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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.

14 votes
4 answers
1k views

Is the "Moebius Stairway" Graph Already Known?

It is a wellknown fact, that Moebius Ladder Graphs have $2n$ vertices, but nowhere could I find any hint of how to generalize them to Graphs with $2n+1$ vertices. Last week I had the idea of giving up …
Manfred Weis's user avatar
  • 13.2k
11 votes
1 answer
390 views

Shortest Paths in the "Cantor Graph"

First, let me explain, what I understand by a "Cantor Graph": it is an infinite, directed graph with self loops and countably many vertices labelled with the natural numbers; every ordered pair of …
Manfred Weis's user avatar
  • 13.2k
11 votes
2 answers
1k views

Densest Graphs with Unique Perfect Matching

Given a graph $G$ with $n$ vertices, that has a perfect matching $M$, what is the maximal number of edges that $G$ can have without contradicting the uniqueness of $M$? Are examples of such extremal …
Manfred Weis's user avatar
  • 13.2k
8 votes
0 answers
433 views

Is there an "Erlangen Program" for Graph Theory?

There are certain graph theoretic problems (especially optimization problems), whose solution-subgraph (i.e. the set of vertices and edges)), is invariant under certain modifications (especially modif …
Manfred Weis's user avatar
  • 13.2k
6 votes
1 answer
368 views

How does the complexity of calculating the Permanent imply the NP completeness of directed 3...

In their paper Two Approximation Algorithms for 3-Cycle Covers of Markus Bläser and Bodo Manthey it is stated that: "...deciding whether an unweighted directed graph has a 3-cycle cover is already NP- …
Manfred Weis's user avatar
  • 13.2k
5 votes
1 answer
236 views

Do the Odd Cycles of a Graph Define a Matroid?

An Odd Cycle Transversal is a set of vertices that, when removed from a graph, renders it bipartite. Question: does the collection of "critical" sets of vertices, whose removal renders a graph bipart …
Manfred Weis's user avatar
  • 13.2k
5 votes
6 answers
435 views

Characterizing Convex Configurations of Quadrupels of Coplanar Points via Linear (In-)equali...

Given 4 points $A$, $B$, $C$ and $D$ in general position in the euclidean plane, is it possible to determine from the 6 distances $AB$, $BC$, $CD$, $AD$, $AC$ and, $BD$ alone, whether every point is a …
Manfred Weis's user avatar
  • 13.2k
5 votes
1 answer
181 views

Resource Constrained Routing with Refueling

What are good algorithms (resp. models) for calculating optimal or near optimal routes while taking into account fuel consumption, options for refueling and, limited tank capacity? Especially modeling …
Manfred Weis's user avatar
  • 13.2k
4 votes
1 answer
244 views

Probability of a vertex being a "degree-celebrity" in a random graph

If $G(n,p)$ is a random graph of the Erdös-Rényi model, what is the probability that $\mathrm{deg}(v)\gt\mathrm{deg}(u)\ \forall u\in\mathrm{adj}(v)$ Please feel free to relate answers to other m …
Manfred Weis's user avatar
  • 13.2k
4 votes
2 answers
416 views

What Kind of Graph is This?

I am currently developing TSP heuristics that aim at symmetrically reducing the original, complete and undirected graph. The overarching rationale is that the reduction is done via a sequence of regul …
Manfred Weis's user avatar
  • 13.2k
4 votes
1 answer
75 views

Existence of optimal tours containing an 'extremal' edge

let $\mathcal{P}$ be a finite set of points in the euclidean plane in general position, and let $$\lbrace p_A,p_B,p_C,p_D\rbrace: \ \|p_C-p_A\|+\|p_D-p_B\|\ \gt \|p_i-p_h\|+\|p_k-p_j\|\quad\forall\ \ …
Manfred Weis's user avatar
  • 13.2k
4 votes
0 answers
143 views

Halin Graphs with Highest Number of Hamilton Cycles

Halin graphs contain a Hamilton cycle and have the interesting property, that, also in the case of arbitrary real edge weights, it is possible to report one of the shortest contained Hamilton cycles i …
Manfred Weis's user avatar
  • 13.2k
4 votes
1 answer
168 views

Finding minimum weight perfect matchings in sparse bipartite graphs

Question: What can be recommended for finding optimal perfect matchings in large bipartite graphs with small vertex degree if the edge-weights are positive real values? I am looking for algor …
Manfred Weis's user avatar
  • 13.2k
4 votes
2 answers
464 views

Geodesics in Graphs:Shortest Paths vs Going as Straight Ahead as Possible

According to Wikipedia http://en.wikipedia.org/wiki/Geodesic, a geodesic " is a generalization of the notion of a "straight line" to "curved spaces " and further " In the presence of an affine connect …
Manfred Weis's user avatar
  • 13.2k
4 votes
0 answers
104 views

"Shape Aware" Trees

Have these kinds of geometric spanning trees already been described? all three are Minimum Spanning Trees of points that are elements of open disks, the only difference being in how the edge-weight …
Manfred Weis's user avatar
  • 13.2k

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