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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.

10 votes
1 answer
341 views

The number of chains of chordal graphs

Consider a saturated chain $G_0 \subset G_1 \subset \cdots \subset G_m$ of graphs on $n$ labelled vertices, where $G_i$ has $i$ edges, and $m = {{n}\choose {2}}$. Altogether there are $m!$ such chains …
Gil Kalai's user avatar
  • 24.7k
25 votes
3 answers
2k views

Some models for random graphs that I am curious about

G(n,p) We are familiar with the standard notion of random graphs where you fixed the number n of vertices and choose every edge to belong to the graph with probability 1/2 (or p) independently. This …
Gil Kalai's user avatar
  • 24.7k
14 votes
0 answers
857 views

A Conjecture About Directed Graphs that are the Union of Two Trees

Let D=(V,E) be a directed graph that is the union of two edge-disjoint directed spanning trees. Suppose that There no subset X of vertices so that there is precisely one directed edge from X to its c …
Gil Kalai's user avatar
  • 24.7k
7 votes
1 answer
346 views

Edge Colorings of Directed Graphs which Respect an Involution

Let G be a graph and let C be a set of coloring. Suppose that there is an involution $\phi$ from C to C. We can think about the element of C as the nonzero elements of some Abelian group and $\phi(x)= …
Gil Kalai's user avatar
  • 24.7k
16 votes
0 answers
1k views

Optimal monotone families for the discrete isoperimetric inequality

Background: the discrete isoperimetric inequality Start with a set $X=\{1,2,...,n\}$ of $n$ elements and the family $2^X$ of all subsets of $X$. For a real number $p$ between zero and one, we consider …
Gil Kalai's user avatar
  • 24.7k
13 votes
3 answers
2k views

Models for graphs representing real-life networks

I am interested in basic models of graphs (stochastic or deterministic) that are offered for real-life networks (like social networks, the Internet, neuron networks). I will be thankful for answers t …
Gil Kalai's user avatar
  • 24.7k
12 votes
0 answers
328 views

The number of labeled pairs of edge disjoint trees and related questions

I wonder what is known on the following: 1) What is the number $T_k(n)$ of $k$-tuples of (pairwise) edge-disjoint trees $(T_1,T_2,\dots, T_k)$ with $n$ labelled vertices? 2) (harder, it seems) What …
Gil Kalai's user avatar
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9 votes
1 answer
588 views

Spanning $k$-trees

##k-trees A $k$-tree is a graph defined as follows: (They were defined by Harary and Palmer.) a) A complete graph with $k$ vertices is a $k$-tree. b) A $k$-tree on $n$ vertices $T$ is obtained by a $ …
Gil Kalai's user avatar
  • 24.7k
9 votes
3 answers
2k views

Weighted Regular Graphs

The following graph theoretic notion appeared in an economics paper entitled: "Prize competition under limited comparability, by Michele Piccione and Ran Spiegler which studies models of economics wer …
Gil Kalai's user avatar
  • 24.7k
63 votes
19 answers
12k views

Generalizations of the four-color theorem

The four color theorem asserts that every planar graph can be properly colored by four colors. The purpose of this question is to collect generalizations, variations, and strengthenings of the four c …
36 votes
21 answers
6k views

Generalizations of Planar Graphs

This is a follow up to Harrison's question: why planar graphs are so exceptional. I would like to ask about (and collect answers to) various notions, in graph theory and beyond graph theory (topology; …
Gil Kalai's user avatar
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