All Questions
39 questions
23
votes
3
answers
3k
views
Cauchy-Schwarz proof of Sidorenko for 3-edge path (Blakley-Roy inequality)
Is there a "Cauchy-Schwarz proof" of the following inequality?
Theorem. Given $f \colon [0,1]^2 \to [0,1]$, one has
$$
\int_{[0,1]^4} f(x,y)f(z,y)f(z,w) \, dxdydzdw \geq \left(\int_{[0,1]^2} f(x,y) \,...
- 986
11
votes
0
answers
195
views
Number of triangle-free graphs with prescribed number of edges
This question is posted from StackExchange since it received no answer there.
Let $f(n, e)$ be the number of triangle-free graphs on $n$ vertices and $e$ edges. From empirical evidence, I am motivated ...
- 384
7
votes
1
answer
514
views
A proper definition of connectivity for hypergraphs
For usual graphs on $n$ vertices, a edge-minimal connected graph is nothing but a spanning tree of this graph. It is well-known that any spanning tree has $n-1$ edges.
I would like to know whether ...
- 121
6
votes
1
answer
234
views
Graphs without short cycles and with linear number of edges
Let $f : \mathbb{N} \rightarrow \mathbb{N}$ be a non-decreasing function and let $X_f$ be the class of graphs where every $n$-vertex graph $G$ is $(C_3, C_4, \ldots, C_{f(n)})$-free, i.e. $G$ contains ...
- 655
6
votes
2
answers
349
views
extremal bipartite graph
I'm facing the following question:
Given a bipartite graph $G = (L \cup R, E)$.
Let $n = |L|$, $m = |R|$, and a parameter $k \in \mathbb{N}$, $n > m > k$.
What is a minimal possible number of ...
- 161
4
votes
1
answer
887
views
Existence of triangle-free graphs for regular graphs of degree at most n/2
It is known that for triangle-free graphs, if they are $d$-regular, then $2d\leq n$, where $n$ is the number of vertices. In words, the degree is less than or equal to half the number of vertices (...
- 43
4
votes
1
answer
437
views
Extremal functions for tournaments
We are looking at directed graphs with no loops or parallel edges, but given two vertices $x$ and $y$, we allow the presence of both the edge $(x, y)$ and $(y, x)$. Thus, if $G$ is a directed graph ...
- 677
4
votes
1
answer
220
views
Maximal number of perfect matchings that pairwise form a Hamiltonian cycle
Definition: Let $MH(n)$ be the maximal number of perfect matchings (1-regular graphs) on $n$ vertices where the union of any two perfect matchings is a Hamiltonian cycle.
Question: Is it true that $MH(...
- 3,004
4
votes
1
answer
376
views
Looking for a counterexample to a strengthening of the union-closed sets conjecture
[Now crossposted at math.stackexchange]
Let $\mathcal{F} = \{\{x_1, x_2\} : 1 \le x_1 \lt x_2 \le n \}$, $n \ge 8$, and let $\mathcal{G} = \{G_1, \ldots, G_n\}$ be a partition of $\mathcal{F}$ in $n$ ...
- 700
4
votes
0
answers
113
views
What properties do graphs avoiding large regular subgraphs have?
Fix a positive integer $r$ and real $\delta \in (0,1)$.
Let $G$ be an undirected graph on $n$ vertices. Suppose that $G$ does not contain an $r$-regular subgraph on at least $\delta n$ vertices (i.e., ...
- 557
4
votes
0
answers
104
views
Maximal number of smallest circuits in a matroid
It is known (see here for example) that, in a simple graph of odd genus $g$ with $n$ vertices and $m$ edges, the number of cycles of lenght $g$ is at most $\frac{n(m-n+1)}{g}$.
Since this can be be ...
- 3,446
3
votes
1
answer
155
views
Does Sidorenko's conjecture hold when the host graph's maxdegree/mindegree is a constant?
Does the following holds?
For every bipartite graph $H$ and every graph $G$ with $\frac{\Delta(G)}{\delta(G)}\leq 2$,
$$t(H,G)\geq t(K_2, G)^{e(H)}.$$
If not sure, is this a equal question as ...
- 359
3
votes
2
answers
276
views
Ramsey-Turán density function is well defined
Define
$$RT(n,K_l,f(n))=ex_l(n,f(n))=\max_G\{e(G): K_l \not\subset G, v(G)=n, \alpha(G)\leq f(n)\}$$
and the Ramsey-Turán density function $f_l:(0,1] \to \mathbb{R}$ as
$$f_l(\alpha)=\lim_{n\to \infty}...
