  Fluctuations of the mean field of a globally coupled dynamical systems are
discussed. The origin of hidden coherence is related with the instability of
the fixed point solution of the self-consistent Perron-Frobenius equation.
Collective dynamics in globally coupled tent maps are re-examined, both with
the help of direct simulation and the Perron-Frobenius equation. Collective
chaos in a single band state, and bifurcation against initial conditions in a
two-band state are clarified with the return maps of the mean-field, Lyapunov
spectra, and also the newly introduced Lyapunov exponent for the
Perron-Frobenius equation. Future problems on the collective dynamics are
discussed.
