Difference between revisions of "Open Problems:42"

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(Created page with "{{Header |title=Graph Frequency Vectors |source=bertinoro11 |who=Noga Alon }} For a graph $G$, a $k$-disc around a vertex $v$ is the subgraph induced by the vertices that are ...")
 
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Revision as of 01:54, 7 March 2013

Suggested by Noga Alon
Source Bertinoro 2011
Short link https://sublinear.info/42

For a graph $G$, a $k$-disc around a vertex $v$ is the subgraph induced by the vertices that are at distance at most $k$ from $v$. The frequency vector of $k$-discs of $G$ is a vector indexed by all isomorphism types of $k$-discs of vertices in $G$ which counts, for each such isomorphism type $K$, the fraction of $k$-discs of vertices of $G$ that are isomorphic to $K$. The following is a known fact observed in a discussion with Lovász. It is proved by a simple compactness argument.

Fact: For every $\epsilon > 0$, there is an $M=M(\epsilon)$ such that for every $3$-regular graph $G$, there exists a $3$-regular graph $H$ on at most $M(\epsilon)$ vertices (independent on $|V(G)|$), such that variation distance between the frequency vector of the $100$-discs in $G$ and the frequency vector of the $100$-discs in $H$ is at most $\epsilon$.

Question: Find any explicit estimate on $M(\epsilon)$. Nothing is currently known.