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In the famous paper by Benjamini-Schramm 2001, they consider the space of rooted graphs with uniformly bounded degrees, this space modulo rooted graph isomorphism is denoted by $\mathcal X$.

This space is equiped with a local metric: the distance between two rooted graphs $(G,o)$ and $(G',o')$ is $2^{-k}$, if $k$ is the maximal integer such that the $k$-ball of the root in $G$ and $G'$ are isomorphic as rooted graphs.

It follows that $\mathcal X$ is a compact metric space. A random rooted graph is a borel probablistic measure on $\mathcal X$. In the paper, they define the so-called Benjamini-Schramm convergence of a sequence of random rooted graphs $(G_n,o_n)\rightarrow (G,o)$ if and only if, for any finite rooted graph $(H,o')$ and $k\geqslant 0$, the probability that $(H,o')$ is rooted isomorphic to the $k$-neighborhood of $o_n\in G_n$ converges to the probability that $(H,o')$ is rooted isomorphic to the $k$-neighborhood of $o\in G$.

It seems to me that their definition of convergence amounts to saying that the measure of any metric balls of $\mathcal X$ converges. But the general definition of weak convergence requires more than that. One of the equivalent definitions write: for any bounded continuous function $f$ on $\mathcal X$, the integral pairing converges: $$\langle f,(G_n,o_n)\rangle\rightarrow \langle f,(G,o)\rangle.$$

My question is whether the Benjamini-Schramm convergence is equivalent to the weak convergence of Borel measure?

Edit: thank Will for pointing out a typo.

p.s. In the present situation, every metric ball is open and closed. Hence the characteristic function of any metric ball of $\mathcal X$ is continuous. So weak convergence would imply Benjamini-Schramm convergence. I want to figure out the reverse direction.

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  • $\begingroup$ How should these items be comparable? The $G_n$ are different objects, so how could one function $f$ live on them all? $\endgroup$
    – user473423
    Commented Sep 11, 2023 at 14:16
  • $\begingroup$ @Echo $f$ lives on $\mathcal X$, not on the $G_n$. $\endgroup$
    – Will Sawin
    Commented Sep 11, 2023 at 14:24
  • $\begingroup$ Isn't weak convergence usually defined with continuous bounded functions, rather than bounded Borel functions? If you allow arbitrary Borel measurable functions then it's impossible for any sequence of countably supported measures on $\mathbb R$ to converge weakly to uniform measure on $[0,1]$ since a countable set is measurable with measure zero. The same logic should show it's not possible for any sequence of finite graphs to converge under the Borel definition to an infinite graph, since the set of finite graphs is countable and hence Borel. $\endgroup$
    – Will Sawin
    Commented Sep 11, 2023 at 14:29
  • $\begingroup$ @WillSawin Sure, that's a typo, and thank you for letting me know something I take for granted before! $\endgroup$
    – Fredy
    Commented Sep 11, 2023 at 16:21

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