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For any set $X$, we let $[X]^2 = \big\{\{x,y\}:x\neq y \in X\big\}$.

If $G=(V,E)$ is a simple, undirected graph, and $v\in V$, let $N(v) = \{z\in V: \{v,z\}\in E\}$. Given any $v\in V$, we use the following notation:

  1. Let $G\setminus\{v\} := (V\setminus \{v\}, E \cap [V\setminus\{v\}]^2)$, and
  2. if $w\in V$ with $\{v,w\}\in E$, let $(G\setminus\{v\})^w := (V\setminus\{v\}, E_{v,w})$ where $E_{v,w}:= (E \cap [V\setminus\{v\}]^2) \cup \{\{w,z\}: z\in N(v)\}$.

A graph $G=(V,E)$ is said to be vertex-critical if $\chi(G\setminus\{v\}) < \chi(G)$ for every $v\in V$. We say $G$ is collapse-critical if $\chi\big((G\setminus\{v\})^w\big) < \chi(G)$ for all $v\in V$ and $w\in V$ such that $\{v,w\}\in E$. It is clear that $\chi(G\setminus\{v\})\leq \chi\big((G\setminus\{v\})^w\big)$, so collapse-criticality implies vertex-criticality.

Question. What is an example of a finite, connected graph that is vertex-critical, but not collapse-critical?

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    $\begingroup$ The only vertex critical graph $G$ with $\chi(G)=3$ is the cycle graph of odd length, but cycle graphs of odd length at least 5 are never collapse critical. $\endgroup$ Commented Sep 20, 2021 at 14:20
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    $\begingroup$ For problems like this, it would be good to use or write some code that tests random graphs for vertex criticality and edge criticality. Testing whether a graph is bipartite can be done in linear time. Computing the chromatic number of a graph is more difficult, but one can write a backtracking algorithm that traverses through all (up-to-renaming the colors) partial $k$-colorings $f:\{v_{1},\dots,v_{r}\}\rightarrow[k]$ of your graph $(V,E)$ with $V=\{v_{1},\dots,v_{n}\}$. $\endgroup$ Commented Sep 20, 2021 at 14:28
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    $\begingroup$ Also, the direction of the inequality between $\chi((G\setminus\{v\})^{w})$ and $\chi(G\setminus\{v\})$ seems to have been reversed. $\endgroup$ Commented Sep 20, 2021 at 14:30
  • $\begingroup$ If you want to test vertex critical graphs $G$ with $\chi(G)>3$, then it is easy to estimate the time it takes for your backtracking algorithm to terminate and compute the chromatic number of your graphs; this will save you some time so you can just compute the most efficient backtrackings. See the paper 'Estimating the Efficiency of Backtrack Programs' by Donald E. Knuth. $\endgroup$ Commented Sep 20, 2021 at 14:57
  • $\begingroup$ Thanks @JosephVanName - indeed I got the inequality of collapse-criticality vs vertex-criticality wrong. Will think about your other points and try some random graphs. $\endgroup$ Commented Sep 20, 2021 at 15:41

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Unless I'm mistaken, your definition of $(G\setminus \{v\})^w$ is symmetrical, and generally referred as an edge-contraction, so collapse critical is the same as edge-contraction critical.

this graph is vertex-critical, but not collapse-critical.

duplicate

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