- 227
3
votes
1
answer
289
views
Graph properties that imply a bounded number of edges
Many combinatorial problems can be reduced to bounding the number of edges in a given graph with $n$ vertices. Each time I encounter such a problem, I check whether the corresponding graph has a ...
- 1,072
3
votes
1
answer
121
views
The least number of edges to add to a tree that would force a certain number of edge-disjoint cycles
Let $c(n,k)$ be the least integer such that if $G$ is a simple graph on $n$ vertices with $n + c(n,k) - 1$ edges then $G$ has $k$ edge-disjoint cycles.
Clearly, $c(n, 1) = 1$ and it not very hard to ...
- 1,034
3
votes
0
answers
70
views
Boundary differences in two graphs
Let $\Gamma, \Xi$ be two graphs with the same set of vertices $V$ with $n$ elements. Assume $\Gamma$ is connected. Write $\Gamma\cup \Xi$ (or $\Gamma\cap \Xi$) for the graph whose set of edges is the ...
- 20.2k
3
votes
0
answers
66
views
An extremal problem in directed path systems
The following is a common rephrasing of the well-known open problem in extremal graph theory to (asymptotically) determine $ex(n, C_8)$:
What is the asymptotically maximum $L = L(n)$ such that ...
- 1,389
2
votes
2
answers
143
views
Regarding a specific Turán number of graphs
I wish to know the latest bound on the number of edges a graph of girth greater than or equal to $t$ can have.
Specifically, I heard somewhere that a graph of girth greater than or equal to $t$ can ...
- 2,089
2
votes
1
answer
356
views
Extremal combinatorics on bipartite graphs
One open question in extremal graph Theory is the so-called Zarankiewicz problem
(see for instance the wikipedia page), which ask for the maximum number of edges in a bipartite graph with a fixed ...
- 1,561
2
votes
1
answer
300
views
Do sparse graphs contain a single regular pair?
An easy corollary of the Szemerédi Regularity Lemma is that dense graphs contain linear sized $\varepsilon$-regular bipartite subgraphs whose density is similar to that of the parent graph. As noted ...
- 805
1
vote
2
answers
391
views
Lower bound for the size of a family of sets
Consider a family $\mathcal{G} = \{ A_1,B_1,\ldots,B_m \}$ of $m+1$ non-empty finite distinct sets with the following property:
$$A_1 \cap B_k = \emptyset, 1 \le k \le m$$
Let $\mathcal{F} = \{A_1 \...
- 700
1
vote
1
answer
125
views
Extremal graph theory - many copies of $K_r$ imply a copy of $r$-chromatic $H$
I know that it must be a simple consequence of the Kővári–Sós–Turán (and Erdős–Stone) theorem, but I am struggling to formulate a proof: Let $H$ be a fixed-size $r$-chromatic graph. Then there exists $...
1
vote
1
answer
75
views
Lower bound for the sum of the number of vertices of some subgraphs of a directed graph
Let $G$ be any simple weakly connected directed graph with vertices $V$, $\vert V \vert = n$. Let $V_1, \ldots, V_m$, $m = \binom{n}{k}$ be all subsets of $V$ of size $k$.
Let $C(V_i)$ be the union of ...
- 700
1
vote
2
answers
360
views
Tight bound of Turan number for K_{1,t,t}
I'm looking for a tight bound for Turan number $ex_2(n,K_{1,t,t})$, where $K_{1,t,t}$ is the complete 3-partite graph with parts of size 1, t, and t.
The motivation is that we now $ex_2(n,K_{t,t})=O(...
- 281
1
vote
2
answers
205
views
Extremal density of a graph without a non-backtracking $2k$-cycle
The current best bound for the maximum possible density of an $n$-node graph with girth (shortest cycle length) $>2k$ is of the form
$$ex(n \ \mid \ C_{\le 2k}) = O(n^{1 + 1/k}),$$
while the ...
- 1,389
1
vote
0
answers
141
views
Specific regularity in bipartite graphs
Let $G(A,B)$ be a bipartite graph with $|A| = |B| = n$, where $n$ is sufficiently large(thus, $o(n)/n,o(n^2)/n^2\ll 1$). The edge density of $G$ is $d = \frac{e(A,B)}{n^2}$, where $e(A,B)$ denotes the ...
