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Questions tagged [graph-theory]

Graph theory is the study of graphs, mathematical structures used to model pairwise relations between objects.

3
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0answers
170 views

zero-sum path problem on a digraph

Consider a digraph $G=(V,A)$ with each arc weighted by either $+1$ or $-1$. A path is called a zero-sum path, if and only if all the arcs in its first half have weight $+1$, and all the arcs in the ...
2
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0answers
193 views

Graph optimization problem with multiple objectives/constraints

Let's assume that we have a directed acyclic graph $G = (V, E)$, non-negative vertex weight functions $w_a(v)$ and $w_b(v)$, and a non-negative edge weight function $t(u,v)$. We can divide vertices in ...
5
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1answer
366 views

Proof that the graph optimization problem is NP-hard

I'm trying to prove that the following optimization problem is NP-hard: Given a graph $G=(V,E)$, non-negative vertex weight functions $w(v)$ and $s(v)$, and a non-negative edge weight function $t(u,v)...
-2
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1answer
196 views

questions on implications Babais quasi P time graph isomorphism result

Babai has reputedly repaired his proof of graph isomorphism in quasipolynomial time.[1] the proof hinges crucially on Johnson graphs. based on the proof, does this mean now that if Johnson ...
1
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1answer
228 views

What is a weakly-simplicial vertex?

While studying chordal bipartite graphs, I have come across weakly simplicial vertices. I have searched for some time to understand what a weakly simplicial vertex is but I haven't succeeded. A ...
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0answers
105 views

Efficiently generate a random subgraph (Gs) with maximum degree K, using only edges from an existing graph G

I am looking find a way of efficiently generating a random, undirected subgraph $G_s$ with $N$ vertices, using a subset of edges from an exisiting undirected graph $G$, also of size $N$, where the ...
6
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2answers
318 views

Has this generalization of the vertex cover problem been researched?

I have the following problem, let's call it the $n$ vertex cover: Given a directed graph $G,$ find a minimum subset of vertices $S$ such that each trail of length $n$ has at least one vertex in $S$....
2
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1answer
201 views

Partition refinement in transition state systems (bisimulation contraction)

I am trying to understand bisimulation contraction of Kripke models. I have read these lecture slides and this Wikipedia page, but I still don't fully understand it. I can understand that the two ...
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0answers
207 views

A linear time algorithm for the all pair longest paths on a special kind of trees

Consider an edge-weighted graph $G=(V,E_G)$, a spanning tree $T=(V,E_T)$ of $G$, and a simple path $P$ in the tree that connects a leaf $\ell\in T$ to the root $r\in T$. A pebble starts in the leaf $\...
5
votes
2answers
146 views

Complexity of Uniform Generation of Perfect Matchings

Jerrum,Valiant and Vazirani on their paper "Random generation of combinatorial structures from a uniform" (http://www.cc.gatech.edu/~vazirani/AppCount.pdf) talk about seeing problems related to ...
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0answers
168 views

Maximize the weight of MST + sum of vertex weights

I am considering a problem where the goal is to choose a subset of size $k$ of the vertices in a graph, such that the weight of their minimum spanning tree + the sum of their vertex weights is ...
12
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1answer
273 views

Generalization of Dilworth's theorem for labeled DAGs

An antichain in a DAG $(V, E)$ is a subset $A \subseteq V$ of vertices that are pairwise unreachable, namely, there are no $v \neq v' \in A$ such that $v$ is reachable from $v'$ in $E$. From Dilworth'...
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0answers
81 views

Complexity of computing the simplicial width of a graph

Let $G=(V,E)$ be a finite undirected graph. A tree decomposition $(T,\lambda)$ of $G$ is a tree $T$ with labeling function $\lambda : T \to 2^{V}$ such that: For every edge $\{v_1,v_2\} \in E$, there ...
0
votes
1answer
85 views

Number of bounded minimum vertex covers

Minimum Vertex Cover problem Input: $G=(V,E)$ and Parameter $k$ Output: Decide whether there exists minimum vertex cover of size at most $k$. Question:- Can we bound the number of minimum vertex ...
4
votes
1answer
109 views

Shortest cycle separator for biconnected planar graphs

An $(s,f)$- balanced separator in a graph $G$ is a set $S$ of $s$ vertices removing which yields connected compoennts of size at most $f|V|$. If the vertices of $S$ form a cycle of length $s$, $S$ is ...
9
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1answer
165 views

What's the proof of this lemma by Hajnal about the randomized query complexity of monotone graph properties?

