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For a continuous mapping $\varphi: X \to X$ on a compact Hausdorff space $X$ we define the entropy as follows:

Given open covering $\mathcal{U}$, $\mathcal{V}$ of $X$, we call $\mathcal{V}$ a refinement of $\mathcal{U}$, if for all $V \in \mathcal{V}$ there exists $U \in \mathcal{U}$ such that $V \subseteq U$. We call

$$ \mathcal{U} \vee \mathcal{V} := \{U \cap V; \, U \in \mathcal{U}, \, V \in \mathcal{V}\}$$

the common refinement of $\mathcal{U}$ and $\mathcal{V}$.

For $n \in \mathbb{N}$ we set $\mathcal{U}_n := \bigvee_{i = 0}^{n-1} \varphi^{-i}(\mathcal{U})$, where $\varphi^{-i}(\mathcal{U}) := \{\varphi^{-i}(U); \, U \in \mathcal{U}\}$. Now denote by $N(\varphi, n, \mathcal{U})$ the minimal cardinality of a refinement of $\mathcal{U}_n$. (Notice that, since every subcover is a refinement and $X$ is compact, $N(\varphi, n, \mathcal{U})$ is always finite.) We now set

$$h_{\mathcal{U}}(\varphi) := \lim_{n \to \infty} \frac{1}{n} \log \big(N(\varphi, n, \mathcal{U})\big)$$

and call

$$ h(\varphi) = \sup \{ h_{\mathcal{U}}(\varphi) ; \, \mathcal{U} \text{ is an open covering of } X\}$$

the (topological) entropy of $\varphi$.

Notice that the constant $h_{\mathcal{U}}(\varphi)$ asymptoically satisfies

$$ N(\varphi, n ,\mathcal{U}) \sim e^{n h_{\mathcal{U}(\varphi)}}$$

for $n \to \infty$, so we can think of $h_{\mathcal{U}(\varphi)}$ as the exponential growth rate of $N(\varphi, n ,\mathcal{U})$ as $n$ grows larger.

Similarly we can define the polynomial (topological) entropy, namely by setting

$$p_{\mathcal{U}} (\varphi) := \lim_{n \to \infty} \frac{\log \big(N(\varphi, n, \mathcal{U})\big)}{\log(n)},$$

then

$$ p(\varphi) = \sup \{ p_{\mathcal{U}}(\varphi) ; \, \mathcal{U} \text{ is an open covering of } X\}$$

is called the polynomial (topological) entropy.

Similar to the classical entropy we now have asymptotically that

$$N(\varphi, n, \mathcal{U}) \sim e^{\log(n) p_{\mathcal{U}}(\varphi)} = n^{p_{\mathcal{U}}(\varphi)}.$$

This justifies the name "polynomial entropy".

Now to my question:

Is there any interesting literature that treats properties of polynomial entropy. I am particularly interested in the polynomial entropy as a topological invariant. It would also be interesting to know of any classes of topological dynamical systems for which $p(\varphi)$ is a complete topologic invariant. (An exemplary class of TDS for which the entropy is a complete topologic invariant is that of Bernoulli shifts.)

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    $\begingroup$ It might help to have some more context on what you're looking for exactly.These slides of Paris-Romaskevich may be useful for you: people.math.osu.edu/gogolyev.1/DynScreen/ParisR_dys.pdf $\endgroup$
    – Yankl
    Commented Apr 5, 2022 at 21:04
  • $\begingroup$ Well, first and foremost I am looking for papers or any type of source that treats polynomial entropy similar to treatments of entropy in classical dynamical system literature. This means I am looking for literature that introduces this lesser-known notion of entropy and proves basic properties. $\endgroup$ Commented Apr 5, 2022 at 22:44
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    $\begingroup$ See e.g. the paper arxiv.org/pdf/2107.13695.pdf $\endgroup$ Commented Apr 6, 2022 at 6:41
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    $\begingroup$ There are these two papers somehow related to your question. This one arxiv.org/abs/2003.01257 talks about generalized entropy. You can consider different growth rates. This one arxiv.org/abs/2203.08336 is focused on Polynomial entropy and comparing Morse-Smale systems on surfaces. $\endgroup$ Commented Apr 21, 2022 at 14:20
  • $\begingroup$ I also suggest you to have a look at this review paper: arxiv.org/pdf/2004.04655.pdf $\endgroup$ Commented May 22, 2022 at 11:34

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