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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.

7 votes
Accepted

Asymptotic Geodesic Flow on Planar Graphs

You can't do better. Lipton and Tarjan's planar separator theorem says that any $n$-node planar graph $G=(V,E)$ contains a set $S$ of $O(\sqrt{n})$ vertices whose removal separates the graph into comp …
Louigi Addario-Berry's user avatar
4 votes

Notation for a graph without any edges?

I don't think there is standard notation for this. If you've already fixed a notation for complement (say a superscript c) then you could use $K_n^c$. But I don't think standard notation exists for th …
Louigi Addario-Berry's user avatar
15 votes

Proving that every graph is an induced subgraph of an r-regular graph

Use induction on $r-\delta$, where $\delta=\delta(G)$ is the smallest degree of any vertex in $G$. If $r-\delta=0$, then you are done. If $r-\delta > 0$ then create two disjoint copies of $G$, say …
Louigi Addario-Berry's user avatar
1 vote

edges minus vertices

My answer comes from the random graphs community. In the book Random Graphs, the quantity "edges minus vertices" is called the excess, which is quite standard terminology at least in random graphs. In …
Louigi Addario-Berry's user avatar
1 vote

How to estimate the growth of the probability that $G(n, M)$ contains a $k$-clique

You might take a look at Chapter VII of Bollobas. In particular, Theorem VII.1.7 -- which is simple enough that he doesn't bother providing a proof -- states that the expected number of $k$-cliques i …
Louigi Addario-Berry's user avatar
5 votes
Accepted

Cover time and intersection time of random walks

In Proposition 5 of Chapter 14 of the unpublished book on Markov chains by Aldous and Fill, they show that for continuous time reversible Markov chains, \[ I \le \max\{ \mathbb{E}_i T_j, i,j \in V\}, …
Louigi Addario-Berry's user avatar
9 votes
Accepted

Similarity of weighted graphs

If you view the weights as edge lengths then you can view each graph as a metric space, and then use the Gromov-Hausdorff distance between the two metric spaces. This may not be at all suitable for yo …
Louigi Addario-Berry's user avatar
10 votes

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

One whole family comes from considering properties that are monotone for connected graphs but can change when the connectivity changes. For example: the diameter of a graph -- defined to be the maximu …
Louigi Addario-Berry's user avatar
7 votes
Accepted

Differences of near diagonal Ramsey numbers.

Edit: Erdős got three things wrong. First of all, it wasn't Faudree, Shelp, and Rousseau, it was Faudree, Shelp, and Burr. Second, it wasn't "recently", it was in the future (with respect to the qu …
Louigi Addario-Berry's user avatar
3 votes

The critical value of percolation on Cayley graphs.

In a paper called Critical Percolation on any Nonamenable Group Has no Infinite Clusters, Benjamini, Lyons, Peres, and Schramm show that ... critical percolation on any nonamenable group has no infini …
Louigi Addario-Berry's user avatar
3 votes
Accepted

Number of Geodesic Paths Passing Through a Vertex in an Expander Graph

My short answer: I think not much is known. But: here is the state of the art on related problems, as far as I am aware. Aldous and Bhamidi consider the following model. Place independent exponentia …
Louigi Addario-Berry's user avatar
6 votes

Does graph asymmetry imply all eigenvalues of the graph Laplacian are simple?

Take two asymmetric $d$-regular graphs $H_1,H_2$, and let $G$ be their disjoint union. Then $d$ will be a repeated eigenvalue. If you want $G$ connected, take the complement of the graph obtained by …
Louigi Addario-Berry's user avatar
2 votes
Accepted

Critical probability for Erdos-Renyi digraphs to be strongly connected

This is basically the main result of a paper by Ilona Palásti: On the strong connectedness of directed random graphs. Studia Sci. Math. Hungar. 1 (1966), 205–214. Here's the MathSciNet summar …
Louigi Addario-Berry's user avatar
2 votes
Accepted

Is there a simple inductive procedure for generating labeled trees uniformly at random, with...

This is an interesting question. For any fixed positive integer $d \geq 2$, write $T_d^{\infty}$ for the complete infinite rooted $d$-ary tree (by this I mean every node has exactly $d$ children). Lu …
Louigi Addario-Berry's user avatar
17 votes
Accepted

Properties of Some Random Graphs

Yes, this model has been studied. You should look at Chapter 9 of Janson, Luczak and Rucinski's Random Graphs book, and in particular at Corollary 9.44. This corollary is in fact a rather well-known t …
Louigi Addario-Berry's user avatar

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