In this paper, Hajnal states the following lemma: Let $\mathscr{G}_{n, m}$ be the set of all bipartite graphs with $n$ vertices in the left part and $m$ vertices in the right part. Suppose $\...
3
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1answer
319 views

Max weight k-clique

Given an edge-weighted directed complete graph $G = (V,A)$, the maximum weight clique of fixed size $k$ ($k$ is a constant) can be identified in polynomial time with a brute-force algorithm, however ...
10
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2answers
423 views

Are social networks typically good expanders?

I am interested in the combinatorial properties of social networks as graphs. People have looked at things such as the distribution of the degrees, the clustering coefficient and the compressibility ...
2
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0answers
279 views

expected number of edges for fixed min cut

It is known that a graph $G=(V,E)$ with $n$ nodes and min cut $k$, must have at least $\frac{1}{2}nk$ edges. Are there any tighter bounds or expectations I can place on $|E|$ if I assume that $G$ ...
9
votes
1answer
331 views

Bipartite matching with degree domination

Given an unweighted bipartite graph $G=(V, E)$. Is it true that there always exists a nonempty matching $M\subseteq E$ (not necessarily maximal), such that for every $(i,j)\in E$ with $i$ matched and $...
4
votes
1answer
126 views

Min cost set of edges to connect 2 subgraphs s.t dist of nodes between subgraphs <= K

I find myself with another graph problem that I can't find the name of. I was wondering if anyone was able to identify if this problem and any efficient algorithms to solve it are known. The ...
7
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0answers
152 views

Positive cut algorithm on bipartite graphs with negative weights

Let $G=(V,E,w)$ be a bipartite graph with weight function $w:E→\{-1,1\}$. Is there an efficient (polynomial) algorithm for finding some positive (not necessarily maximum) cut of $G$, if one exists? If ...
10
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0answers
281 views

Triangle arrangement problem

Suppose you are given an undirected graph $G$, with each vertex representing an equilateral triangle with sides of unit length. Does there exist an arrangement of these triangles in two dimensions (...
5
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0answers
127 views

Graph Isomorphism Algorithm of Vertex Transistive Graphs and other

What are the best known Graph-Isomorphism algorithms for below graph classes- 1.vertex-transitive, 2. edge-transitive, 3.arc-transitive (or symmetric) 4.distance-transitive. Are they GI Complete?...
1
vote
1answer
118 views

K-fold Traveling salesman problem - A variant of TSP

Consider a weighted graph $K_n$ and where the weights between vertices $i,j$ is $w_{ij}$. Consider a path, $\sigma$, passing through each vertex only once. Here $\sigma_i$ is the vertex in the $(i\%n)^...
1
vote
1answer
853 views

Longest path from every vertex in a tournament

I have a tournament (directed complete graph) with $V$ vertices. For every vertex I want to find the longest path starting in it (so the longest path starting in the first vertex, longest path ...
1
vote
0answers
1k views

Fastest Algorithm for the Minimum Edge Covering Problem

Given an undirected weighted graph, G, where all the weights are non-zero positive numbers, my algorithm must produce a sub-graph G' that satisfies the following constraints: G' must include all the ...
0
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0answers
161 views

Q: How to design d-regular graphs with maximum minimum bisection bandwidth?

Let G=(V,E) be an undirected graph with n nodes and degree d at every node. Let us assume n to be even for simplicity. Given a partition of the nodes into two sets of size n/2 each, the bisection ...
1
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0answers
147 views

What is the *generally accepted* definition for the percolation threshold on finite-sized graphs?

For "regular" graphs (lattices, if you will), it's easy to define the percolation threshold $p_c$ as the critical probability beyond which the infinite graph will contain an infinitely large cluster ...
3
votes
1answer
203 views

Finding a graph that minimizes the number of nodes for a given number of paths

There is a problem that is of great interest for communications and optics, but I do not know if there is an easy solution of it. We are looking for an oriented graph that goes from a node A (starting ...
1
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0answers
95 views

Detecting bridges in Hypergraph S-t Reachability

Is there a fast algorithm to detect possible bridge arcs in hyperpaths from set $S$ of nodes to node $t$ in a hypergraph $G$. That is, given a hypergraph $G$, source nodes $S$, and target node $t$, ...
4
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0answers
128 views

Maximize number of edges covered by an independent set of vertices

Smallest vertex cover which is also an independent set asks about finding an independent set that covers all edges. This problem is known as the independent vertex cover problem and is equivalent to ...
8
votes
2answers
568 views

Dichotomy of the spectra of directed graphs

Compared to spectra of undirected graphs, which correspond to symmetric matrices, the spectra of directed graphs is not very well known: It is known that a directed graph $G = (V,E)$ has an adjacency ...
4
votes
1answer
633 views

Smallest vertex cover which is also an independent set

The vertex cover and independent set as a subset of nodes are always considered in a dual relationship. Have they been looked at together? What I mean is: start from a minimum vertex cover, and if it ...
1
vote
0answers
91 views

