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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.

9 votes
2 answers
496 views

Can a parent and child node have the same type in a well-founded digraph tree?

$\newcommand\toward{\rightharpoonup}$It would help me to understand something in a current research project if someone could provide an example of directed graph $\langle G,\toward\rangle$ with the fo …
Joel David Hamkins's user avatar
47 votes
7 answers
5k views

Is it easy to produce hard-to-color graphs?

This question arises from my recent visit to my daughter's second-grade class, where I led some discussion and activities on graph coloring (see Math for seven-year-olds). In one such activity, each c …
Joel David Hamkins's user avatar
31 votes
3 answers
2k views

Is the Rado graph a Cayley graph? If so, what is the group like? (And other questions...)

The countable random graph, also known as the Rado graph, is characterized as the unique countable graph in which every two disjoint finite sets $A$ and $B$ of vertices admit a vertex $p$ connected to …
Joel David Hamkins's user avatar
19 votes
2 answers
985 views

Which graphs are elementarily equivalent to their own disjoint sums?

In Stefan Geschke's recent question, one of the solutions observed that the graph consisting of a single infinite beaded chain, a $\mathbb{Z}$-chain where each integer is connected to its nearest neig …
Joel David Hamkins's user avatar
32 votes
9 answers
5k views

How many groups of size at most n are there? What is the asymptotic growth rate? And what of...

Question. How many (isomorphism types of) finite groups of size at most n are there? What is the asymptotic growth rate? And the same question for rings, fields, graphs, partial orders, etc. Motiva …
Joel David Hamkins's user avatar
76 votes
6 answers
9k views

Which graphs are Cayley graphs?

Every group presentation determines the corresponding Cayley graph, which has a node for each group element, and arrows labeled with the generators to get from one group element to another. My main …
Joel David Hamkins's user avatar