All Questions
65 questions with no upvoted or accepted answers
24
votes
0
answers
760
views
How much of the plane is 4-colorable?
In 1981, Falconer proved that the measurable chromatic number of the plane is at least 5. That is, there are no measurable sets $A_1,A_2,A_3,A_4\subseteq\mathbb{R}^2$, each avoiding unit distances, ...
- 7,633
14
votes
0
answers
522
views
Reconstruction conjecture and partial 2-trees
Reconstruction conjecture says that graphs (with at least three vertices) are determined uniquely by their vertex deleted subgraphs. This conjecture is five decades old.
Searching relevant literature,...
- 281
11
votes
0
answers
228
views
Is there a term for this graph subset?
Suppose $G$ is a (finite) graph which is $k$-vertex colourable (i.e. $\chi(G)\leqslant k$). Suppose $S$ is a set of vertices of $G$ with the following property:
If $c:V(G)\rightarrow [k]$ is a vertex ...
- 211
8
votes
0
answers
181
views
Self-avoiding walks on strips
A strip is a locally finite graph which admits a quasi-transitive (i.e. finitley many orbits on vertices) action of $\mathbb Z$. A self avoiding walk is a walk which visits no vertex more than once.
...
- 2,671
8
votes
0
answers
149
views
Edge-colorings of plane graphs: do you know references where the following questions are studied?
Let $G$ be a plane graph (or more generally, a graph embedded on a surface) with a proper edge-coloring of $G$ with $k$ colors $\{1,\ldots,k\}$. I am interested in studying the cyclic permutations of ...
- 537
8
votes
0
answers
2k
views
What is the best lower bound for the domination number in regular graphs of girth 5?
The following theorem is a classical result (see [Alon and Spencer, The probabilistic method, 2nd ed., Theorem 1.2.2]):
Theorem: Let $G$ be a graph on $n$ vertices with minimum degree $d$. Then $G$ ...
- 161
8
votes
0
answers
152
views
Disjoint Rooted Paths with Specified Patterns
Let $S:=$ { $s_i : i \in [k]$ } and $T:=$ { $t_i : i \in [k]$ } be disjoint subsets of vertices of a graph $G$. Furthermore, let $A$ be a subset of $S_k$ (the symmetric group on $[k]$). A set of ...
- 32.1k
7
votes
0
answers
97
views
What is known about chromatic polynomial of hypergraph at $-1$
Let $H$ be a hypergraph and let $P_H$ denote its chromatic polynomial. I am interested in the best results interpreting $P_H(-1)$. I am interested both in the general case (which I think is hard) as ...
- 7,875
7
votes
0
answers
74
views
Graphs all of whose cuts are positive
Let $(V, E, w)$ a weighted graph, with vertices $V$, edges $E$, and signed weight $w:E\to \mathbb R$.
I am interested to know other popular properties that are known to imply, or are equivalent to, ...
- 2,041
7
votes
0
answers
171
views
What is known about the distribution of lengths of the cycle you get by adding an edge to a uniform spanning tree?
Let $G$ be a finite, connected graph. Let $T$ be a uniform spanning tree, and let $e$ be a uniformly random edge not in $T$. When we add $e$ to $T$, we get a subgraph with a unique cycle, $C$. I am ...
- 1,462
7
votes
0
answers
281
views
Relations between Betti numbers for clique complex
Given a clique complex $K$ constructed from a discrete set of vertices (i.e. its faces are isomorphic to the set of cliques in the 1-skeleton of $K$.), it seems that the Betti numbers $\beta_k$ ...
- 8,071
7
votes
0
answers
229
views
Has anyone seen these binary trees (Catalan-type related to the Gegenbauer polynomials and Motzkin paths)?
The OEIS entry A121448 enumerates binary trees with $n$ edges and $k$ vertices with outdegree 1.
Has anyone seen these trees?
The o.g.f. for this entry, $G(x,t)$, is essentially a discriminant ...
- 10.5k
6
votes
0
answers
373
views
Circle numbers on edges of a graph
Let $k$ vertices in a graph be given. Some pairs of vertices are connected by an edge, each edge is labeled either $\{1,2\}$, $\{1,3\}$, or $\{2,3\}$. We can circle some of the numbers on the edges. ...
- 277
6
votes
0
answers
116
views
Chromatic numbers for coloring-constrained graphs
I am interested in any and all articles about chromatic numbers applying to constrained colorings of a graph. For example, if a graph must be (properly) colored so that there is a 2-color path ...
- 91
6
votes
0
answers
359
views
Have topographs been studied before?
This is my first post on MO so I hope this question is suitable. I have quite a few definitions which I will need to state before my questions at the end of this post. Please let me know if anything ...
- 661
5
votes
0
answers
141
views
If chromatic polynomials for two graphs agree, can I always find an edge such that the two deletion-contraction minors have same chromatic polynomial?
Suppose I have non-isomorphic graphs $G$ and $H$ (which have at least one edge), but such that their chromatic polynomials are the same. Can I then always find an edge $e$ in $G$ and $f$ in $H$ such ...
- 15.8k
5
votes
0
answers
231
views
Schröder and graphical logic?
