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Questions tagged [graph-minors]

A graph $H$ is called a minor of a graph $G$ if $H$ can be obtained from $G$ by contracting edges, deleting edges, and deleting isolated vertices.

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1answer
111 views

Is every finite graph an induced minor of $\omega^2$?

Let $G=(V,E)$ be a simple, undirected graph. Suppose that ${\cal S}$ is a collection of non-empty, connected, and pairwise disjoint subsets of $V$. Let $G({\cal S})$ be the graph with vertex set ${\...
2
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0answers
28 views

Flat or linkless embeddings of graph with fixed projection

The problem of finding whether a given planar diagram of a graph, with over- and under-crossings, is a linkless embedding or not has unknown complexity (Kawarabayashi et al., 2010). My first question ...
3
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1answer
132 views

Induced minors of $\{0,1\}^\omega$

Let $G=(V,E)$ be a simple, undirected graph. Suppose that ${\cal S}$ is a collection of non-empty, connected, and pairwise disjoint subsets of $V$. Let $G({\cal S})$ be the graph with vertex set ${\...
2
votes
1answer
80 views

Compactness of Hadwiger number

Is there an infinite, simple, undirected graph $G=(V,E)$ such that there is $n\in\mathbb{N}$ with the following properties? $K_n$ is a minor of $G$, but $K_{n+1}$ is not a minor of $G$, and if $F$ ...
7
votes
1answer
109 views

Two disjoint trees

Let $G$ be a graph and let $A_1, A_2 \subseteq V(G)$ be disjoint sets of vertices. Let us call $(A_1, A_2)$ independent if there exist vertex-disjoint trees $T_1, T_2 \subseteq G$ within $G$ which ...
3
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0answers
103 views

Hadwiger number of Erdös-Faber-Lovasz graphs

For any set $X$, let $[X]^2 = \big\{\{a,b\}:a,b \in X, a\neq b\big\}$. We call a finite, simple, undirected graph $G=(V,E)$ an $n$-Erdös-Faber-Lovasz (EFL-) graph if there are $n$ subsets $S_1,\...
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1answer
118 views

Large complete minors of $\mathbb{Z}^\omega$

Let $x,y\in \mathbb{Z}^\omega$ and let $x,y\in\mathbb{Z}^\omega$ form an edge if there is $i\in\omega$ such that $|x_i - y_i|=1$ and $ x_k = y_k$ for all $k\in \omega\setminus\{i\}$. $K_\omega$, the ...
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1answer
90 views

Complete minors of the grid graphs $\mathbb{Z}^n$

Let $n>1$ be an integer. We say that two points $(x_1,\ldots,x_n),(y_1,\ldots,y_n)\in\mathbb{Z}^n$ are a member of the edge set $E_n$ if and only if $$\sum_{i=1}^n|x_i-y_i| = 1.$$ Question. Given ...
1
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1answer
141 views

Is this totally unimodular family?

Is it possible to prove this matrix family only contains totally unimodular matrices? The matrix has dimensions $\frac{3n(n-1)}2$ rows and $n+\frac{n(n-1)}2$ columns. To every pair $(i,i')$ with $1\...
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0answers
99 views

Ordinal corresponding to well-quasi-order on graphs

Let $K$ be an infinite cardinal. Then, by the Robertson–Seymour theorem, the set of graphs with fewer than $K$ vertices and edges form a well-quasi-order. In terms of $K$, what is the maximal order ...
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1answer
179 views

Bounds on degrees of minors obtained by edge contractions of regular graphs

Given a connected $d$-regular graph $G=(V,E)$, generate a sequence of minors by performing only edge contractions and loop deletions (as, e.g., in Karger's algorithm) until the graph collapses to a ...
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0answers
44 views

Increasing the Hadwiger number by identifying non-adjacent points

This is a specialisation of a more general, still unanswered question. Suppose $G$ is a finite simple graph. Let $h(G)$ denote the Hadwiger number, that is, the maximum $n\in\mathbb{N}$ such that $...
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1answer
68 views

Hadwiger critical graphs of arbitrarily high chromatic number

This is an update to an older question admitting a trivial example to answer it. Suppose $G$ is a finite simple graph. Let $h(G)$ denote the Hadwiger number of $G$; that is, the maximum $n\in\mathbb{...
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1answer
78 views

Identifying two non-adjacent vertices and the effect on the Hadwiger number [closed]

Suppose $G$ is a finite simple graph. Let $h(G)$ denote the Hadwiger number of $G$; that is, the maximum $n\in\mathbb{N}$ such that $K_n$ is a minor of $G$. What is an example of a graph $G_0=(V_0, ...
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193 views

Two types of criticality

Suppose $G$ is a finite simple graph. Let $h(G)$ denote the Hadwiger number (synonym: connected pseudoachromatic number, cf. [Abrams-Berman 2014, p. 315]) of $G$; that is, the maximum $n\in\mathbb{N}$ ...
6
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1answer
71 views

Characterizing SP-DAGs by Forbidden Minors?

