All Questions
Tagged with extremal-set-theory extremal-combinatorics
29 questions
2
votes
1
answer
142
views
Bounds for ground set of Steiner system (inverse EKR style problem)
Imagine we have $r$ subsets of a ground set $S$, each of size $k$, such that each set of size $l$ is contained in at most one of the $r$ sets. What can we say about the minimum value of $|S|$? I am ...
- 5,409
5
votes
1
answer
221
views
How many base elements can a sunflower-free system have?
A collection of $t$ sets $A_i$ is called a t-sunflower if $A_i \cap A_j = Z $ for all $i \neq j$ for some fixed $Z$. A well-known conjecture of Erdős and Rado says that there is a constant $C_t$ such ...
- 3,499
2
votes
0
answers
99
views
A variant of the social golfer problem and the kirkman schoolgirl problem
I came across the following simple question that seems to be open:
Let $U$ be a set of $n$ elements.
Let $P_1$ be a partition of $U$ into $k\le n$ "blocks" (i.e. disjoint subsets) and let $...
1
vote
0
answers
45
views
How small must partitions be to ensure overlapping blocks?
Consider the set family $F$ of all $t$-element subsets of $[n]$, for some positive integer $n$.
Let $P_1$ be a partition of $F$ into $k$ blocks.
Let $P_2 \ne P_1$ be another partition of $F$ into $k$ ...
6
votes
3
answers
511
views
Partitioning the 3-sets of [n]={1,...,n} into families
Let $F_1,...,F_m$ be a partition of the 3-element subsets of $[n]$ into families such that no three subsets in any one family $F_i$ are all contained in one 4-element subset of $[n]$. What is the ...
- 32.1k
6
votes
1
answer
277
views
A Sauer-Shelah-like lermma for prefix tree
I proved a variant of the Sauer-Shelah lemma and I was wondering if something like that is already known.
Let $S \subseteq \{0,1\}^n $. We say that a set of coordinates $K \subseteq [n]$ is shattered ...
- 6,652
3
votes
0
answers
124
views
Minimum number of couples of sets with non-empty intersection in a union closed family
Every union closed family $\mathcal{F}$, $\emptyset \notin \mathcal{F}$, with $|\mathcal{F}| = n$ sets, must have at least $\frac{2}{3}\binom{n}{2}$ unordered couples of sets with at least one element ...
- 1,000
6
votes
2
answers
392
views
Coloring of a graph representing the power set
For a positive integer $n$, let $\mathcal{P}$ be the power set of $[n]$. Consider the graph $G$ with $\mathcal{P}$ as its vertex set, and, for $S_1,S_2 \in \mathcal{P}$, the edge $(S_1,S_2)$ exists ...
- 1
1
vote
0
answers
104
views
Number of intersections that must occur in any partition of a given size
Let $\mathcal{S}$ be the set of all $n$-element subsets of $[2n]:=\{1,\dots,2n\}$.
Consider a partition $\mathcal{P}$ of $\mathcal{S}$ into $m$ blocks $P_1,\dots,P_m$, where all except at most one of ...
3
votes
0
answers
95
views
Minimum number of partitions of a set such that the same pair must occur in a block in at least half of them
For positive integers $k$ and $n$, let ${S} = \{1,\dots,k\ n\}$. Consider $\ell \ge 3$ partitions $P_1,\dots,P_\ell$ of ${S}$, where each $P_i$ splits ${S}$ into $n$ blocks all of size $k$.
Question: ...
- 31
2
votes
0
answers
87
views
Restricted Erdos-Ko-Rado theorem reference
Consider a family $\mathfrak{F}$ of $k$ element subsets of $\{1,2,..,n\}$, where $n\geq 2k$, such that any two members of $\mathfrak{F}$ have nonempty intersection. The Erdos-Ko-Rado theorem asserts ...
- 1,949
2
votes
0
answers
59
views
Is the finite projective plane stable as an extremal set system?
Let $\Sigma$ be a set of $|\Sigma| = n$ subsets of the universe $[n]$, each of size $k$, with the property that any two of these subsets intersect on at most one element. It is easy to see that the ...
- 1,389
1
vote
1
answer
168
views
Number of distinct 3-sets formed as subsets of a 4-uniform set family
Let $\mathcal{F} \subseteq \binom{[n]}{4}$ be a collection of size-4 subsets of $\{1,\ldots,n\}$, i.e., a 4-uniform set family.
Let $\mathcal{F'}$ be the collection of size-3 sets for which a ...
- 6,051
2
votes
1
answer
144
views
Abundance in union closed families
For any finite set $S$ and every partition $S_1, \dots, S_n$ of $S$, let $P(S_1, \dots, S_n)$ be the family consisting of all possible unions of $S_1, \dots, S_n$. Clearly, $P(S_1, \dots, S_n)$ is a ...
