fluctuations of the mean field do not
necessarily scale strength, and
he coined the term "violation of the law of large numbers" for this
behaviour. A bit later, Ershov and Potapov observed essentially the
same phenomenon in systems of globally coupled tent maps and explained
it by semi-rigorous arguments. So far there is no rigorous explanation
of this phenomenon. On the other hand, Esa Järvenpää
proved in 1997 for systems of analytic expanding circle maps that this
phenomenon does not occur at small coupling strength, and a bit later
I showed that just C^2-smoothness of the invariant densities is needed.
In my talk I plan to first summarize these results in a probabilistic
and work in progress (with J.-B. Bardet and R. Zweimüller) where
we study a parametrized family of two-to-one uniformly expanding
piecewise fractional linear maps of
an interval. The maps are coupled via the parameter which is a sigmoid
function of the mean field. Since the maps leave a space of probability
measures invariant whose distribution functions are Herglotz functions, the
Perron-Frobenius operators of the maps have some "hidden" monotonicity
properties. This allows to detail the dynamics of the (nonlinear) SCPFO for
a broad range of coupling strengths and to show that the operator has a
bifurcation from a unique stable fixed point to a pair of stable fixed
points
separated by a kind of hyperbolic fixed point. For all these parameters the
finite systems have a unique mixing smooth invariant density. The dynamics
of the SCPFO can be related to the large deviations behaviour of the finite
systems at fixed time when the system size tends to infinity. The large
deviations behaviour of the invariant densities is still unknown to us.
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