Attractors of Weakly Damped Forced KdV Equations with External Periodic Excitation

Consider the KdV equations with the external periodic excitation u t+uu x+u xxx +γu=f(t) , with f(t+T)=f(t). Prove the existence of attractors of resulting discrete semigroup ｛S(τ+mT)｝ m∈N .