  A long-standing question is what invariant sets can be shared by two maps
acting on the same space. A similar question stands for invariant measures. A
particular interesting case are expanding Markov maps of the circle. If the two
involved maps are commuting the answer is almost complete. However very little
is known in the non-commutative case. A first step is to analyze the structure
of the invariant sets of a single map. For a mapping of the circle of class
$C^{\alpha}$, $\alpha>1$, we study the topological structure of the set
containing all compact invariant sets. Furthermore for a fixed such mapping we
examine locally, in the category sense, how big is the subset of all maps that
have at least one non trivial joint invariant compact set. Lastly we show the
strong dimensional relation of the maximal invariant set of a given Markov map
contained in a subinterval of $[0,1)$ and the set of all right endpoints of its
invariants sets contained in the same subinterval as well as the continuity
dependence of the dimension on the endpoints of the subinterval.
