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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.

1 vote
1 answer
267 views

History of the Vertex Disjoint Cycle Cover with Minimal Edgeweight Sum

Questions: who first posed the problem of determining a collection of (directed) cycles, whose edgeweight sum is minimal and, for which each vertex belongs to exactly one of the cycles? who came up …
Manfred Weis's user avatar
  • 13.2k
2 votes
0 answers
28 views

Graph of the optimal spanning trees of a complete weighted graph

Given a complete weighted graph $G(V,E);\ E=\lbrace\lbrace u,v\rbrace\,|\,u,v\in V\rbrace$, the set $\mathbb{T}$ of the spanning trees of $G$ is the disjoint union of sets of spanning trees, whose und …
Manfred Weis's user avatar
  • 13.2k
0 votes
1 answer
37 views

Graph connectivity after deleting an f-factor

Let $G(V,E)$ be an undirekted $k$-vertex-connected, $k$-regular graph and let $F$ be an $f$-factor of $G$ consisting of a set of $f$-vertex-connected components, $f<k$. Question: what is the v …
Manfred Weis's user avatar
  • 13.2k
1 vote
3 answers
109 views

Freely accessible collections of graph data

This question aims at providing links to definitions of graphs that either come from real-world problem or research that can be accessed and used freely; good examples in that vein are TSPLIB95 or the …
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
1 vote
0 answers
125 views

Pitfalls with modeling problems as graphs

The motivation for this question is that I could solve problem by extracting a graph structure from it and then applying a standard graph-algorithm and transfering the solution back to the interpretat …
Manfred Weis's user avatar
  • 13.2k
-1 votes
1 answer
73 views

Effect of a Specific Restriction on the Integrality of Min-cost Flow Solutions

I want to model the following situation: there is one production site (modelled by the source), a collection of depots (modelled by nodes without demand) and, of course many more customers (modelled b …
Manfred Weis's user avatar
  • 13.2k
2 votes
1 answer
1k views

Detecting Negative Cycles in Undirected Graphs

I recently faced the problem of quickly detecting negative cycles in undirected, weighted graphs. Resorting to the Bellman-Ford Algorithm, as commonly suggested, turned out to be very inefficient and …
Manfred Weis's user avatar
  • 13.2k
0 votes
0 answers
40 views

Subgraph induced by negative cycles detected by Bellman-Ford algorithm

What is known about the properties of the subgraph induced by the negative cycles are defined by the predecessor relation that is established during the execution of the Bellman-Ford algorithm and arc …
Manfred Weis's user avatar
  • 13.2k
1 vote
0 answers
36 views

Directed vertex-disjoint 3-cycle covers from Windy Postman tours?

It is a result of Zaw Win (paywall) that optimal Windy Postman tours in eulerian digraphs can be calculated in polynomial time. Bodo Manthey has shown that directed 3-Cycle Covers are APX hard to appr …
Manfred Weis's user avatar
  • 13.2k
1 vote
1 answer
87 views

Finding Optimal Vertex Weights without Linear Programing

Are there algorithms for finding $\omega_1\dots,\omega_n$, so that $\omega_i+\omega_j\le \|e_{ij}\|\ \forall i,j\quad\wedge\quad\sum{\omega_i}=max$ that are not based on linear programing, e.g. …
Manfred Weis's user avatar
  • 13.2k
3 votes
1 answer
626 views

"Eccentricity" in the Definition of Graph Center

On the Wikepdia Page Graph Center I saw that the center of graph is the set of vertices with minimal eccentricity, i.e the set of vertices, whose maximal distance to other vertices is minimal. On the …
Manfred Weis's user avatar
  • 13.2k
3 votes
1 answer
58 views

Name for Biconnected Tree+Cycle Graph

Is there an established name for graphs, that can be decomposed into a tree with at least three leaf nodes and a connected two-regular graph with the tree's leaf nodes as vertices? examples of …
Manfred Weis's user avatar
  • 13.2k
2 votes
0 answers
38 views

Can Orientability of Manifolds be Generalized to TSP Instances?

It is well known, that there are two basic kinds of manifolds, orientable and non-orientable ones; the most simple examples being obtained by identifying a pair of opposite sides of a rectangular stri …
Manfred Weis's user avatar
  • 13.2k
0 votes
0 answers
50 views

Examples of Binary Functions that Yield Regular Graphs with Invertible Adjacency Matrix

Question: What are, provided their existence, examples of functions $f$ with the following properties: \begin{align}f:& \ \mathbb{N}\times\mathbb{N}\ni(i,j)&\mapsto\ \quad\quad\quad\qu …
Manfred Weis's user avatar
  • 13.2k

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