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Let $X$ be a compact Hausdorff space without isolated points and $f:X\rightarrow X$ be a homeomorphism such that the $\mathbb{Z}$-action on $X$ that is defined by $f$ is minimal. Namely, for each $x\in X$, the orbit of $x$ defined below:

$$ [x]_f = \big\{ f^n(x): n\in \mathbb{Z} \big\} $$ is dense in $X$. For each subset $A\subseteq X$, we define the orbit of $A$ the same way:

$$ [A]_f = \big\{ f^n(a): a\in A,\, n\in\mathbb{Z} \big\} $$ In this case, is it always true that, or is there a necessary condition that implies that there exists a proper subset $A\subseteq X$ such that $[A]_f=X$?

This is a rather naive example. In a compact metric space $(X, d)$, suppose there exists $r>1$ such that for any $x, y\in X$, $d\big( f(x), f(y) \big) \geq rd(x, y)$, or we call $f$ is $r$-expansive. Then clearly for any (proper) open ball $B$, the diameter of $f^n(B)$ will increase as $n$ increases, and hence $[B]_f$ must be the entire $X$ since the diameter of $X$ is finite in this case. However, in this example, I am considering a compact metric space $(X, d)$ that has an $r$-expansive homeomorphism, but in general do not know if any compact metric space can have an $r$-expansive homeomorphism.

Counter-examples of a compact metric space that does not have any $r$-expansive (or even $1$-expansive) homeomorhpism, and counter-examples to my question above are both welcomed.

Updates: This post shows that in the set-up above an $f$-invariant subset always exists even when $f$ is not invertible.

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  • $\begingroup$ There are no expansive compact metric spaces (consisting of more than one point). You can always take your $A$ to be the complement to a singleton. $\endgroup$ Commented Dec 6 at 21:26
  • $\begingroup$ By "expansive" compact metric space, do you mean the one that has an expansive homeomorphism? If without details, it is not clear to me that $\big[ X\backslash \{x\} \big]_f$ is always equal to $X$. Please see the link I included in the post, which shows that there is always an $f$-invariant closed subset of $X$. In particular, such an $f$-invariant closed subset could be a fixed point of $f$ when $f$ is contractive. $\endgroup$
    – Kaku Seiga
    Commented Dec 6 at 21:44
  • $\begingroup$ If $f$ is not the identity take any orbit of $f$ containing at least two different points $x,y$ with $fx=y$. Then $A=X\setminus\{y\}$ is a proper subspace of $X$ with $[A]_f=X$. $\endgroup$ Commented Dec 6 at 22:12
  • $\begingroup$ Your are right. I just forgot how I define the orbit. Perhaps in the next question I can look for a subset of $X\backslash \{y\}$ that has more properties. $\endgroup$
    – Kaku Seiga
    Commented Dec 6 at 22:34

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