Set families with a forbidden subposet

Forbidden induced subposets in the grid

In this short paper, we prove the following generalization of a result of Methuku and Pálvölgyi. Let $P$ be a poset, then there exists a constant $C_{P}$ with the following property. Let $k$ and $n$ be arbitrary positive integers such that $n$ is at least the dimension of $P$, and let $w$ be the size of the largest antichain of the grid $[k]^{n}$ endowed with the usual pointwise ordering. If $S$ is a subset of $[k]^{n}$ not containing an induced copy of $P$, then $|S|\leq C_{P}w$.

On families of subsets with a forbidden subposet

Let $\F\subset 2^{[n]}$ be a family of subsets of $\{1,2,..., n\}$. For any poset $H$, we say $\F$ is $H$-free if $\F$ does not contain any subposet isomorphic to $H$. Katona and others have investigated the behavior of $\La(n,H)$, which denotes the maximum size of $H$-free families $\F\subset 2^{[n]}$. Here we use a new approach, which is to apply methods from extremal graph theory and probability theory to identify new classes of posets $H$, for which $\La(n,H)$ can be determined asymptotically as $n\to\infty$ for various posets $H$, including two-end-forks, up-down trees, and cycles $C_{4k}…

Set families with forbidden subposets

Let $F$ be a family of subsets of $\{1,\ldots,n\}$. We say that $F$ is $P$-free if the inclusion order on $F$ does not contain $P$ as an induced subposet. The \emph{Turán function} of $P$, denoted $π^*(n,P)$, is the maximum size of a $P$-free family of subsets of $\{1,\ldots,n\}$. We show that $π^*(n,P) \le (4r + O(\sqrt{r}))\binom{n}{n/2}$ if $P$ is an $r$-element poset of height at most $2$. We also show that $π^*(n,S_r) = (r+O(\sqrt{r}))\binom{n}{n/2}$ where $S_r$ is the standard example on $2r$ elements, and that $π^*(n,B_2) \le (2.583+o(1))\binom{n}{n/2}$, where $B_2$ is the $2$-dimension…

Forbidden subposet problems for traces of set families

In this paper we introduce a problem that bridges forbidden subposet and forbidden subconfiguration problems. The sets $F_1,F_2, \dots,F_{|P|}$ form a copy of a poset $P$, if there exists a bijection $i:P\rightarrow \{F_1,F_2, \dots,F_{|P|}\}$ such that for any $p,p'\in P$ the relation $p<_P p'$ implies $i(p)\subsetneq i(p')$. A family $\mathcal{F}$ of sets is \textit{$P$-free} if it does not contain any copy of $P$. The trace of a family $\mathcal{F}$ on a set $X$ is $\mathcal{F}|_X:=\{F\cap X: F\in \mathcal{F}\}$. We introduce the following notions: $\mathcal{F}\subseteq 2^{[n]}$ is $l$-trac…

Induced and non-induced forbidden subposet problems

The problem of determining the maximum size $La(n,P)$ that a $P$-free subposet of the Boolean lattice $B_n$ can have, attracted the attention of many researchers, but little is known about the induced version of these problems. In this paper we determine the asymptotic behavior of $La^*(n,P)$, the maximum size that an induced $P$-free subposet of the Boolean lattice $B_n$ can have for the case when $P$ is the complete two-level poset $K_{r,t}$ or the complete multi-level poset $K_{r,s_1,\dots,s_j,t}$ when all $s_i$'s either equal 4 or are large enough and satisfy an extra condition. We also sh…

A simple discharging method for forbidden subposet problems

The poset $Y_{k+1, 2}$ consists of $k+2$ distinct elements $x_1$, $x_2$, \dots, $x_{k}$, $y_1$,$y_2$, such that $x_1 \le x_2 \le \dots \le x_{k} \le y_1$,~$y_2$. The poset $Y'_{k+1, 2}$ is the dual of $Y_{k+1, 2}$ Let $\rm{La}^{\sharp}(n,\{Y_{k+1, 2}, Y'_{k+1, 2}\})$ be the size of the largest family $\mathcal{F} \subset 2^{[n]}$ that contains neither $Y_{k+1,2}$ nor $Y'_{k+1,2}$ as an induced subposet. Methuku and Tompkins proved that $\rm{La}^{\sharp}(n, \{Y_{3,2}, Y'_{3,2}\}) = Σ(n,2)$ for $n \ge 3$ and they conjectured the generalization that if $k \ge 2$ is an integer and $n \ge k+1$, the…

