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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.

4 votes
Accepted

Planar Graphs with #Vertices = #Faces

Unlike some other planar graphs, these ones always contain at least one triangular face and at least one vertex of degree $\le 3$. The reason for the first property is that, by Euler's formula, the nu …
David Eppstein's user avatar
4 votes
Accepted

What is the relation between Treewidth and Order of graph?

The treewidth of an $n$-vertex graph is always at most $n-1$ (because it is defined as one less than the cardinality of the largest bag in an optimal tree decomposition, and each bag is a set of $n$ o …
David Eppstein's user avatar
5 votes
Accepted

Which graphs are prime under the Cartesian product?

For connected graphs, they are the graphs in which every two edges are connected by a sequence of pairwise relations using one or both of the following two types of relation: Edge $xy$ and $uv$ are …
David Eppstein's user avatar
3 votes

Properties of bipartite graphs

No. For instance a 3x3 grid is a median graph (it has a unique median for every three vertices, a stronger version of your property 2 which does not require uniqueness) but it is not chordal bipartite …
David Eppstein's user avatar
2 votes

Is there currently a known way to construct an injective mapping that transforms finite grap...

Map any $n$-vertex graph $G$ into a collection of $n$ points in $(n-1)$-dimensional space that are all at unit distance from each other, together with a line segment connecting two points whenever the …
David Eppstein's user avatar
9 votes

Request for examples of 4-regular, non-planar, girth at least 5 graphs

If a planar graph has girth four or more, it can have at most $2n-4$ edges, but every 4-regular graph has exactly $2n$ edges, so every 4-regular graph with girth $\ge 4$ is nonplanar. That is, your re …
David Eppstein's user avatar
4 votes
Accepted

vertex independent set and the maximal clique

If $G$ is an odd cycle (as commented above), the answer is no. If it's an $(n+1)$-clique, it meets your degree condition but not your condition on the clique number. And if it's neither a clique nor a …
David Eppstein's user avatar
3 votes
Accepted

How to calculate the maximum number of rainbows for arbitrary graphs?

For arbitrary graphs, the problem is NP-complete. It includes as a special case the problem of coloring the edges of a 3-regular graph with three colors, so that each vertex is rainbow, which was show …
David Eppstein's user avatar
3 votes

Complement of a Cayley graph

The complete graph on $|G|$ vertices is a Cayley graph for $S=G\setminus\{1_G\}$. Its complement, the graph without edges, is not a Cayley graph.
David Eppstein's user avatar
4 votes
Accepted

On a special kind of graph connectig n point to n points.

These are the crown graphs.
David Eppstein's user avatar
7 votes
Accepted

Graph Theory: question regarding a class of digraph

Qiaochu Yuan has already provided an answer in terms of functions, but if you prefer to think graph-theoretically these things are known as directed pseudoforests.
David Eppstein's user avatar
4 votes

Interesting families of sparse graphs?

Minimally rigid graphs in d dimensions. For d=2 these are the Laman graphs but I don't think any kind of clean combinatorial description of these graphs is known for higher dimensions.
David Eppstein's user avatar
2 votes

Graphs with fractal properties?

Your idea of transforming nodes into sets of nodes sounds a lot like graph grammars — see e.g. this Wikipedia article.
David Eppstein's user avatar
5 votes
Accepted

A graph connectivity problem (restated)

It appears to be NP-complete even when m=1: see The Complexity of the Matching-Cut Problem, Maurizio Patrignani and Maurizio Pizzonia, WG 2001, http://dx.doi.org/10.1007/3-540-45477-2_26
David Eppstein's user avatar
10 votes

Recognition of graph families.

It's too big to fit into one table, but I believe what you want is the Information System on Graph Classes and their Inclusions. In particular, for each of over 1200 graph classes listed on this site, …
David Eppstein's user avatar

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