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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.

0 votes
1 answer
37 views

Cycle-Sculpturing with Minimal Vertex-Deletion

given a simple, finite and symmetric graph $G(V,E)$ with $n$ vertices and at least $n$ edges Question: how can the smallest set of vertices $W\subset V$ be calculated for which the graph induced by $ …
Manfred Weis's user avatar
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1 vote
1 answer
54 views

Complexity of maximum weight-sum matching for cycle graphs

I need to determine a matching with maximal weight-sum for a cycle graph with positive, negative and zero edge-weights. Question: What is the fastest way of calculating such a matching? Because of t …
Manfred Weis's user avatar
  • 13.2k
0 votes
0 answers
23 views

Building hypercubes from the bottom up

let $H^k$ denote a $k$-dimensional hypercube in a complete symmetric graph $G(V,E)$ without self-loops and parallel edges; let $|V|=2^n$ be the number of vertices. setting $\mathbb{H}^0 := V$, i.e. th …
Manfred Weis's user avatar
  • 13.2k
2 votes
1 answer
224 views

Name for generalization of trees to digraphs

One definition of tree in graph theory could be as follows: A tree is a(n undirected) graph for which there is a unique (undirected) path between any pair of vertices. This suggest a possible defini …
Manfred Weis's user avatar
  • 13.2k
0 votes
0 answers
32 views

Enumeration of flat integral $K_4$

Question: What is known about the enumeration of all $(a,b,c,d,e,f)\in\mathbb{N}^6_+: \\ \quad\operatorname{GCD}(a,b,c,d,e,f)=1\ \\ \land\ \exists \lbrace x_1,x_2,x_3,x_4\rbrace\subset\mathbb{E}^2:\ …
Manfred Weis's user avatar
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2 votes
2 answers
202 views

Isometric embeddings of metric $K_{n+1}$ in $\mathbb{R}^n$

Question: is it always possible to embed a complete, symmetric and metric graph $G$ with $n+1$ vertices isometrically in $\mathbb{R}^n$? I'm convinced it must be true, but can't remember having seen …
Manfred Weis's user avatar
  • 13.2k
2 votes
1 answer
124 views

Constructing optimal Hamilton cycles from optimal Hamilton paths

Question: can the shortest Hamilton cycle in a complete symmetric graph with weighted edges be constructed from the shortest Hamilton path in the same graph by connecting its ends and then exchanging …
Manfred Weis's user avatar
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0 votes
0 answers
89 views

Relation of minimum spanning trees to the shortest Hamiltonian path problem

Spanning trees can be decomposed into a minimal set of maximal path graphs, whose vertices have degree two exactly if they also have degree two in the spanning tree; lets call these paths tree paths. …
Manfred Weis's user avatar
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2 votes
2 answers
122 views

Existence of certain regular graphs

Question: what can be said about the existence of $2k$ regular graphs, $1\lt k$ that have a $1$-factor and a $2$-factor? Provided their existence, what is/are the smallest for $k$? The graphs must b …
Manfred Weis's user avatar
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1 vote
1 answer
93 views

Algorithmic complexity of calculating maximum weight $k$-regular subgraphs

Question: what is known about the complexity of calculating the heaviest $k$-regular subgraph of a weighted symmetric graph if edge-weights can also be negative? Please note that in contrast to $k$- …
Manfred Weis's user avatar
  • 13.2k
0 votes
1 answer
75 views

Dual equivalence of minimum feedback-vertex sets and cycle packing

it is known that the duals of feedback-set problems are set-packing problems; in the context of digraphs the feedback set are either a minimal set of vertices or edges that hit every oriented cycle; t …
Manfred Weis's user avatar
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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
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0 votes
1 answer
40 views

Reconstructing a 2-factor from its edge set

Let $G(V,E)$ be a symmetric graph with $n$ vertices and $m$ edges that has a $2\text{-factor}$ with edge set $F$, i.e. $F$ are the edges of an undirected vertex-disjoint cycle cover of $G$. Question: …
Manfred Weis's user avatar
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0 votes
0 answers
40 views

Subtour-gluing constraints for ILP formulation of TSPs

If one doesn't want to introduce additional variables to the ILP of a TSP instance, one has to add exponentially many so-called subtour-elimination constraints; in practical calculations subtour-elimi …
Manfred Weis's user avatar
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0 votes
0 answers
76 views

Regular graphs without non-trivial $f$-factor

Question: are there any known examples of $k$-regular graphs that have no regular $f$-faktor, $1\le f\lt k;\ k\ge 3$, resp., can their existence or nonexistence be proved?
Manfred Weis's user avatar
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