So it's well-known that an alternative way to define a series-parallel (undirected graph) is by the forbidden minor $K_4$. Is there a known analog of this definition for directed graphs — ...
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1answer
38 views

Complete minors in a “redoubled” graph

For any finite, simple, undirected graph $G=(V,E)$ denote by $G_2 = (V_2, E_2)$ the graph, in which $V_2$ and $E_2$ are defined as follows: $V_2 = \big(V\times\{1\}\big) \cup \big(V\times \{2\}\big)$...
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1answer
48 views

Complete minors in graphs of bounded diameter

For positive integers $m, d\in \mathbb{N}$ consider the following statement: $\mathsf{S}(m,d):$ There is $N\in\mathbb{N}$ such that the complete graph $K_m$ on $m$ vertices is a minor of any finite ...
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1answer
86 views

Is the “surface-minor” ordering of plane graphs a well-quasi-ordering?

A plane graph is a finite simple graph with a fixed embedding into the two-sphere. The embedding induces an embedding on a minors of a plane graph (i.e. a graph obtained by successive removal of ...
2
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1answer
132 views

Minors of graphs with infinite chromatic number

Let $G=(V,E)$ be an infinite simple, undirected graph with $\chi(G) \geq \aleph_0$. Is there a minor $M$ of $G$ such that $M\not\cong G$, and $\chi(M)=\chi(G)$ ?
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1answer
100 views

Complete minors and minimal degree

Is there for every positive integer $n\in\mathbb{N}$ a finite, simple, undirected graph $G=(V,E)$ with the following property? $G$ does not have a complete minor with more than $\frac{\delta(G)}{n}$...
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2answers
143 views

Does anyone know a specific polynomial-time algorithm to detect if a given signed graph contains an odd-K4 as a signed minor?

By signed graph, I mean each edge is designated either odd or even (e.g. as in Guenin's result for weakly bipartite graphs).
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0answers
56 views

Is there an established notion of 'signed treewidth' for signed graphs?

By a signed graph, I mean a graph where each edge is designated as either odd or even (as in Guenin's result for weakly bipartite graphs). It is well-known that for (unsigned, or usual) graphs, ...
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0answers
152 views

Hadwiger's conjecture in the language of graph homomorphisms

Consider the following statement: (S): If $G$ is not a complete graph, then there is a minor $M$ of $G$ such that $M \not \cong G$, and there is a graph homomorphism $f:G\to M$. Hadwiger's ...
2
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1answer
105 views

Size of forbidden minors for treewidth

For any $k$, the class of graphs of treewidth at most $k$ can be characterized by a finite set of forbidden minors. For treewidth $1$ and $2$, the set is of size $1$. Then for treewidth $3$, the set ...
6
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1answer
80 views

Graph minors, and Kronecker product

Let $X$ and $Y$ be graphs and consider the Kronecker product: $Z = X \otimes Y$. Is it true that if $X$ excludes an $M$-minor, $Z$ excludes an $M \otimes Y$ minor? I am particularly interested in the ...
1
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3answers
196 views

Hadwiger number and minimal degree

Suppose $G$ is a finite simple graph and $\eta(G)$ is the maximum $n\in\mathbb{N}$ such that $K_n$ is a minor of $G$. If $\delta(G)$ is the minimal degree of $G$, do we have $\delta(G)\leq\eta(G)$?
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1answer
211 views

Is there a polynomial-time algorithm to check if a signed graph contains an odd-K5 minor?

I suspect this exists, if anyone has a reference please that would be very helpful. By signed graph, I mean each edge is designated either odd or even (e.g. as in Guenin's result for weakly bipartite ...
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0answers
76 views

An algorithm to compute the number of graphs of size n with a given graph as a minor

I'm looking for any results regarding computing the number of graphs of size $n$ which have a given graph $H$ as a minor. Are there any known algorithms which are more efficient than a brute force ...
3
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2answers
188 views

Asymptotics of list size in Robertson-Seymour theorem

A planar graph cannot have $K_5$ and $K_{3,3}$ as minors. Robertson-Seymour theorem generalizes this by stating for every genus $g$ there is a finite list of forbidden minor graphs that are ...
3
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1answer
279 views

Is there a version of Robertson-Seymour's graph minor theorem known to apply to signed graphs?