Supersaturation and stability for forbidden subposet problems

We address a supersaturation problem in the context of forbidden subposets. A family $\mathcal{F}$ of sets is said to contain the poset $P$ if there is an injection $i:P \rightarrow \mathcal{F}$ such that $p \le_P q$ implies $i(p) \subset i (q)$. The poset on four elements $a,b,c,d$ with $a,b \le c,d$ is called butterfly. The maximum size of a family $\mathcal{F} \subseteq 2^{[n]}$ that does not contain a butterfly is $Σ(n,2)=\binom{n}{\lfloor n/2 \rfloor}+\binom{n}{\lfloor n/2 \rfloor+1}$ as proved by De Bonis, Katona, and Swanepoel. We prove that if $\mathcal{F} \subseteq 2^{[n]}$ contains $…

Characterizing subclasses of cover-incomparability graphs by forbidden subposets

In this paper we continue investigations of cover-incomparability graphs of finite partially ordered sets (see \cite{Bres,Bres2,Bres3,Bres4} and \cite{Max,MaxDH}). We consider in some detail the distinction between cover-preserving subsets and isometric subsets of a partially ordered set. This is critical to understanding why forbidden subposet characterizations of certain classes of cover-incomparability graphs in \cite{Bres} and \cite{Bres3} are not valid as presented. Here we provide examples, investigate the root of the difficulties, and formulate and prove valid revisions of these charact…

Supersaturation, counting, and randomness in forbidden subposet problems

In the area of forbidden subposet problems we look for the largest possible size $La(n,P)$ of a family $\mathcal{F}\subseteq 2^{[n]}$ that does not contain a forbidden inclusion pattern described by $P$. The main conjecture of the area states that for any finite poset $P$ there exists an integer $e(P)$ such that $La(n,P)=(e(P)+o(1))\binom{n}{\lfloor n/2\rfloor}$. In this paper, we formulate three strengthenings of this conjecture and prove them for some specific classes of posets. (The parameters $x(P)$ and $d(P)$ are defined in the paper.) $\bullet$ For any finite connected poset $P$ and $\va…

线性格中的禁止子偏序集问题

Forbidden subposet problems in the linear lattice

研究线性格中的弱强禁止子偏序集问题，证明Bukh-Griggs-Lu猜想的q-模拟失败，并给出若干精确极值结果，包括菱形猜想的q-模拟肯定解答。

关于避免子偏序集的家族数量

On the number of families avoiding a subposet

研究了避免子偏序集的家族数量，证明其上限与最大诱导子家族大小有关，并获得相关超饱和结果。

The method of double chains for largest families with excluded subposets

For a given finite poset $P$, $La(n,P)$ denotes the largest size of a family $\mathcal{F}$ of subsets of $[n]$ not containing $P$ as a weak subposet. We exactly determine $La(n,P)$ for infinitely many $P$ posets. These posets are built from seven base posets using two operations. For arbitrary posets, an upper bound is given for $La(n,P)$ depending on $|P|$ and the size of the longest chain in $P$. To prove these theorems we introduce a new method, counting the intersections of $\mathcal{F}$ with double chains, rather than chains.

The covering lemma and $q$-analogues of extremal set theory problems

We prove a general lemma (inspired by a lemma of Holroyd and Talbot) about the connection of the largest cardinalities (or weight) of structures satisfying some hereditary property and substructures satisfying the same hereditary property. We use it to show how results concerning forbidden subposet problems in the Boolean poset imply analogous results in the poset of subspaces of a finite vector space. We also study generalized forbidden subposet problems in the poset of subspaces.

关于禁止子偏序集的彩虹Turán问题

Rainbow Turán problems for forbidden subposets

研究禁止子偏序集问题的彩虹版本，通过偏序集彩虹强制建立$La^*$与$La^*_R$函数联系，确定树形偏序集的$La_R^*(n,T)$渐近性质，还得到反链和完全二分偏序集相关结果。

On an extremal problem involving a pair of forbidden posets

Resolving a conjecture of Methuku and the first author we determine the size of the largest family of subsets of an $n$-element set avoiding both $Y_k$ and $Y_k'$ as induced subposets. The result follows as a consequence of the analogous result on a cyclical grid poset.

