# Repeller

Within nonlinear systems dynamics, repellers correspond, qualitatively speaking, to the opposite behaviour of attractors: given a fixedpoint or a cyclic trajectory of a dynamic system, they are called repeller type trajectories if small perturbations can make the system evolve to trajectories that are far from the original one, thus these are unstable regimes characterized by positive feedback. If there are several attractors in a phase space then their attraction regions are separated by unstable point sets representing repellers, so that all or almost all neighboring phase trajectories are repelled from these parts. We can say then that stable equilibria are attractors with negative feedback, unstable equilibria are repellers governed by positive feedback where the positive feedback can result in the butterfly effect, a nonlinear amplification of some small event into a large change process.