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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.

2 votes
1 answer
144 views

Distance queries to reconstruct an unknown graph

Let $G$ be a finite, simple, connected, undirected graph on $n$ vertices. Suppose your goal is to determine $G$ uniquely via queries. Each query choses a $v_i$, and returns the shortest distances (num …
Joseph O'Rourke's user avatar
5 votes
2 answers
715 views

Bound on graph domination number when min degree is 7

I have a graph $G$ whose minimum vertex degree is $\delta=7$. I am seeking an upper bound on the domination number $\gamma(G)$ in terms of the number of vertices $n$ of $G$. I found a paper by Edwin C …
Joseph O'Rourke's user avatar
8 votes
6 answers
965 views

Random planar, bipartite graphs

I have a need to generate random planar graphs none of which have an odd cycle, i.e., bipartite graphs. I know there is a substantial two-decade literature on random planar graphs, little with which I …
Joseph O'Rourke's user avatar
53 votes
1 answer
6k views

Why are there 1024 Hamiltonian cycles on an icosahedron?

Fix one edge $e$ of the graph (1-skeleton) of an icosahedron. By a computer search, I found that there are 1024 Hamiltonian cycles that include $e$. [But see edit below re directed vs. undirected!] Wi …
Joseph O'Rourke's user avatar
12 votes
1 answer
591 views

Characterizing graphs by their "walkers"

Let $G$ be a (large) graph and $W$ another (smaller) graph. $W$ is what I call a walker. Let me use "vertices" and "edges" for $G$ and "nodes" and "arcs" for $W$. $W$ has a distinguished node, its ce …
Joseph O'Rourke's user avatar
7 votes
3 answers
808 views

When can the Cayley graph of the symmetries of an object have those symmetries?

Let $P$ be an object in $\mathbb{R}^n$ with symmetry group $G$. Let $C$ be the a Cayley graph of $G$. When can $C$ be embedded in $\mathbb{R}^m$ so that the embedded graph has the same symmetry …
Joseph O'Rourke's user avatar
3 votes
1 answer
373 views

Diagonal shortcuts to minimize all-pairs shortest-paths in grid graph

Augment the grid graph $G$ on lattice points $[1,n]^2$, which connects each point to its four distance-$1$ vertical and horizontal neighbors. Augment $G$ to $G'$ by adding in one of the two $\sqrt{2}$ …
Joseph O'Rourke's user avatar
3 votes
0 answers
159 views

Rubber-band graph embedding under gravity

Let $G$ be a simple $3$-connected plane graph with the vertices of its outer face pinned to the $xy$-plane. View each edge as an ideal rubber band, and each internal node given a weight proportional t …
Joseph O'Rourke's user avatar
2 votes
0 answers
224 views

Graphs with the same Laplacian eigenvalues

Let $L$ be the Laplacian matrix for a simple graph $G$ of $n$ vertices, and $\lambda_0,\ldots,\lambda_{n-1}$ its $n$ eigenvalues. Q. What is the cardinality of the class of $n$-vertex graphs $ …
Joseph O'Rourke's user avatar
17 votes
3 answers
2k views

Laplacians on graphs vs. Laplacians on Riemannian manifolds: $\lambda_2$?

A graph $G$ is connected if and only if the second-largest eigenvalue $\lambda_2$ of the Laplacian of $G$ is greater than zero. (See, e.g., the Wikipedia article on algebraic connectivity.) Is th …
Joseph O'Rourke's user avatar
6 votes
5 answers
1k views

Generate random graphs that satisfy the triangle inequality

I would like to generate random graphs that might be geometric graphs in some (unknown) dimension. So I would like every triangle in the graph to satisfy the triangle inequality on its (random) edge l …
Joseph O'Rourke's user avatar
8 votes
3 answers
599 views

Decimating the infinite grid graph

Let $G$ be the graph whose nodes are the points of $\mathbb{Z}^d$ in the nonnegative orthant (i.e., all coordinates are $\ge 0$), with edges connecting each pair of points separated by unit distance. …
Joseph O'Rourke's user avatar
24 votes
6 answers
3k views

Shortest grid-graph paths with random diagonal shortcuts

Suppose you have a network of edges connecting each integer lattice point in the 2D square grid $[0,n]^2$ to each of its (at most) four neighbors, {N,S,E,W}. Within each of the $n^2$ unit cells of thi …
Joseph O'Rourke's user avatar
7 votes
1 answer
337 views

Maximal disarrangement of $n \times n$ numbers

This question is inspired by Martin Erickson's question, "Labeling a Square Array." I'll start by quoting Martin: the $n^2$ cells of an $n \times n$ array are labeled with the integers $1, \dots …
Joseph O'Rourke's user avatar
20 votes
2 answers
1k views

Erdős, Harary, Tutte's "dimension of graph": Progress in last 48 yrs?

I just ran across this delightful paper by an amazing triumvirate: Paul Erdős, Frank Harary, and William Tutte. "On the dimension of a graph." Mathematika 12.118-122 (1965): 20. (Cambridge link) …
Joseph O'Rourke's user avatar

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