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Questions about the branch of combinatorics called graph theory (not to be used for questions concerning the graph of a function). This tag can be further specialized via using it in combination with more specialized tags such as extremal-graph-theory, spectral-graph-theory, algebraic-graph-theory, topological-graph-theory, random-graphs, graph-colorings and several others.

1 vote

Is the empty graph a tree?

Some comments above discuss the question from the point of view of homology. I'd like to expand on these. You might consider a tree to be an abstract simplicial complex of dimension at most 1 that ha …
Russ Woodroofe's user avatar
0 votes

When does a graph underlie the Hasse diagram of a poset?

This is not an answer, but some context. A somewhat related notion is that of comparability graphs: these are the graphs for which there is a poset $P$ on the vertex set such that $\{x,y\}$ is an edge …
Russ Woodroofe's user avatar
6 votes

Does the hypergraph of subgroups determine a group?

As @Keith Kearnes says, the negative answer ought to be somewhere in Roland Schmidt's book. Unless I'm mistaken, it suffices to find two non isomorphic groups with isomorphic coset lattices. Indeed, …
Russ Woodroofe's user avatar
3 votes
Accepted

Proving that the complement of a bipartite graph has chromatic number equal to clique number

According to this Wikipedia entry the statement that $\chi(\overline{G}) = \omega(\overline{G})$ for all bipartite $G$ is actually equivalent to König's Theorem.
Russ Woodroofe's user avatar
5 votes

A flag complex is contractible iff the underlying graph is....?

A nice graph theory lemma for showing homotopy equivalence that builds on the "clique starring" already discussed is stated as Lemma 3.2 of Alexander Engström's paper arXiv:math/0508148. I'll rephras …
Russ Woodroofe's user avatar