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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

What is the cycle structure of a graph?

Another possible answer is the cycle space of a graph, which is a vector space and so supports the application of many tools from linear algebra.
Joseph O'Rourke's user avatar
3 votes

four color proof

Just as background, definitely not an answer to your question: You are probably aware of the paper, "A new proof of the four colour theorem," by N. Robertson, D. P. Sanders, P. D. Seymour and R. Thom …
Joseph O'Rourke's user avatar
3 votes

Recognition of graph families.

Permit me to direct you to the Wikipedia page on "Forbidden graph characterization," which contains a long table of graph classes that have a forbidden subgraph characterizations. For example, chordal …
Joseph O'Rourke's user avatar
4 votes

Minimum dimension for sphere packing a graph in Euclidean space

This is a longish comment rather than a complete answer. Long ago Fred Roberts introduced the notion of the "boxicity" of a graph, and later the related notion of "sphericity" of a graph was studied. …
Joseph O'Rourke's user avatar
5 votes

determining k-edge-connectivity of a graph

Generally $k$-connectivity is computed using max-flow min-cut algorithms. I cannot quote you complexities off the top of my head, but you should be able to find the number of edges that disconnect an …
Joseph O'Rourke's user avatar
8 votes
Accepted

Why is this graph not generically globally rigid?

                  Image created using Cinderella.
Joseph O'Rourke's user avatar
5 votes

How to find central vertex in a graph?

Mathematica has a function GraphCenter[] that computes the center of a graph (the set of vertices with minimum eccentricity--exactly your definition). You can find a description in the documentation h …
Joseph O'Rourke's user avatar
4 votes

Degree of faces in a regular graph

This will likely only serve to sharpen your question, but I will just observe that there is no upper bound on the number of edges of a face of a 4-regular planar graph:            The octagon vertices …
Joseph O'Rourke's user avatar
5 votes
Accepted

Convex representation of (planar) graphs

Although what constitutes a "practical justification" is in the eye of the beholder, I would think this might qualify. There is a notion called geometric routing that is used to solve network routing …
Joseph O'Rourke's user avatar
2 votes

Two graph constructions: new, old?

As a public service, so to speak, here is $C'_4$, if I've followed the construction correctly:      
Joseph O'Rourke's user avatar
7 votes

Is there anything similar to the four color theorem for 3-dimensional objects?

This is an easy result, not at all comparable to the $4$-color theorem, but it perhaps has the flavor you are seeking: A collection of tetrahedra forming a pure simplicial complex may be "solid 4- …
Joseph O'Rourke's user avatar
14 votes

Always a planar-drawn cycle through $n$ points

Here is a quote from the first paper cited below: Steinhaus posed a version of your question, which has become known as simple polygonization of a set of points:   1Agarwal, Pankaj K., Ferran …
Joseph O'Rourke's user avatar
2 votes

Shortest path in a weighted graph with coloured edges

There is quite a bit of work on finding shortest paths with turn penalties. In a typical model, such as that used in the paper cited below, "Turn costs are stored in tables that are assigned to nodes. …
Joseph O'Rourke's user avatar
2 votes

Bound on the number of unlabeled cographs on n vertices

The paper "Enumeration and limit laws of series-parallel graphs" by Manuel Bodirsky, Omer Gimenez, Mihyun Kang, and Marc Noy, establishes that the number of labeled series-parallel graphs on $n$ verti …
Joseph O'Rourke's user avatar
1 vote

In search for isotropic graphs: Straight lines and parallels

Perhaps it will help to explore the world of pseudoline arrangements. A pseudoline is a simple curve in the projective plane that is topologically a line. Each pair of pseudoines in an arrangment meet …
Joseph O'Rourke's user avatar

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