Dynamical Systems [Micha L Misiurewicz] Basic Notions
Dynamical Systems [Micha lMisiurewicz] Basic Notions, Examples The main objects studied in the theory of dynamical systems are maps f : X X, where X is a space with some structure. For instance, this structure can be topological,→ differentiable, or there may be a measure on X. Therefore three basic cases we will consider are: (a) X is a topological space (usually compact) and f is continuous, (b) X is a smooth (Cr, 1 r ) manifold (also usually compact) with or without boundary and f is smooth≤ (C≤r), ∞ (c) X is a space with a σ-field and a measure µ on it, the measure is a probability one, that is µ(X) = 1; f preservesF µ, that is µ(f −1(A)) = µ(A) for every A . Other way of saying that f preserves µ is that µ is f-invariant, or that f is an endomorphism∈ F of (X, ,µ). F 0 In each of cases (a)-(c), we consider the iterates of f, defined by induction: f = idX , and then f n+1 = f n f. Thus, f n = f f. This means that we look at the set of ◦ ◦···◦ n times n all iterates of f, f n∈Z+ . In other words, we look at the action of the semigroup Z+ on the space X.{ The} special case which| is{z considered} as often as the general one (and maybe even more often) is when the map f is invertible. We require then that f −1 is also within the class of maps we consider. That means that in case (a) f is a homeomorphism, in case (b) f is a diffeomorphism (of class Cr), and in case (c) f is an invertible measure preserving transformation (an automorphism) of X.
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