Concentration Bounds for functions of matrices

This is a question about properties of large directed graphs which are preserved when we randomly sample edges. Imagine I have an infinite sequence of positively weighted directed graphs. The ...
2
votes
1answer
205 views

generate a graph with fixed min cut

Is there a constructive way to generate a graph with a fixed min cut equal to $k$? One approach is to generate a random graph and then try to make edges alterations (additions, deletions, swaps) to ...
7
votes
1answer
278 views

Minimum Spanning tree on a complete “random” graph

Consider a complete undirected graph with $n$ vertices, $K_n$. Let weight of an edge between vertices $i\; \& \;j$ be a random variable $E_{ij}$. Let $E_{ij} \sim exp(\lambda)$, where $exp(\lambda)...
2
votes
1answer
237 views

Are there any results on the following “generalized matching” problems?

Given a graph $G = (V, E)$, one can view a matching $M$ on the graph as a partition of $V$ into vertex sets $S_{i, j}$ for $j \in \{1, 2\}$, where each $S_{i, j}$ induces a subgraph in $G$ isomorphic ...
1
vote
1answer
82 views

Subclasses or characterizations of modular or pseudo-modular planar graphs

We say that a graph G is modular if for every three vertices x,y,z there exists a vertex w that lies on a shortest path between every two of x, y, z. Pseudo-modular (or "3-Helly") graphs are defined ...
8
votes
1answer
271 views

Are there poly time algorithms to determine if a graph is almost bipartite?

Given an undirected graph G, we can say that G is almost bipartite if deleting k edges (or vertices) would make it bipartite. Are there poly time algorithms to determine if a graph is exactly or ...
4
votes
1answer
121 views

What automorphisms on a Markov Chain imply a uniform limiting distribution?

Consider an irreducible aperiodic Markov chain $M$, modeled as a connected directed graph with weighted edges. The existence of certain (graph) automorphisms on this Markov chain imply various ...
6
votes
1answer
98 views

Is every well-founded simplification order a well-partial order?

I'm contemplating the proof of Kruskal's Tree Theorem, as presented in the book "Term Rewriting and All That." They use it to prove that every simplification order is well-founded: first by showing ...
7
votes
2answers
176 views

Something-Treewidth Property

Let $s$ be a graph parameter (ex. diameter, domination number, etc) A family $\mathcal{F}$ of graphs has the $s$-treewidth property if there is a function $f$ such that for any graph $G\in \mathcal{...
3
votes
2answers
351 views

Does the problem “partition a vertex-weighted graph into $k$ balanced connected parts” have a standard name?

Consider the following problem: Given an integer $k$ and a vertex-weighted graph $G=(V,E)$, find a partition of $V$ into $V_1,\ldots,V_k$ such that each subgraph induced by $V_i$ is connected, ...
12
votes
2answers
190 views

Reference for a class of graphs that preserve subgraph distances when ordered

Let us say that a graph $G$ has the property $M$ if its vertices can be ordered $v_1, v_2, \ldots v_n$ in such a way that the graph $H_i$ induced by the vertices $\{v_1, \ldots, v_i\}$ has $dist_{H_i} ...
4
votes
1answer
142 views

Bidirectional A*: is an update of the distance estimation feasible while searching?

The A* algorithm is some kind of an enhanced Dijkstra. Keep in mind that I want an optimal algorithm here and A* is optimal if the heuristic does not overestimate the distance to the goal. The ...
2
votes
1answer
173 views

Intersection graphs of squares and rectangles

Is it known if the class of intersection graphs of rectangles is equal to the class of intersection graphs of squares (not necessarily unit)?
0
votes
1answer
122 views

The number of edges in the ith shortest path in a directed graph

$G$ - directed graph, $n$ - count of nodes According to Eppstein's Algorithm in this paper, the ith shortest path in a digraph may have $\Omega(ni)$ edges. Anybody can explain how this estimate is ...
0
votes
1answer
177 views

Online/approximate weighted and capacitated bipartite matching

I wish to take a look at online/approximate weighted and capacitated bipartite matching problem. Consider $G=\{L\cup R, E\}$, $|L|=n_1$, $|R|=n_2$, $|E|=m$ and $E\subseteq L\times R$. For each $r_i\...
6
votes
1answer
306 views

Ordering of a DAG minimizing some definition of cost

Consider a DAG $(V,A)$ with a topological ordering $(v_1,v_2,\ldots,v_n)$. I define the cost of this ordering as the maximum over all $1\leq i\leq n$ of $|\{j\leq i \mid \exists k>i: (v_j,v_k)\in A\...