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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.

57 votes
4 answers
15k views

Connectivity of the Erdős–Rényi random graph

It is well-known that if $\omega=\omega(n)$ is any function such that $\omega \to \infty$ as $n \to \infty$, and if $p \ge (\log{n}+\omega) / n$ then the Erdős–Rényi random graph $G(n,p)$ is asymptoti …
Matthew Kahle's user avatar
27 votes
1 answer
3k views

Monochromatic triangles in every two-coloring of the plane?

An old problem (possibly due to Erdős and Graham?): given a triangle $T$ and a two-coloring of the plane, does there necessary exist a monochromatic congruent copy of $T$? Here "monochromatic" means …
Matthew Kahle's user avatar
21 votes
11 answers
4k views

What are some good examples of non-monotone graph properties?

It seems that many, if not almost all, of the properties studied in graph theory are monotone. (Property means it is invariant under permutation of vertices, and monotone means that the property is e …
Matthew Kahle's user avatar
20 votes
2 answers
881 views

Is there an analogue of the Erdős–Gallai theorem for simplicial complexes?

The Erdős–Gallai theorem gives a necessary and sufficient condition for a finite sequence of natural numbers to be the degree sequence of a simple graph. In particular $d_1 \ge d_2 \ge \dots \ge d_n$ …
Matthew Kahle's user avatar
18 votes
2 answers
1k views

In an Erdős–Rényi random graph, what is the threshold for the property "every edge is contai...

Let $G(n,p)$ denote the Erdős–Rényi random graph, where $n$ is the number of nodes and $p$ is the probability for each edge. I'm interested in precisely what range of $p$ the random graph has at least …
Matthew Kahle's user avatar
14 votes
5 answers
663 views

Do there exist sparse graphs with large crossing number?

Does there exist a sequence of graphs $\{ G_n \}$ such that $G_n$ has $n$ vertices, the number of edges of $G_n$ is $O(n)$, and the crossing number of $G_n$ is $\Omega(n)$? In particular, do rando …
Matthew Kahle's user avatar
11 votes
2 answers
3k views

Algorithm for embedding a graph with metric constraints

Suppose I have a graph $G$ with vertex set $V$, edge set $E \subseteq {V \choose 2}$, a poistive integer $d$, and a weight function $w:E \to \mathbb{R}^{+}$. Is there a nice algorithmic way to decide …
Matthew Kahle's user avatar
10 votes
1 answer
455 views

For what range of edge probability does the following property hold for random graphs?

Let $G(n,p)$ denote the Erdős–Rényi model of random graph. For a given function $p = p(n)$ we say that $G \in G(n,p)$ asymptotically almost surely has property $\mathcal{P}$ if $$\mbox{Pr}[G \mbox{ …
Matthew Kahle's user avatar
10 votes
2 answers
441 views

Higher-dimensional Fáry's theorem?

Fáry's theorem says that every finite simple planar graph admits a planar embedding with straight line edges. For which $(k,d)$ is it true that every finite $k$-dimensional simplicial complex embeddab …
Matthew Kahle's user avatar
6 votes
2 answers
724 views

Has the following kind of (minimum degree $d$) random graph been studied?

The following random construction is simple enough that I am guessing it must have been studied. Fix $d \ge 3$, and let $n > d$. For each of the $n$ vertices, pick exactly $d$ other vertices to conne …
Matthew Kahle's user avatar
4 votes
0 answers
263 views

What are the best bounds to date on the maximum girth of a cubic graph?

The girth of a graph is the length of its smallest cycle. Studying the maximum possible girth for a $k$-regular graph on $n$ vertices is a very well-studied problem. In the 1988 paper "Ramanujan grap …
Matthew Kahle's user avatar