Borel complexity of the set of generic points of dynamical systems that satisfy a weak specification property
Borel complexity of the set of generic points of dynamical systems that satisfy a weak specification property
It was shown by H. Ki and T. Linton that for any integer b≥2, the set of numbers that are normal in base b is a Π03-complete set. Other authors have since used this result to arrive at the complexity of other sets of normal numbers. Let (X,T,μ) be a dynamical system, that is, X is a Polish space, T:X→X is continuous, and μ is an invariant Borel probability measure on X. We generalize the result of H. Ki and T. Linton by showing that the set of generic points of (X,T,μ) is a Π03-complete set if (X,T) satisfies a weakening of the specification property and there are at least two invariant measures. We also prove a stronger result for systems which are subshifts of shift spaces on a finite or countable alphabet. We extend the result from the case of a single measure μ to a closed, connected, proper set V of measures. Here the corresponding set G(V) is also Π03-hard, but to show G(V) is a Π03 set requires X being compact. We give an example of a non-compact (X,T) for which G(V) is Π11-complete. As a consequence of our results we answer a question of Sharkovsky and Sivak.