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Give two metric space $X,Y$, we denote by $d_{GH}(X,Y)$ the Gromov- Haudorff distance. It seems that it only relies on $X,Y$ not on the ambient metric space $Z$ where we embed $X,Y$ (because in the notation, there is no $Z$). How can we prove this is well-defined?

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$d_{GH}(X, Y)$ is computed by looking at all possible "ambient spaces" $Z$. The phrasings one often sees, like

The infimum over all embeddings of $X, Y$ into a common space $Z$,

can make it sound like there is a "special" $Z$. But rather, we're looking at all pairs of embeddings into all (possible) spaces.

EDIT: Specifically, here's a clearer way to define $d_{GH}(X, Y)$.

$d_{GH}(X, Y)$ is the infimum of the set of $\delta$ such that, for some metric space $Z$ and some embeddings $f, g: X, Y\rightarrow Z$, we have $d_{H}^Z(f(X), g(Y))=\delta$ (where "$d_H^Z$" denotes the usual Hausdorff distance in $Z$).

In particular, we look at all possible embeddings of $X$ and $Y$ into all possible spaces. There is no single $Z$ we're looking at!


Incidentally, it's not hard to prove - and fairly useful - that we can restrict attention to those $Z$s whose underlying set is $X\sqcup Y$ and whose metric restricted to $X$ (resp. $Y$) is just the original $X$-metric (resp. $Y$-metric).

Again, that is:

Theorem. $d_{GH}(X, Y)$ is equal to the infimum of the set of $\epsilon$ such that, for some metric space $Z$ with underlying set $X\sqcup Y$ whose metric on the $X$ part (resp., $Y$ part) is the same as the $X$ metric (resp., $Y$ metric), we have $d_H^Z(X, Y)=\epsilon$.

Note that this is made much clearer if we distinguish between the set $A$ and the metric space $\mathcal{A}=(A,d_A)$. Namely, we have:

Theorem, rephrased. $d_{GH}((X, d_X),(Y, d_Y))$ is equal to the infimum of the set of $\epsilon$ such that, for some metric $d$ on $X\sqcup Y$ with $d\upharpoonright X=d_X$ and $d\upharpoonright Y=d_Y$, we have $\epsilon=d_H^{(X\sqcup Y, d)}(X, Y)$.

Again, though, we're looking at all possible metrics on $X\sqcup Y$ which restrict to $d_X$ and $d_Y$ on $X$ and $Y$, respectively.

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  • $\begingroup$ How to define a metric on $X\sqcup Y$? $\endgroup$ – Upc Aug 1 '17 at 20:33
  • $\begingroup$ @XavierYang There's not a single one. We look at all possible metrics on that set which extend the $X$-metric on the $X$-part and the $Y$-metric on the $Y$-part. $\endgroup$ – Noah Schweber Aug 1 '17 at 21:16
  • $\begingroup$ @XavierYang I've edited, hopefully this makes things clearer. The point is that we are always, no matter how we define the Gromov-Hausdorff definition, taking an infimum over lots of different metric spaces; there's no "one metric space $Z$," or "one metric on $X\sqcup Y$," which plays a special role here. $\endgroup$ – Noah Schweber Aug 1 '17 at 21:24
  • $\begingroup$ Thanks, now it is clear, theoretically. But pratically, how can I calculate the GH distance between e.g. $S^1$ and $S^2$ or $S^1$ and $\mathbb{R}$ (of course, all are equipped with usual metric)? $\endgroup$ – Upc Aug 1 '17 at 21:39
  • $\begingroup$ @XavierYang With reasonable spaces, you can often guess the correct answer; the verification, however, is generally not that simple. The power of $d_{GH}$ lies in its applications, not in its ease of calculation. $\endgroup$ – Noah Schweber Aug 1 '17 at 21:44

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