Forbidden induced subposets of given height

Let $P$ be a partially ordered set. The function $\mbox{La}^{\#}(n,P)$ denotes the size of the largest family $\mathcal{F}\subset 2^{[n]}$ that does not contain an induced copy of $P$. It was proved by Methuku and Pálvölgyi that there exists a constant $C_{P}$ (depending only on $P$) such that $\mbox{La}^{\#}(n,P)<C_{P}\binom{n}{\lfloor n/2\rfloor}$. However, the order of the constant $C_{P}$ following from their proof is typically exponential in $|P|$. Here, we show that if the height of the poset is constant, this can be improved. We show that for every positive integer $h$ there exists a co…

Generalized forbidden subposet problems

A subfamily $\{F_1,F_2,\dots,F_{|P|}\}\subseteq {\cal F}$ of sets is a copy of a poset $P$ in ${\cal F}$ if there exists a bijection $ϕ:P\rightarrow \{F_1,F_2,\dots,F_{|P|}\}$ such that whenever $x \le_P x'$ holds, then so does $ϕ(x)\subseteq ϕ(x')$. For a family ${\cal F}$ of sets, let $c(P,{\cal F})$ denote the number of copies of $P$ in ${\cal F}$, and we say that ${\cal F}$ is $P$-free if $c(P,{\cal F})=0$ holds. For any two posets $P,Q$ let us denote by $La(n,P,Q)$ the maximum number of copies of $Q$ over all $P$-free families ${\cal F} \subseteq 2^{[n]}$, i.e. $\max\{c(Q,{\cal F}): {\cal…

无冠族与禁子偏序集满足$e(P)\in \{1,2\}$

Crown-free families and forbidden subposets with $e(P)\in \{1,2\}$

本文研究偏序集参数$e(P)$与$\pi^-(P)$的关系，新增$e(P)=1$的偏序集，构造弱$O_6$-自由族，证明相关不等式并构造$e(P)=2$且下密度任意大的偏序集。

A note on the size of N-free families

The $\mathcal{N}$ poset consists of four distinct sets $W,X,Y,Z$ such that $W\subset X$, $Y\subset X$, and $Y\subset Z$ where $W$ is not necessarily a subset of $Z$. A family $\mathcal{F}$ as a subposet of the $n$-dimensional Boolean lattice, $\mathcal{B}_n$, is $\mathcal{N}$-free if it does not contain $\mathcal{N}$ as a subposet. Let $\text{La}(n, \mathcal{N})$ be the size of a largest $\mathcal{N}$-free family in $\mathcal{B}_n$. Katona and Tarján proved that $\text{La}(n,\mathcal{N})\geq {n \choose k}+A(n,4,k+1)$, where $k=\lfloor n/2\rfloor$ and $A(n, 4, k+1)$ is the size of a single-erro…

Abelian groups yield many large families for the diamond problem

There is much recent interest in excluded subposets. Given a fixed poset $P$, how many subsets of $[n]$ can found without a copy of $P$ realized by the subset relation? The hardest and most intensely investigated problem of this kind is when $P$ is a diamond, i.e. the power set of a 2 element set. In this paper, we show infinitely many asymptotically tight constructions using random set families defined from posets based on Abelian groups. They are provided by the convergence of Markov chains on groups. Such constructions suggest that the diamond problem is hard.

Generalized laminar families and certain forbidden matrices

Recall that in a laminar family, any two sets are either disjoint or contained one in the other. Here, a parametrized weakening of this condition is introduced. Let us say that a set system $\mathcal{F} \subseteq 2^X$ is $t$-laminar if $A,B \in \mathcal{F}$ with $|A \cap B| \ge t$ implies $A \subseteq B$ or $B \subseteq A$. We obtain very close asymptotic bounds in terms of $n$ on the maximum size of a $2$-laminar family $\mathcal{F} \subseteq 2^{[n]}$. A construction for $3$-laminar families and a crude analysis for general $t$ are also given.