Here a signed graph is one where each edge is signed either odd or even. A cycle is odd or even according to the sum of the signs of its edges. For a given signed graph, a resigning may be performed ...
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0answers
192 views

A separation property of graphs of bounded tree-width

The following separation property of trees is well-known and in fact easy to prove (see e.g. the paper "Covering a hypergraph of subgraphs" by Noga Alon, Lemma 2.2) Let $T$ be a tree and $r, m$ non-...
3
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1answer
92 views

What is the relation between Hadwiger number and Treewidth?

Is there any general relation between Hadwiger number and Treewidth of a graph? Intuitively I think Hadwiger number is greater than or equal to Treewidth, but I couldn't prove it.
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1answer
91 views

What is the relation between size of maximum clique and branchwidth?

Let $bw(G)$ be the branchwidth of graph $G$ and $\omega(G)$ be the size of maximum clique in $G$. I think the following inequality holds: $$ \omega(G)\leq bw(G) $$ Intuition: Assume (in reverse of ...
5
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1answer
328 views

A claim from “Graph minors - a survey” by Robertson and Seymour

Can someone give me a proof sketch for this: Let $\mathscr{P}_n$ be the set of all graphs which do not contain a path on $n$ vertices as a subgraph. Define the type of a graph inductively as: the type ...
1
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1answer
163 views

Are there good ways of relating a minor to the full determinant?

Say $A$ is a $(n-1)\times (n-1)$ matrix and we augment it by a $n^{th}$ row and a column and get a $n \times n$ matrix $B$. Is there a nice way to relate $det(B)$ and $det(A)$ and the added row and ...
7
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2answers
265 views

Number of edges in linklessly embeddable graphs

What is the maximum number of edges of an $n$-vertex linklessly embeddable graph? A more general question is the following. What is the maximum number of edges of an $n$-vertex graph with Colin de ...
3
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1answer
190 views

Contracting a planar graph to a (multiply-edged)-tree

Given a planar graph (no loops, no multiple edge), is it always possible to perform edge contractions* in order to obtain a graph $T$ which has no loops, and if one ignores parallel edges, $T$ is a ...
2
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1answer
266 views

A variant of Kruskal's theorem

For $X$ and $Y$ finite sequences of finite trees, let us say that $X$ is everywhere contained in $Y$ ($X\subseteq_{ec}Y$) iff, for every $y\in Y$, there is some $x\in X$ such that $x$ is a minor of $y$...
4
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1answer
267 views

Do graphs with large number of paths contain large chain minor?

Definition: A "$k$-chain" is a multi-graph obtained from a path of length $k$ by duplicating every edge. Note that the number of paths between two endpoints of a $k$-chain is $2^k.$ Question: Let $G$...
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2answers
426 views

Do graphs with large number of cycles always contain large necklace minor?

Let "$k$-necklace" denote the (multi)graph obtained from a cycle of length $k$ by duplicating every edge. Note that the number of cycles in $k$-necklace is at least $2^k.$ Question : Suppose a ...
3
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1answer
652 views

Big binary tree as an induced subgraph

I believe this is true: Suppose $G$ is a graph. If $G$ has a subdivision of a large binary tree, prove that $G$ has an induced subgraph which is a subdivision of a large binary tree or the line ...
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0answers
119 views

Upper bound on size of obstruction set for wye-delta-wye reducible graphs

A graph is $Y \Delta Y$-reducible if it can be reduced to an empty graph by the following operations: $Y \leftrightarrow\Delta$ transforms; Replacing multiple edges with single edges (parallel ...
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0answers
491 views

Planar minor graphs

The theorem of Robertson-Seymour about graph minors says that there exists no infinite family of graphs such that none of them is a minor of another one. Apparently, it came as a generalization of ...
4
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2answers
521 views

Minor-closed classes of graphs with large numbers of excluded minors

Robertson and Seymour tell us that any minor-closed family of graphs has a finite collection of excluded minors. Standard examples include planar graphs with two excluded minors ($K_5$ and $K_{3,3}$) ...
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0answers
1k views

About Hadwiger's conjecture

Reading the Wikipedia article about Hadwiger's conjecture, I found this open problem really interesting. In this article it is written that "in a minimal $k$-coloring of any graph $G$, contracting ...
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3answers
1k views

Obstructions for embedding a graph on a surface of genus g

Kuratowski's theorem tells us the complete graph $K_5$ and the bipartite graph $K_{3,3}$ are the only obstructions to a graph being planar, ie embeddable in the plane with no edge-crossings. Is the ...
5
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1answer
275 views

Ref request: A graph G contains H as a minor iff it contains one of finitely many graphs as a topological minor

For definitions of graph minors and topological minors, see wikipedia's article on graph minors. Theorem: For every graph H, there is a finite set of graphs, say S(H), such that G contains H as a ...