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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.

5 votes

What are the applications of hypergraphs?

Ford, Green, Konyagin, Maynard, and Tao - Long gaps between primes mentions a hypergraph covering theorem of Pippenger and Spencer. This theorem is generalized and used in a sieving method to find l …
LSpice's user avatar
  • 12.9k
2 votes

Diameter of random segment intersection graph?

Here is a way of thinking. It may turn out to be misleading, but it may also suggest a way to think properly about this problem. Lets take a random instance of a matching and reconstruct it edge by …
The Masked Avenger's user avatar
3 votes
Accepted

Can we find 3 disjoint directed Hamiltonian cycles in the cube?

It looks like the titled question (d=3) is not directly answered: I will hint at how to show the answer is no. At each vertex, there are two ways that the decomposition can go. I like to call them b …
The Masked Avenger's user avatar
2 votes

Random graphs with boundary in a game (Tsuro)

I would not be surprised if the probability was near the probabilty for a random matching between the points of a given set forming a special submatching. For a board holding k by k many tiles, this …
The Masked Avenger's user avatar
4 votes

The diameter of a certain graph on the positive integers

Maybe this will work. Given positive integers a and b, choose c large enough so that c^2 > a+b. also, choose c so that c^2 -a -b is odd and factors as (e+d)(e-d). Then a has an edge with c^2 - a, b h …
The Masked Avenger's user avatar
1 vote

4-regular graph with every edge lying in a unique 4-cycle

Consider two labeled squares, vertices on one labeled from the abcd alphabet, the other labeled from 1234. We are going to identify one or more pairs of vertices while maintaining the constraint that …
The Masked Avenger's user avatar
5 votes
Accepted

Relationship between triangle free graphs and their minimum degree

In addition to the literature mentioned in the other answers, one can try some arguments based on counting. Let $u$ and $v$ be two of $n$ vertices in a triangle-free graph, and further assume they ar …
Sergiy Kozerenko's user avatar
3 votes

Wait time to grid network disconnection with failing edges

Instead of answering the question, I will suggest some modifications that should make the model easier to analyze. I will denote the number of vertices as v, and the number of edges as dv for the d-d …
The Masked Avenger's user avatar
1 vote

Coloring vertices in a cubic lattice graph and counting edges between vertices of identical ...

This is more a collection of potentially useful ideas and intuitions, with no guarantee of correctness and no proof. If you take a coloring and tweak it by switching the colors on two vertices of opp …
The Masked Avenger's user avatar
0 votes

Graphs with dangling edges

There exist partial algebras ( having nontotal basic operations ), multi-typed structures, hypergraphs, and various flavors of categories, so why not partial multigraphs? Unfortunately, your descri …
The Masked Avenger's user avatar
1 vote
Accepted

How many edges can you put in a graph such that every edge belongs to a minimal $k$-cycle?

Here is a rough analysis to start things off. k=3 corresponds to a complete graph, and k=4 to a complete bipartite graph with the vertex set split as evenly as possible. (I don't see an easy proof of …
Community's user avatar
  • 1
1 vote

Estimate size of graph by taking random walks

Unless you use some exhaustive criterion for stopping, such as Anton Petrunin's suggestion of deleting vertices while maintaining connectivity and not stopping until all vertices are gone, I do not se …
The Masked Avenger's user avatar
2 votes

Reference Request for: Finding Large Bipartite Subgraphs via Destruction of Odd Cycles in Gr...

Not a reference, but you might enjoy tinkering with this. Decide for your graph on two parameters C and M. For each of C-many trials, two-color the vertices. In each trial, look at the vertices v w …
The Masked Avenger's user avatar
1 vote

Counting the number of rooted trees given the distance distribution at each level

Even with a distinguished root and insisting that all isomorphisms respect this distinguished root, this will be a challenging enumeration. For $k \leq 2$ the problem is straightforward: The count is …
The Masked Avenger's user avatar