Two Posets of Noncrossing Partitions Coming From Undesired Parking Spaces

Consider the noncrossing set partitions of an $n$-element set which either do not contain the block $\{n-1,n\}$, or which do not contain the singleton block $\{n\}$ whenever $1$ and $n-1$ are in the same block. In this article we study the subposet of the noncrossing partition lattice induced by these elements, and show that it is a supersolvable lattice, and therefore lexicographically shellable. We give a combinatorial model for the NBB bases of this lattice and derive an explicit formula for the value of its Möbius function between least and greatest element. This work is motivated by a rec…

On crown-free families of subsets

The crown $\Oh_{2t}$ is a height-2 poset whose Hasse diagram is a cycle of length $2t$. A family $\F$ of subsets of $[n]:=\{1,2..., n\}$ is {\em $\Oh_{2t}$-free} if $\Oh_{2t}$ is not a weak subposet of $(\F,\subseteq)$. Let $\La(n,\Oh_{2t})$ be the largest size of $\Oh_{2t}$-free families of subsets of $[n]$. De Bonis-Katona-Swanepoel proved $\La(n,\Oh_{4})= {n\choose \lfloor \frac{n}{2} \rfloor} + {n\choose \lceil \frac{n}{2} \rceil}$. Griggs and Lu proved that $\La(n,\Oh_{2t})=(1+o(1))\nchn$ for all even $t\ge 4$. In this paper, we prove $\La(n,\Oh_{2t})=(1+o(1))\nchn$ for all odd $t\geq 7$.

Forbidden Families of Configurations

A simple matrix is a (0,1)-matrix with no repeated columns. For a (0,1)-matrix $F$, we say that a (0,1)-matrix $A$ has $F$ as a configuration if there is a submatrix of $A$ which is a row and column permutation of $F$ (trace is the set system version of a configuration). Let $\ncols{A}$ denote the number of columns of $A$. Let ${\cal F}$ be a family of matrices. We define the extremal function $forb(m,{\cal F})=\max{\ncols{A} : A is m-rowed simple matrix and has no configuration F\in{\cal F}}$. We consider some families ${\cal F}=\{F_1,F_2,\ldots, F_t\}$ such that individually each $\forb(m,F_…

Forbidden Families of Minimal Quadratic and Cubic Configurations

A matrix is \emph{simple} if it is a (0,1)-matrix and there are no repeated columns. Given a (0,1)-matrix $F$, we say a matrix $A$ has $F$ as a \emph{configuration}, denoted $F\prec A$, if there is a submatrix of $A$ which is a row and column permutation of $F$. Let $|A|$ denote the number of columns of $A$. Let $\mathcal{F}$ be a family of matrices. We define the extremal function $\text{forb}(m, \mathcal{F}) = \max\{|A|\colon A \text{ is an }m-\text{rowed simple matrix and has no configuration } F\in\mathcal{F}\}$. We consider pairs $\mathcal{F}=\{F_1,F_2\}$ such that $F_1$ and $F_2$ have no…

$Q_2$-free families in the Boolean lattice

For a family $\mathcal{F}$ of subsets of [n]=\{1, 2, ..., n} ordered by inclusion, and a partially ordered set P, we say that $\mathcal{F}$ is P-free if it does not contain a subposet isomorphic to P. Let $ex(n, P)$ be the largest size of a P-free family of subsets of [n]. Let $Q_2$ be the poset with distinct elements a, b, c, d, a<b, c<d; i.e., the 2-dimensional Boolean lattice. We show that $2N -o(N) \leq ex(n, Q_2)\leq 2.283261N +o(N), $ where $N = \binom{n}{\lfloor n/2 \rfloor}$. We also prove that the largest $Q_2$-free family of subsets of [n] having at most three different sizes has at …

Diamond-free Families

Given a finite poset P, we consider the largest size La(n,P) of a family of subsets of $[n]:=\{1,...,n\}$ that contains no subposet P. This problem has been studied intensively in recent years, and it is conjectured that $π(P):= \lim_{n\rightarrow\infty} La(n,P)/{n choose n/2}$ exists for general posets P, and, moreover, it is an integer. For $k\ge2$ let $\D_k$ denote the $k$-diamond poset $\{A< B_1,...,B_k < C\}$. We study the average number of times a random full chain meets a $P$-free family, called the Lubell function, and use it for $P=\D_k$ to determine $π(\D_k)$ for infinitely many valu…

$\mathcal{L}$-相交或配置禁用族在集合系统与有限域上向量空间中的研究

$\mathcal{L}$-intersecting or Configuration Forbidden Families on Set Systems and Vector Spaces over Finite Fields

