Questions tagged [graph-theory]

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

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Generating a tower defense maze, aka Finding the K most vital nodes (“nodewise interdiction”) in an unweighted grid-graph

In a tower defense game, you have an NxM grid with a start, a finish, and a number of walls. Enemies take the shortest path from start to finish without passing through any walls (they aren't usually ...
22
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3answers
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Number of distinct nodes in a random walk

Commute time in a connected graph $G=(V,E)$ is defined as the expected number of steps in a random walk starting at $i$, before node $j$ is visited and then node $i$ is reached again. It is basically ...
21
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5answers
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Reasons for which a graph may be not $k$ colorable?

While reasoning a bit on this question, I've tried to identify all the different reasons for which a graph $G = (V_G,E_G)$ may fail to be $k$ colorable. These are the only 2 reasons that I was able to ...
21
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5answers
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About properties of adjacency matrix when a graph is planar

1- Is there any specific properties for adjacency matrix when a graph is planar? 2- Is there any thing special for computing the permanent of adjacency matrix when a graph is planar?
21
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2answers
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Coloring Planar Graphs

Consider the set of planar graphs where all the internal faces are triangles. If there is an interior point of odd degree the graph cannot be three colored. If every interior point has even degree can ...
21
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3answers
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Graphs in which every minimal separator is an independent set

Background: Let $u, v$ be two vertices of an undirected graph $G=(V,E)$. A vertex set $S\subseteq V$ is a $u,v$-separator if $u$ and $v$ belong to different connected components of $G-S$. If no proper ...
21
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2answers
946 views

Finding a 5-cycle in a sparse graph efficiently.

(crossposted from MathOverflow) Hi, I was reading this thread: https://mathoverflow.net/questions/16393/finding-a-cycle-of-fixed-length I want to find a 5-cycle in a graph. Actually, what I really ...
21
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4answers
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What are infinite graphs good for?

I have just read on the German Wikipedia that an infinite graph is a graph with an infinite number of nodes or an infinite number of edges. I only know applications and algorithms for finite graphs. ...
21
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0answers
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$\Delta = 57, d=2$ Moore Graph

I am looking into the last open question regarding the existence of Moore Graphs of diameter 2. A problem that has been open in combinatorics for more than 55 years. You may recall that Hoffman and ...
20
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2answers
490 views

Is it necessary to call matrix multiplication $n$ times to find a claw

A claw is a $K_{1,3}$. A trivial algorithm will detect a claw in $O(n^4)$ time. It can be done in $O(n^{\omega+1})$, where $\omega$ is the exponent of fast matrix multiplication, as follows: take the ...
20
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2answers
996 views

Succinct circuit representation of graphs

The complexity class PPAD (e.g. computing various Nash equilibria) can be defined as the set of total search problems polytime reducible to END OF THE LINE: END OF THE LINE: Given circuits S and P ...
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6answers
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Network / Social network analysis visualization tools?

I was using Jung ( http://jung.sourceforge.net/ ) to visualize page rank and found it a little slow and difficult to scale it beyond 100 nodes. I was wondering what other tools people use for network /...
20
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1answer
597 views

Minimum chordless odd-cycle graph completion: is it NP-hard?

The following interesting problem came up in my research recently: INSTANCE: Graph $G(V, E)$. SOLUTION: A chordless odd-cycle completion, defined as a superset $E'$ of the edge set $E$ so that ...
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11answers
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Models of random graphs, for real computer networks

I am interested in models of random graphs which are similar to the graphs of real computer networks. I am not sure if the common well-studied $G(n,p)$ model ($n$ vertices, each possible edge is ...
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3answers
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graphs where vertex coloring is in P but independent set is NP complete

Is there an example of a class of graphs for which the vertex coloring problem is in P but the independent set is problem is NP complete?
19
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3answers
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Counting the Number of Simple Paths in Undirected Graph

How can I go about determining the number of unique simple paths within an undirected graph? Either for a certain length, or a range of acceptable lengths. Recall that a simple path is a path with no ...
19
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3answers
552 views

What separates easy global problems from hard global problems on graphs of bounded treewidth?

Plenty of hard graph problems are solvable in polynomial time on graphs of bounded treewidth. Indeed, textbooks typically use e.g. independet set as an example, which is a local problem. Roughly, a ...
19
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2answers
983 views

Is feedback vertex set problem is solvable in polynomial time for 3-degree bounded graphs?

