Pullback Attractors of Nonautonomous Semidynamical Systems

A nonautonomous semidynamical system is a skew-product semi-flow consisting of a cocycle mapping on a state space which is driven by a semidynamical system on a base space. It is shown that the driving system can be extended backwards in time on a compact invariant set, such as a global attractor, as a set-valued semidynamical system. Pullback attraction and invariance with respect to the cocycle mapping are then reformulated in terms of the set-valued backwards extension of the driving system.