- 359
1
vote
0
answers
63
views
Is there any other norms besides cut norm defined on graphon?
Let $\mathcal{W}$ denote the space of all bounded symmetric measurable functions
$W : [0, 1]^2 \rightarrow \mathbb{R}.$ For any $W\in\mathcal{W}$ we say it is a kernel and define its cut norm $\lVert ...
- 359
0
votes
1
answer
1k
views
Vertex cover of regular graph
(1.) How small can set $S$ of vertices in any regular undirected graph $G$ on $n$ vertices with degree $\Omega(n^\alpha)$ where $\alpha\in(0,1)$ can be such that every edge in the graph is incident on ...
user76479
0
votes
1
answer
143
views
A general Turan-like question
Thinking of an edge as of a $2$-clique, it's natural to consider a slightly more general question than Turan considered in his celebrated theorem: given $r \le k \le n$, what is the maximal possible ...
0
votes
1
answer
424
views
Reference for Turan Density
I am working a 3-graph problem. I convert it to calculate Turan density, that is $lim_{n\to \infty}\frac{ex_3(n,F)}{\binom{n}{3}}$, where F is a3-graph. I'd like to know are there some methods and ...
- 281
0
votes
0
answers
57
views
Does Forcing conjecture equals to assume the host graph is regular?
Given two graphs $H$ and $G$, the homomorphism density $t(H, G)$ is defined as the proportion of mappings from the vertices of $H$ to the vertices of $G$ that preserve adjacency. Formally,
$$
t(H, ...
- 359
0
votes
0
answers
45
views
Another version of Sidorenko's conjecture(?)
I would like to ask a question about Sidorenko's conjecture. Here is the background of my question:
Quasi-random graphs
A sequence of graphs $(G_n)$ is called quasi-random if it satisfies certain ...
- 359
0
votes
0
answers
51
views
Inverse problem of "graph limits to graphon"
A graphon is a measurable symmetric function $W: [0,1]\to [0,1].$ By Lovasz's book "Large networks and graph limits" we know for any graph sequence $G_1, G_2, \dots G_i,\dots$ there exists a ...
- 359
0
votes
0
answers
67
views
Does Sidorenko's conjecture hold when the host graph's edge density not too small?
Does the following hold?
For every bipartite graph $H$ and every graph $G$ with $e(G)\geq 0.1(v(G))^2$,
$$t(H,G)\geq t(K_2, G)^{e(H)}.$$
If not sure, is this a equal question as Sidorenko's conjecture ...
- 359
0
votes
0
answers
68
views
Proving we can minimize the number of crossings by having a planar embedding of $K_{2,2}$ encircle another out of any 2 such embeddings
Say that we draw a graph in the following way: we first draw $n$ planar embeddings of $K_{2,2}$ (that is, we first draw $n$ quadrilaterals) such there are no edges which cross. Then for each of the $...
- 1
0
votes
0
answers
43
views
Locally uniformly convexity in kernels (generalized definition of graphon) with cut norm
Let $\mathcal{W}$ denote the space of all bounded symmetric measurable functions
$W : [0, 1]^2 \rightarrow \mathbb{R}.$ For any $W\in\mathcal{W}$ we say it is a kernel and define its cut norm $\lVert ...
- 359
0
votes
0
answers
52
views
Does "epsilon-regular" equal to "cut distance less than epsilon"?
Let $G$ be a bipartite graph (vertex number sufficient large) with bipartition $(U,W)$ and edge density $d$. Does these two statement equal?
$G$ is $\varepsilon$-regular, i.e. $\big|e_G(X,Y)-d|X||Y|\...
- 359
0
votes
0
answers
49
views
Property of edge-vertex transitive graphs
Recently I am reading a paper (https://arxiv.org/abs/1504.00858) with respect to edge-vertex transitive graphs. What is the property of the graph that is edge transitive and vertex transitive? I know ...
- 359
0
votes
0
answers
39
views
Does this "linear-approximated" version of Graph Counting Lemma hold?
Let $0\leq d\ll\varepsilon,\frac{1}{e},\frac{1}{v}\leq 1.$ Let $G$ be a $n$-vertices graph ($n$ is sufficient large, $1/n\ll d$) and for any $A,B\subseteq V(G)$, the edge density $d(A,B)\geq d.$ Then ...
- 359