Small deterministic and random perturbations of dynamical systems or
stochastic processes will be considered. If the original system has a
unique stationary regime (stationary distribution) then, under some natural
additional assumptions, the perturbed system will be close, in a sense, to
the original one even on long time intervals. But if the non-perturbed
system has multiple invariant measures, the perturbations, in an
appropriate time scale, may lead to a drift in the cone of invariant
measures. This drift determines the long-term evolution of the perturbed
system. I will show how this general idea works in concrete problems:
Perturbations of systems with a finite number of "asymptotically stable"
invariant distributions, of oscillators, of Landau-Lifshitz magnetization
equation and its generalizations will be considered. The Neumann problem
for second order elliptic equations with a small parameter will be also
examined.
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