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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.

22 votes

Interesting and accessible topics in graph theory

I have found that the Art Gallery Problem engages middle- and high-school students, and quickly leads to the unknown, which itself can be eye-opening to students. (On the latter point, students tend t …
15 votes
Accepted

Travelling Salesman Problem: Can the nearest neighbor algorithm be $n$ times longer than the...

The nearest-neighbor (NN) heuristic (among others) is analyzed in this paper: Johnson, David S., and Lyle A. McGeoch. "The traveling salesman problem: A case study in local optimization." Local se …
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
13 votes
Accepted

Difference Sets

A keyword in this area is homometric, and a key paper this one: Joseph Rosenblatt and Paul D. Seymour. "The Structure of Homometric Sets." SIAM. J. on Algebraic and Discrete Methods, 3, 343-35 …
Joseph O'Rourke's user avatar
12 votes
Accepted

Which degree sequences are planar graphical?

It is always difficult to say what is "currently known," but at least around 2008, the paper "A Characterization of the degree sequences of 2-trees." Prosenjit Bose, Vida Dujmovi, Danny Krizanc, …
Joseph O'Rourke's user avatar
12 votes
Accepted

Shortest Path in Plane

The problem you posed is known in the literature as the weighted region problem. It was the focus of Joe Mitchell's Ph.D. thesis, under the direction of Papadimitriou, and their results were eventuall …
Joseph O'Rourke's user avatar
12 votes

Embedding of planar graphs

The recent paper below (and its references) may help. They mention that every planar graph with max degree $4$ (except for the octahedron) admits a $2$-bend embedding. Deciding whether a graph can b …
Joseph O'Rourke's user avatar
10 votes

Small 4-chromatic coin graphs

Here is Flo Pfender's 11-coin graph (in his first comment):           
Joseph O'Rourke's user avatar
10 votes

Concepts in topology successfully transferred to graph theory and combinatorics with non-tri...

There has been very interesting work on defining curvature on a discrete graph. For example: Knill, Oliver. "A graph theoretical Gauss-Bonnet-Chern theorem." arXiv:1111.5395 (2011). (Abstract.) …
10 votes

A tree with prime vertices

Not an answer, just a drawing of the tree including the OP's $2 \rightarrow 191$ path:          
Joseph O'Rourke's user avatar
10 votes

Which curves and surfaces are realizable by linkages? references?

Erik Demaine and I also included a proof for $d=2$ in Geometric Folding Algorithms: Linkages, Origami, Polyhedra, Chapter 3. There we asked if there is a planar (non-crossing) linkage that "signs you …
Joseph O'Rourke's user avatar
9 votes
Accepted

Elegant representations of graphs in R^3

You might start by exploring the various tools that are available. For example, Mathematica's GraphPlot3D[] does a nice job with small graphs. Here is a 5-node graph:        And here is a somewhat den …
Joseph O'Rourke's user avatar
9 votes

Visualizing polyhedra from their 1-skeletons

In response to the request for "a visualization of the hexahedral graph 5":            Just to illustrate the point that there are multiple realizations of any polyhedral graph:           
Joseph O'Rourke's user avatar
9 votes
Accepted

Smallest Connected Graph for Given Degree Sequence

A theorem of Hakimi says that any pair of degree-equivalent graphs can be obtained one from the other by a sequence of "elementary $2$-switchings" (probably known under many other names), which involv …
Joseph O'Rourke's user avatar
8 votes

Generalizations of the four-color theorem

There is a recent generalization to $k$-uniform hypergraphs that are embeddable in $\mathbb{R}^d$ without edge intersections. "For $k=d=2$ the problem specializes to graph planarity": Carl Georg Heis …

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