本文推导了有限域向量空间中相交$k$-Sperner族大小的紧上界，建立了子集族与子空间族在$\mathcal{L}$-相交或配置禁用性质上界的一般关系，并据此推广了Erdős匹配与单纯形猜想，推翻了Ihringer的相关猜想。

通过（简单）禁止子有向图求解广义锦标赛和广义二分锦标赛某些族中的核完美问题

Solving the kernel perfect problem by (simple) forbidden subdigraphs for digraphs in some families of generalized tournaments and generalized bipartite tournaments

本文通过禁止子有向图刻画了广义锦标赛和广义二分锦标赛若干族中的核完美与临界核不完美有向图，并证明判定CKI问题可多项式求解，回应了Bang-Jensen的猜想。

Families of Subsets Without a Given Poset in the Interval Chains

For two posets $P$ and $Q$, we say $Q$ is $P$-free if there does not exist any order-preserving injection from $P$ to $Q$. The speical case for $Q$ being the Boolean lattice $B_n$ is well-studied, and the optiamal value is denoted as $\lanp$. Let us define $\La(Q,P)$ to be the largest size of any $P$-free subposet of $Q$. In this paper, we give an upper bound for $\La(Q,P)$ when $Q$ is a double chain and $P$ is any graded poset, which is better than the previous known upper bound, by means of finding the indpendence number of an auxiliary graph related to $P$. For the auxiliary graph, we can f…

Set families with a forbidden pattern

A balanced pattern of order $2d$ is an element $P \in \{+,-\}^{2d}$, where both signs appear $d$ times. Two sets $A,B \subset [n]$ form $P$-pattern, which we denote by $\operatorname{pat}(A,B) = P$, if $A\triangle B = \{j_1,\ldots ,j_{2d}\}$ with $1\leq j_1<\cdots < j_{2d}\leq n$ and $\{i\in [2d]: P_i = + \} = \{i\in [2d]: j_i \in A \setminus B\}$. We say ${\cal A} \subset {\cal P}[n]$ is $P$-free if $\operatorname{pat}(A,B)\neq P$ for all $A,B \in {\cal A}$. We consider the following extremal question: how large can a family ${\cal A} \subset {\cal P}[n]$ be if ${\cal A} $ is $P$-free? We pro…

Poset-free Families and Lubell-boundedness

Given a finite poset $P$, we consider the largest size $\lanp$ of a family $\F$ of subsets of $[n]:=\{1,...,n\}$ that contains no subposet $P$. This continues the study of the asymptotic growth of $\lanp$; it has been conjectured that for all $P$, $π(P):= \lim_{n\rightarrow\infty} \lanp/\nchn$ exists and equals a certain integer, $e(P)$. While this is known to be true for paths, and several more general families of posets, for the simple diamond poset $\D_2$, the existence of $π$ frustratingly remains open. Here we develop theory to show that $π(P)$ exists and equals the conjectured value $e(P…

An upper bound on the size of diamond-free families of sets

Let $La(n,P)$ be the maximum size of a family of subsets of $[n]=\{1,2,...,n\}$ not containing $P$ as a (weak) subposet. The diamond poset, denoted $B_{2}$, is defined on four elements $x,y,z,w$ with the relations $x<y,z$ and $y,z<w$. $La(n,P)$ has been studied for many posets; one of the major open problems is determining $La(n,B_{2})$. Studying the average number of sets from a family of subsets of $[n]$ on a maximal chain in the Boolean lattice $2^{[n]}$ has been a fruitful method. We use a partitioning of the maximal chains and introduce an induction method to show that $La(n,B_{2})\leq(2.…

精确结果及Duke-Erdős禁止太阳花问题的极值族结构

Exact results and the structure of extremal families for the Duke--Erdős forbidden sunflower problem

本文精确解决了Duke-Erdős禁止太阳花问题的极值族结构，针对t=2、奇数s及k≥5的情况，揭示了极值族与Erdős-Rado问题的联系。

线性映射族的禁止交集问题

Forbidden intersection problems for families of linear maps

本文研究线性映射族的禁止交集问题，利用 junta 逼近、谱方法与超压缩不等式，证明 GL(V) 中 (t-1)-交集无关族的大小上界及等号结构。

禁止相交问题的稀疏近似方法

Spread approximations for forbidden intersections problems

本文提出基于 r-spread 族的 spread 近似方法，并利用其解决禁止相交问题，首次在 t 随 n 增长的新范围内证明置换 t-相交与 Erdős–Sós 问题的上界 (n-t)!。