I was actually surprised by a comment by John Baez over at the n-Category Cafe about his surprise that Ernst Schröder, a mathematician of whom he had known through Schröder's work on mathematical ...
- 10.5k
5
votes
0
answers
102
views
An upper bound on the minimum number of vertices in a girth 5 graph of chromatic number $k$
Is there a known upper bound on the minimum number of vertices in a graph with girth 5 and chromatic number $k$? Could you also give references for this?
- 51
4
votes
0
answers
89
views
How to measure the optimality of the induced order by a median order of a tournament on a big subset
Median orders are great tools for dealing with a-priori unknown orientations of edges in tournaments, because they provide us with local properties on oriented edge density.
I've been wondering if ...
- 71
4
votes
0
answers
228
views
How many arrangements of $n$ points with $k$ edge lengths exist in $d$ dimensions?
[Asking on behalf of a high school mathematics course, but responses written at any level are welcome!]
I was recently reading over a nice puzzle called the four points, two distances problem:
...
- 7,831
4
votes
0
answers
282
views
Reference for results about planar graphs
A colleague and I are writing a paper in which we need to make use of some basic facts about planar graphs. I would strongly prefer to simply give references for the results if possible, because the ...
- 2,339
4
votes
0
answers
207
views
Have wiring diagrams been generalized to arbitrary digraphs?
A "combinatorial wiring diagram" is a way to define a permutation by a drawing of a particular planar digraph. For example, this wiring diagram corresponds to the permutation $(3412)$:
In Coxeter ...
- 1,389
4
votes
0
answers
123
views
A color interpolation lemma
I need the following "color interpolation lemma". Actually I know a way to prove it, but I'm not very satisfied with that proof.
Lemma. Let $G=(V,E)$ be a (properly) colored graph with colors $1, \...
- 2,479
4
votes
0
answers
111
views
A relaxation of proper coloring
I am wondering if the following relaxation of proper coloring appears somewhere. I have tried some searching and have found a few relaxations of proper coloring, but none the coincides with what I ...
- 7,875
3
votes
0
answers
61
views
Is this bipartite equivalent of 1-walk-regular graphs known?
A graph $G$ is 1-walk-regular if
for each vertex $v$ the number of closed walks of length $\ell$ starting (and ending) at $v$ depends only on $\ell$ but not on $v$.
for each edge $vw$ the number of ...
- 13.6k
3
votes
0
answers
144
views
Counting homologically non-trivial and trivial cycles in $n \times n$ square lattice torus of a given length $l \geq n$
This should be a fairly standard question but I can't really seem to find a reference.
Consider an $n \times n$ square lattice torus $\mathbb T$. Given a length $l \geq n$, what is the number of ...
- 269
3
votes
0
answers
346
views
Terminology for transforming a directed acyclic graph into a tree
I am looking for the term of converting a directed acyclic graph (DAG) into a tree by traversing its topologically ordered nodes and copying the subtrees of the nodes with in-degree $> 1$.
Such a ...
- 265
3
votes
0
answers
55
views
Efficiently drawing graph of maximum degree $3$ with at most $o(n^2)$ crossings
Let $G$ be simple finite graph of order $n$ and maximum
degree $3$.
Can we efficiently draw $G$ with at most $o(n^2)$ crossings?
"Efficiently" means in time polynomial in $n$ or at worst
$\exp{o(n^2)...
- 25.4k
3
votes
0
answers
342
views
Finding many disjoint sub-trees with many leaves
Let $T$ be a rooted binary tree with $L$ leaves, and let $\ell$ be a natural number smaller than $L$. The question is what is the maximal number of disjoint rooted sub-trees with at least $\ell$ ...
- 419
2
votes
0
answers
65
views
Structure Theory for Tree Decompositions
I that $G=(V,E,W)$ is a weighted graph with positive edge weights and a finite set of vertices $K$. Let $0\le k,M\le K$ be a fixed integer.
Is is known when $G$ admits the following type of ...
- 21
2
votes
0
answers
165
views
Has Mac Lane's article "When can a graph be mapped on a torus?" been published anywhere?
I came across the following abstract of an article: Mac Lane, S., When can a graph be mapped on a torus?, Bull. Amer. Math. Soc. 42(9), 629 (1936). Abstract #341. MR1563375, JFM 62.0694.07.
Q. Does ...
- 1,446
2
votes
0
answers
86
views
Optimal paths in set-weighted graphs
Let $G = (V,E)$ be an $n$-vertex graph, let $R$ be a finite set (to be specific, let us assume that $R = [n]$), and let $W : V \rightarrow 2^R$. Let us call the pair $(G, W)$ a set-weighted graph.
Now ...
- 655
2
votes
0
answers
124
views
Graphs which are built from complete graphs : Reference request
Let $V$ be a set of $n$ vertices. Fix $3 \le k \le n$. Let $\binom V k$ be the set of all $k$ element subsets of $V$.
We add the edges in $V$ as follows: Let $\mathcal S \subseteq \binom V k$ be ...
- 1,269
2
votes
0
answers
140
views
Lower bound for the chromatic number in terms of minimum feedback vertex set
Let $MFVS(G)$ denote the size of minimum feedback vertex set of $G$.
