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Let $(X, d)$ be a metric space and define $K(X, d) = \{A \subseteq X : A \neq \emptyset$ and $A$ is compact in $(X, d)\}$. Let $d_h : K(X, d) \times K(X, d) \rightarrow \mathbb{R}$ be Hausdorff distance. We know that then $(K(X, d), d_h)$ is a metric space.

My question is this:

Find an example of a metric space $(X, d)$ so that we can determine that $(K(X, d), d_h)$ is isomorphic to some well known metric space. In particular, I want to find a metric space $(X, d)$ such that $K(X, d)$ is isomorphic to $([0, 1], d_2)$ where $d_2$ is euclidean metric on $\mathbb{[0, 1]}$. Since it seems to me that such metric space doesn't even exist (although I'm not sure how to prove it). Any example where we know the structure of $K(X, d)$ would be great.

Only example where I was able to determine the structure of $K(X,d)$ is this:

Let $X$ be a finite set, in particular $X = \{x_1,...,x_n\}$ for some $n \in \mathbb{N}$. Let $d : X \times X \rightarrow \mathbb{R}$ be a discrete metric on $X$. Then every subset of $X$ is finite so it is compact, therefore $K(X, d) = \mathcal{P}(X)$. Suppose $A,B \subseteq X$ and $A \neq B$. Without loss of generality we can suppose there exists $x \in A \setminus B$. Now for every $y \in B$ we have $d(x, y) = 1$. From this and the fact that $d$ is discrete metric we can conclude $d_h(A, B) = 1$. Since $d_h$ is a metric, if $A = B$ we have $d_h(A, B) = 0$, therefore we have shown $d_h$ is a discrete metric. It is easy to show now that $(K(X, d), d_h)$ is isomorphic to $(Y, d^{'})$ where $Y = \{y_1,...,y_{2^n}\}$ and $d'$ is discrete metric.

Goki
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  • For an explicit example, when $X$ is the Cantor space, $K(X)$ is the Cantor space again – Alessandro Codenotti Apr 26 '22 at 15:42
  • I also think that $K(X)$ is never isomorphic to $[0,1]$: when $X$ is compact connected $K(X)$ is infinite dimensional, and when $X$ is disconnected the disconnectedness should be inherited by $K(X)$ – Alessandro Codenotti Apr 26 '22 at 15:44
  • When $X$ is compact metric, even though $K(X)$ is huge as mentioned above, the closed subspace $C(X)\subseteq K(X)$ of compact connected subspaces of $X$ is much more well behaved and there are explicit geometric models for it in nice cases. For example $C([0,1])$ is homeomorphic to a closed disk in $\Bbb R^2$ – Alessandro Codenotti Apr 26 '22 at 15:46
  • Thank you very much, I was not aware of this facts. Can you suggest any literature where i can find proofs to some of these or some more proporties in general that hausdorff metric has? – Goki Apr 26 '22 at 16:00
  • suggest any literature --- See the comments to Applications of Hausdorff distance and 'hyper-topologies. A useful introductory account can be found in Point Sets by Eduard Čech (1969) -- see Chapter III, specifically pp. 121-124 and some of the exercises on p. 135. – Dave L. Renfro Apr 26 '22 at 17:47
  • @Goki for $K(C)\cong C$ when $C$ is the Cantor space, this is exercise 7.11 in Kechris Classical Descriptive Set Theory (the idea is to use the characterization in 7.4). Everything else can be found in Hyperspaces by Wanes-Nadler, infinite dimensionality of $K(X)$ for compact connected $X$ is 14.12, while geometric models for $C(X)$ take up the whole of chapter II – Alessandro Codenotti Apr 26 '22 at 18:55

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