Feedback Vertex Set is NP-complete for general graphs. It is known to be NP-complete for degree-8 bounded graphs due to a reduction from vertex cover. The Wikipedia article says that it is poly-time ...
19
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1answer
947 views

Construction of graphs where every pair of vertices have an unique common neighbor

Let $G$ be a simple graph on $n$ vertices $(n > 3)$ with no vertex of degree $n − 1$. Suppose that for any two vertices of $G$, there is a unique vertex adjacent to both of them. It is an exercise ...
19
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1answer
417 views

Finding good induced subgraph

You are given a graph $G = (V,E)$ with $n$ vertices. It might be bipartite if you want. There are $m$ sets of edges $E_1,\ldots, E_m \subseteq E$ (say disjoint). I am interested in the problem of ...
19
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1answer
798 views

Count the number of spanning trees fast

Let $t(G)$ denote the number of spanning trees in a graph $G$ with $n$ vertices. There is an algorithm that computes $t(G)$ in $O(n^3)$ arithmetic operations. This algorithm is to compute $\frac{1}{n^...
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2answers
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Data structure for shortest paths

Let $G$ be an unweighted undirected graph with $n$ vertices and $m$ edges. Is it possible to preprocess $G$ and produce a data structure of size $m \cdot \mathrm{polylog}(n)$ so that it can answer ...
19
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2answers
799 views

Axioms for Shortest Paths

Suppose we have an undirected weighted graph $G = (V, E, w)$ (with non-negative weights). Let us assume that all shortest paths in $G$ are unique. Suppose we have these $\binom{n}{2}$ paths (sequences ...
19
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2answers
746 views

maintaining a balanced spanning tree of a growing undirected graph

I am looking for ways to maintain a relatively balanced spanning tree of a graph, as I add new nodes/edges to the graph. I have an undirected graph that starts as a single node, the "root". At each ...
19
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1answer
360 views

Minor closed properties that are explicitly MSO expressible

Below, MSO denotes the monadic second order logic of graphs with vertex-set and edge-set quantifications. Let $\mathcal{F}$ be a minor closed family of graphs. It follows from Robertson and Seymour'...
18
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1answer
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Is the dominating set problem restricted to planar bipartite graphs of maximum degree 3 NP-complete?

Does anyone know about an NP-completeness result for the DOMINATING SET problem in graphs, restricted to the class of planar bipartite graphs of maximum degree 3? I know it is NP-complete for the ...
18
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3answers
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Solving Superstring Exactly

What is known about exact complexity of the shortest superstring problem? Can it be solved faster than $O^*(2^n)$? Are there known algorithms that solve shortest superstring without reducing to TSP? ...
18
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1answer
486 views

Is induced subgraph isomorphism easy on an infinite subclass?

Is there a sequence of undirected graphs $\{C_n\}_{n\in \mathbb N}$, where each $C_n$ has exactly $n$ vertices and the problem Given $n$ and a graph $G$, is $C_n$ an induced subgraph of $G$? is ...
18
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2answers
696 views

Complexity of the recovery of an adjacency matrix from its square

I am interested in the following problem: Given an $n\times n$ matrix, is there an undirected graph on $n$ vertices whose adjacency matrix squared is that matrix? Is the computational complexity of ...
17
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4answers
590 views

Hard Problems for higher genus graphs

Planar graphs have genus zero. Graphs embeddable on a torus have genus at most 1. My question is simple : Are there any problems that are polynomially solvable on planar graphs but NP-hard on graphs ...
17
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2answers
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Forbidden minors for bounded treewidth graphs

This question is similar to one of my previous questions. It is known that $K_{t+2}$ is a forbidden minor for graphs of treewidth at most $t$. Is there a nicely-constructed, parameterized, ...
17
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2answers
377 views

Generalization of locally bounded treewidth graphs

Is the following graph class known in the literature? The class of graphs is parameterized by positive integers $d$ and $t$ and contains each graph $G=(V,E)$ such that for each vertex $v\in V$, the ...
17
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3answers
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Properties of Random Directed Graphs with Fixed Out-Degree

I am interested in properties of random directed graphs with fixed out-degree $d$. I am imagining a random graph model where each vertex chooses d neighbors (say, with replacement) u.a.r. ...
17
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1answer
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Forbidden minors for bounded genus graphs