- 109k
2
votes
1
answer
197
views
Number of members of a separating union-closed family whose universe has given cardinality
If I'm not wrong, it is easy to prove the following statement:
If $n \leq 4$ is a natural number, if $\mathcal{F}$ is a union-closed family of non-empty sets, if the universe of $\mathcal{F}$ (i.e. ...
- 32.1k
13
votes
2
answers
374
views
Set family $\mathcal{F}$ such that for all $A,B,C \in \mathcal{F}$ both $A \cap B \not \subseteq C$ and $C \not \subseteq A \cup B $
This question initially arose out of a question in asymptotic matroid theory. The matroid question has since been answered in a different way, but the extremal set theory question remains unanswered ...
- 32.1k
5
votes
0
answers
163
views
A Combinatorial Problem on Extremal Set Theory
Given a ground set $[n]$, under what condition of parameters $a,b,c$ does a family of subsets $\mathcal{F}\subseteq 2^{[n]}$ with the following property exist?
(i) $\forall S\in \mathcal{F}$, $|S|=a$....
- 37.7k
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(...
CommunityBot
- 1
8
votes
1
answer
1k
views
On a result of Frankl and Wilson
In the paper 'Intersection theorems with geometric consequences' (Combinatorica 1981) P. Frankl and R. M. Wilson consider families $\mathcal{F}$ of $k$-subsets of $\{1,\dots,n\}$ with the restriction ...
- 32.1k
3
votes
0
answers
133
views
Kruskal-Katona for multisets?
Following Fedor Petrov's remarks, here is a "set-theoretic version" of the
question I asked a while ago.
For integer $n\ge 1$, denote by $\mathcal M_n$ the family of all (finite)
multisets with the ...
CommunityBot
- 1
12
votes
2
answers
425
views
Set system with different differences
What is the maximal number of sets in a set system $\mathcal{A}$ of subsets of an $n$ element set such that for every $i \neq j $ and $A_i,A_j \in \mathcal{A}$ the difference $A_i \setminus A_j$ is ...
- 3,004
4
votes
0
answers
114
views
Kruskal-Katona for homocyclic groups?
I need a version of the Kruskal-Katona theorem (better still, of the Lovasz "approximate" version thereof) for the elementary abelian / homocyclic groups, in the following spirit:
What is the ...
- 23k
6
votes
1
answer
613
views
Given k, what is the minimum n such that n choose n/2 is greater than k? [closed]
I'm not an expert in combinatorics, but it sometimes comes up in my research with students in computer science (which is already pretty far away from my speciality of abstract homotopy theory). I just ...
- 96.4k
2
votes
1
answer
877
views
Combinatorics-the maximum number of subsets with a given property
Let $X$ be a set with $n$ elements. I would like to know the maximum number of subsets of $X$ such that the number of elements in the symmetric difference between any two of these subsets is at most $...
- 23.7k
3
votes
0
answers
102
views
What is the maximal number of partitions with this maximal intersection property?
Let $X = \{ 1, \dots, n = sk \}$ be a finite set. Let $\mathscr P, \mathscr Q$ be equi-partitions of $X$ into $k$ sets of size $s$. Denote by $V(\mathscr P, \mathscr Q)$ the maximum size of ...
- 726
3
votes
1
answer
215
views
Minimal family of k-sets containing all t-sets
Let $n \ge k \ge t \in \mathbb{N}$, and consider a universe $U$ of size $n$. Let $\mathcal{F}$ be a family of $k$-subsets of $U$, such that every $t$-subset of $U$ is contained in at least one member ...
- 419
0
votes
1
answer
181
views
Generalized Helly theorem for $t$-intersecting families
Given a family $\mathcal{F}$ of sets over ground set $X$, let $\tau(\mathcal{F})$ be the transversal number (aka blocking number), that is the cardinality of the smallest set of points $E \subseteq X$ ...
- 19.1k
7
votes
1
answer
774
views
Upper bound for the size of a $k$-uniform $s$-wise $t$-intersecting set system
Given integers $n \geq k \geq t \geq 1$ and an integer $s$, let $m(n,k,s,t)$ denote the maximum size of a family $\mathcal F$ of $k$-subsets of $[n]$, i.e. $\mathcal F \subseteq \binom{[n]}{k}$, such ...
- 3,692
8
votes
0
answers
1k
views
The state of art of the sunflower lemma
I am interesting in the sunflower system and its applications in computer science.
Given a Universe $U$ and a collection of $k$ sets $A_i$ is called a k-sunflower system if $A_i \cap A_j = Y $ for ...
- 393