  We introduce the Blume-Emery-Griffiths (BEG) model in a social networks to
describe the three-state dynamics of opinion formation. It shows that the
probability distribution function of the time series of opinion is a
Gaussian-like distribution. We also study the response of BEG model to the
external periodic perturbation. One can observe that both the interior
thermo-noise and the external field result in phase transition, which is a
split phenomena of the opinion distributions. It is opposite between the effect
acted on the opinion systems of the amplitude of the external field and of the
thermo-noise.