It is well known that $K_5$ and $K_{3,3}$ are forbidden minors for planar graphs. There are hundreds of forbidden minors for graphs embeddable on a torus. The number of forbidden minors for graphs ...
17
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2answers
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Gentle introduction to graph isomorphism for bounded valance graphs

I am reading about classes of graphs for which Graph Isomorphism ($GI$) is in $P$. One of such case is graphs of bounded valence (maximum over degree of each vertex) as explained here. But I found it ...
17
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1answer
340 views

Degree sets for linear extension graphs

A linear extension $L$ of a poset $\mathcal{P}$ is a linear order on the elements of $\mathcal{P}$, such that $x \leq y$ in $\mathcal{P}$ implies $x \leq y$ in $L$ for all $x,y\in\mathcal{P}$. A ...
17
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2answers
646 views

H-free cut problem

Suppose you are given a connected, simple, undirected graph H. The H-free cut problem is defined as follows: Given a simple, undirected graph G, is there a cut (partition of vertices into two ...
17
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2answers
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Cover Time of Directed Graphs

Given a random walk on a graph the cover time is the first time (expected number of steps) that every vertex has been hit (covered) by the walk. For connected undirected graphs, the cover time is ...
17
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1answer
545 views

Connectivity of graphs by edge and vertex removal

Let us say that a graph $G$ is $(a,b)$-connected if the removal of any $a$ vertices and any $b$ edges from $G$ leaves always a connected graph. For example, a $k$-connected graph, according to the ...
17
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2answers
858 views

Graph classes where Hamiltonian Cycle and Hamiltonian Path problems have different complexity

While searching The information System on Graph Classes and their Inclusions, I found several graph classes for which the Hamiltonian Cycle problem is NP-complete while the complexity of Hamiltonian ...
17
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1answer
629 views

Rapidly mixing Markov chains on 3-colorings of a cycle

The Glauber dynamics is a Markov chain on the colorings of a graph in which at each step one attempts to recolor a randomly chosen vertex with a random color. It does not mix for the 3-colorings of a ...
16
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3answers
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Are vertex colourings--in a sense--edge colourings?

We know that edge colourings of a graph $G$ are vertex colourings of a special graph, namely of the line graph $L(G)$ of $G$. Is there a graph operator $\Phi$ such that vertex colourings of a graph ...
16
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4answers
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Graph problems which are NP-Complete on directed graphs but polynomial on undirected graphs

I'm looking for problems which are known to be NPC for directed graphs but has a polynomial algorithm for undirected graphs. I've seen the question regarding the other way around here “Directed” ...
16
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2answers
476 views

About generalized planar graphs and generalized outerplanar graphs

Any planar, respectively, outerplanar graph $G=(V,E)$ satisfies $|E'|\le 3|V'|-6$, respectively, $|E'|\le 2|V'|-3$, for every subgraph $G'=(V',E')$ of $G$. Also, (outer-)planar graphs can be ...
16
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2answers
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Finding k shortest Paths with Eppstein's Algorithm

I'm trying to figure out how the Path Graph $P(G)$ according to Eppstein's Algorithm in this paper works and how I can reconstruct the $k$ shortest paths from $s$ to $t$ with the corresponding heap ...
16
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2answers
528 views

Representing non-planar graphs with overlapping circles

We know that we can represent any planar graph by a set of circles in the plane, known as a coin graph. Each circle represents a vertex and there is an edge between two vertices if and only if the ...
16
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2answers
621 views

Is there any problem in $\mathsf{\Sigma^P_2}$ which is solvable in bounded tree width graphs?

I'm looking for a problem which belongs to $\mathsf{\Sigma^P_2}$ in general graphs but is in $\mathsf{P}$ in bounded tree width graphs, In fact I think this problems are harder than using normal ...
16
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1answer
623 views

Why are perfect graphs called perfect?

Sorry, if this is a naive question, but I could not find the justification in any of the major text books like Bondy-Murty, Diestel or West. Perfect graphs have many beautiful properties, but what is ...
16
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1answer
641 views

Complexity of recognizing vertex-transitive graphs

I am not knowledgeable in the area of complexity theory involving groups so I apologize if this is a well known result. Question 1. Let $G$ be a simple undirected graph of order $n$. What is the ...
16
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1answer
731 views

Reference for (odd-hole,antihole)-free graphs?

X-free graphs are those that contain no graph from X as an induced subgraph. A hole is a cycle with at least 4 vertices. An odd-hole is a hole with an odd number of vertices. An antihole is the ...

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