All Questions
Tagged with extremal-graph-theory reference-request
21 questions
3
votes
1
answer
158
views
Sharp upper bound of the number of edges for graphs of thickness two
A graph $G=(V,E)$ has thickness $2$ if $E$ can be written as a disjoint union $E=E_1\cup E_2$ so that $G_1:=(V,E_1),G_2:=(V,E_2)$ are planar graphs. For instance, $K_5$ has thickness $2$. It is known ...
- 1,075
2
votes
1
answer
178
views
Discrete maximization of geometric mean - reference request
This is a follow-up to my previous MO question:
A discrete optimization problem related to the AM-GM inequality
Let $n,k$ be integers such that $1\le k\le n$. Define the quantity
$$
P(n,k):=\max\ a_1\...
0
votes
2
answers
309
views
Reference for a topological result
I am reading the short paper due to Erdös and Bollobás "On a Ramsey-Turán type problem", where they obtain a lower bound for the number of edges on an $n$-graph without $K_4$ as a subgraph ...
- 1,561
5
votes
1
answer
107
views
Maximal graphs with a property that is invariant w.r.t. vertex removal
Let $P$ be a property of graphs such that if a graph $G$ has $P$, then any graph obtained from $G$ by removal of a vertex also has $P$.
Let $g(n)$ be the maximum size of a graph of order $n$ having $P$...
- 34.4k
0
votes
0
answers
55
views
Comparing spectral radius of two graphs using the entry of Perron vector
Suppose we have a graph $G$.
Let $A$ be the adjacency matrix of $G$ and $x$ be the corresponding Perron vector.
Let $x = (x_1,x_2,\cdots,x_n)^t$, where $x_i$ corresponds to the vertex $i \in V(G)$.
We ...
- 199
3
votes
0
answers
108
views
Harper's theorem on the general Hamming graph
Let $G$ be a graph, and for every $S \subseteq V$, let $N(S)$ denote the neighborhood of $S$ (i.e., the set of vertices that have neighbors in $S$). The vertex expansion of $G$ is
$$ \min_{S\subseteq ...
- 419
11
votes
1
answer
396
views
Dense triangle-free graphs and their independent sets
Recall that a graph is triangle-free if it does not contain a copy of $K_3$. Also, for a graph $G$, $\alpha(G)$ shall denote its independence number. Lastly, we will write $o(1)$ to denote quantities ...
- 3,499
1
vote
0
answers
79
views
Partitioning of a set family that avoids small intersections
Let $\mathcal{F}$ be the family of all $k$-element subsets of $[n]$. What is the smallest $\ell$ such that we can partition $\mathcal{F}$ into $\ell$ families $F_1,\dots,F_\ell$ with the property that ...
- 135
1
vote
2
answers
419
views
Graphs constructed from sums of perfect matchings
Consider the following natural procedure for constructing graphs from perfect matchings in graphs with even number of vertices.
Let $V$ be the set of vertices of cardinality $|V|=2n$ and let $\mathcal{...
- 965
3
votes
1
answer
139
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Turán density of an unbalanced complete $r$-partite $r$-graph
In a survey by Füredi and Simonovits called "The history of degenerate (bipartite) extremal graph problems," Theorem 10.5 states the following:
Let $\mathcal K = K^{(r)}(a_1, \dots, a_r)$ ...
- 131
4
votes
2
answers
2k
views
The number of monochromatic triangles
It is well known that the minimum number of monochromatic triangles in a red/blue coloring of the edges of the complete graph $K_n$ is given by Goodman's formula
$$M(n)=\binom n3-\left\lfloor\frac n2\...
- 13.4k
10
votes
1
answer
526
views
Maximum number of triangles no two of which have a common edge
For $n\in N_+$, define $f(n)$ to be the maximum number of triangles in a graph $G$ with $n$ vertices, taken over all $n$-vertex graphs having the property where no two triangles have a common edge.
Do ...
- 475
3
votes
1
answer
766
views
Smallest triangle-free graph with chromatic number 5
The Grötzsch graph is triangle-free and has chromatic number 4. At 11 vertices it is the (unique) smallest graph with these properties.
What is the smallest number of vertices needed for a triangle-...
- 9,114
3
votes
1
answer
179
views
Reference Request: designing a tree of "main roads" in a graph
Let $G = (V, E)$ be an undirected finite connected graph. Let $u$ be a specified vertex of $G$. Then the sum of distances
$$
\sum_{v \in V} d_G(u,v)
$$
is defined. Now we want to decrease this value, ...
- 1,551
2
votes
0
answers
44
views
Estimates for the drop in the sum of all pairwise weighted distances effected by a decrease of the weight of an edge
Consider simple connected undirected graphs $G = (V, E)$ equipped with a function $w\colon E\rightarrow \{x\in \mathbb{R}\colon x\geq 0\}$.
Define a function $d_{G,w}\colon V\times V\rightarrow\...
- 1,551
1
vote
0
answers
54
views
Constructing graphs from subsets of a minimal alphabet
From an alphabet of $N$ letters, choose $n$ pairwise distinct subsets $ v_1,\dots,v_n$ of a fixed size $k$ and define a graph on $V=\{v_1,\dots,v_n\}$, which has an edge for each pair of vertices that ...
- 13.4k
7
votes
0
answers
232
views
The smallest order of a 4-chromatic graph of given girth
Let $n_4(g)$ denote the smallest order of a $4$-chromatic graph with girth $g$. It is known that $n_4(4)=11$ [2] and $n_4(5)=21$ [1]. By a famous proof of Erdös, it is known that $n_4(g)$ is well-...
- 537
1
vote
1
answer
299
views
maximal sets of vertices that avoids a clique
I am looking for some known algorithm that finds, for a given graph, all the maximal sets of vertices that avoid a clique of some given size $k$. I'd prefer one written in MATLAB, but other languages ...
- 11
0
votes
1
answer
398
views
Forbidden Tripartite Graphs
I was looking at extremal graph theory. I have understood the proofs of upper bounds for the Zarankiewicz problem which basically states: What can you say about the edges of a graph with $n$ vertices ...
- 1,129
2
votes
0
answers
78
views
Maximum cardinality general factor of a graph
Given a graph $G=(V,E)$ and a set of integers $B(v)$ associated to each vertex, a general factor of $G$ is a set of edges $F\subseteq E$ such that the degree of each vertex $v\in V$ in the graph $(V, ...
5
votes
2
answers
383
views
Extremal graph theory for directed graphs
In extremal graph theory, there are results such as
$$t(C_4,G)\geq t(K_2,G)^4,$$
where $G$ is an undirected graph, $C_4$ is a cycle graph on 4 nodes, $K_2$ is a complete graph of $2$ nodes, and $t(\...
- 131