We believe we proved $\chi(G) \ge (|G| - MFVS(\overline{G}))/2$
and this bound is sharp.
Is this known or trivial result?
This ...
- 25.4k
2
votes
0
answers
30
views
Graph vertex label dynamics, statistical model reference request
I am modeling some type of social interaction, and came up with the following natural question.
Let $K_n$ be the complete graph on $n$ vertices, with some initial edge labeling in some alphabet $A$.
...
- 15.8k
2
votes
0
answers
50
views
An equation involving fractional covering number of hypergraphs
Let $\mathcal{H}=(S,\mathcal{X})$ be a hypergraph, where $S = \{ s_1, \ldots, s_n \}$, and $\mathcal{X} = \{ X_1, \ldots, X_m \}$.
The dual hypergraph $\mathcal{H}^*$ of $\mathcal{H}$ is the ...
- 655
2
votes
0
answers
78
views
Maximum cardinality general factor of a graph
Given a graph $G=(V,E)$ and a set of integers $B(v)$ associated to each vertex, a general factor of $G$ is a set of edges $F\subseteq E$ such that the degree of each vertex $v\in V$ in the graph $(V, ...
2
votes
0
answers
116
views
Isomorphic subcategories of directed graphs and presets
For the purposes of this post, a digraph (directed graph) has neither loops nor multiple parallel edges, and a preset is an ordered pair consisting of a set $S$ and a preorder (viz., a reflexive and ...
- 10.5k
2
votes
0
answers
216
views
Polyhedral embeddings of large face-width where all faces have the same length
Where can I find examples of polyhedral embeddings of simple graph with large face-width, such that all the faces have the same length?
By polyhedral embedding I mean an embedding of the graph on a ...
- 884
2
votes
0
answers
642
views
Hamiltonian paths in subgraphs of rectangular lattice graphs
Is following decision problem NP-hard / NP-complete:
Having vertex-induced subgraph of rectangular lattice graph determine if any Hamiltonian path exists
Having vertex-induced subgraph of rectangular ...
1
vote
1
answer
177
views
Spectral characterization of complete or complete bipartite graphs
The Lemma 6 in this paper mention the following spectral characterization of complete or complete bipartite graphs:
Let $G$ be a connected graph with $\ge 2$ vertices. Then $\lambda_2=...=\lambda_{n-...
- 243
1
vote
0
answers
74
views
Keller's cubing conjecture but with arbitrary cubes of side $1$
These days I have been reading about Keller's cube tyling conjecture, which asks if in any covering of $\mathbb{R}^n$ by translates of $[0,1]^n$ with disjoint interiors there are two cubes sharing one ...
- 10.6k
1
vote
0
answers
385
views
Counting number of spanning trees of the complete bipartite with given vertex-degrees
For given $n_1,n_2 \in \mathbb{N}$ let $K_{n_1,n_2}$ be the complete bipartite graph. I have seen a few sources proving that the number of spanning trees $t(K_{n_1,n_2})$ is given by $n_1^{n_2-1} n_2^{...
- 1,295
1
vote
0
answers
152
views
Is this graph theory paper in German translated into English?
I recently read such a paper and want to understand the proof idea of this article. However since it is in German and I have not studied German before, I'd like to ask whether this paper has an ...
- 1,851
1
vote
0
answers
35
views
Term or reference for a set of integer edge weights to guarantee distinct weighted degrees
I am looking for a term or reference describing sets $S$ of $\binom{n}{2}$ non-negative integers such that, for every bijection $w: E(K_n)\to S$ and every pair of distinct vertices $u$ and $v$ in $V(...
- 11
1
vote
0
answers
134
views
Counting unions of unlabelled connected graphs
My question can be stated as follows: let $X$ be a hereditary family of unlabelled graphs closed under disjoint unions. Suppose we know, for each $n$, the number $c_n$ of connected graphs in X on $n$ ...
- 183
1
vote
0
answers
121
views
Cheeger constant of truncated hypercube
Look at the $d$-dimensional hypercube and truncate it. This means one replaces each vertex by a cycle (of length $d$) in such a way the the new graph is 3-regular.
Question 1: What is the asymptotic ...
- 4,432
1
vote
0
answers
81
views
Matchings in infinite, not necessarily bipartite, graphs
Aharoni, Nash-Williams, and Shelah have extended the famous marriage theorem for finite bipartite graphs due to Hall to arbitrary graphs.
Is there a similar generalization of Tutte's theorem on ...
- 48.9k
1
vote
0
answers
29
views
Reference request for blocking sets in graphs
Let $G = (V,E)$ be a DAG and $S$ be a subset of $V$. I will call $S$ $k$-blocking if on every path of length $k~$ there is at least one node from $S$.
This parameter sounds reasonable enough, so I ...
- 673
1
vote
0
answers
151
views
Asymptotic estimation of numbers of unlabeled graphs whose degrees of vertices are bounded
It is known(Enumeration of graphs with a given and bounded degree sequence) that there is no a closed form formula for number of (labeled) graphs with bound on degree of vertices. Thus what I want to ...
- 8,071