Colourings without monochromatic disjoint pairs

The typical extremal problem asks how large a structure can be without containing a forbidden substructure. The Erdős-Rothschild problem, introduced in 1974 by Erdős and Rothschild in the context of extremal graph theory, is a coloured extension, asking for the maximum number of colourings a structure can have that avoid monochromatic copies of the forbidden substructure. The celebrated Erdős-Ko-Rado theorem is a fundamental result in extremal set theory, bounding the size of set families without a pair of disjoint sets, and has since been extended to several other discrete settings. The Erdős…

矩阵空间中的禁止交定理

Forbidden Intersection Theorems for Matrix Spaces

研究一般线性群中矩阵族避免特定维数核交的问题，证明当t小于常数倍n时，极大家族仅由t-共治族及其对偶构成，并给出Frankl-Rödl型构造表明t>n/2时行为改变。

Approximation by juntas in the symmetric group, and forbidden intersection problems

A family of permutations $\mathcal{F} \subset S_{n}$ is said to be $t$-intersecting if any two permutations in $\mathcal{F}$ agree on at least $t$ points. It is said to be $(t-1)$-intersection-free if no two permutations in $\mathcal{F}$ agree on exactly $t-1$ points. If $S,T \subset \{1,2,\ldots,n\}$ with $|S|=|T|$, and $π: S \to T$ is a bijection, the $π$-star in $S_n$ is the family of all permutations in $S_n$ that agree with $π$ on all of $S$. An $s$-star is a $π$-star such that $π$ is a bijection between sets of size $s$. Friedgut and Pilpel, and independently the first author, showed tha…

入射旗的多项式族与显式非对角 Ramsey 图

Polynomial families of incident flags and explicit off-diagonal Ramsey graphs

本文从多项式族构造显式非对角 Ramsey 图，通过插值和标量限制得到指数界，并给出确定性多项式时间算法。

通过混合模式构造具有丰富极值Turán结构的有限超图族

Finite Hypergraph Families with Rich Extremal Turán Constructions via Mixing Patterns

本文证明任意有限极小r-图模式集合都对应一个有限禁用r-图族，其极值Turán构造恰为通过膨胀和递归混合这些模式所得的最大r-图，并据此构造出具有指数多个极值结构且非有限稳定的3-图族，以及可行区域函数在正Hausdorff维数Cantor集上取最大值的3-图族。

无射线图的通用族

Universal families of rayless graphs

研究无射线图的通用族存在性与复杂度，证明无限基数下存在强通用族，并探讨禁止子图对复杂度的影响。

禁止圈与θ图条件下达到最大无符号拉普拉斯谱半径的图的刻画

Characterization of graphs attaining the maximum signless Laplacian spectral radius under forbidden cycles and theta graphs

本文刻画了禁止特定小圈和θ图时，无悬挂点图中达到最大无符号拉普拉斯谱半径的极值图，并完整解决了模3三个同余类下的固定边数问题。

Stability for the Complete Intersection Theorem, and the Forbidden Intersection Problem of Erdős and Sós

A family $F$ of sets is said to be $t$-intersecting if $|A \cap B| \geq t$ for any $A,B \in F$. The seminal Complete Intersection Theorem of Ahlswede and Khachatrian (1997) gives the maximal size $f(n,k,t)$ of a $t$-intersecting family of $k$-element subsets of $[n]=\{1,2,\ldots,n\}$, together with a characterisation of the extremal families. The forbidden intersection problem, posed by Erdős and Sós in 1971, asks for a determination of the maximal size $g(n,k,t)$ of a family $F$ of $k$-element subsets of $[n]$ such that $|A \cap B| \neq t-1$ for any $A,B \in F$. In this paper, we show that fo…

On the number of containments in $P$-free families

A subfamily $\{F_1,F_2,\dots,F_{|P|}\}\subseteq \mathcal F$ is a copy of the poset $P$ if there exists a bijection $i:P\rightarrow \{F_1,F_2,\dots,F_{|P|}\}$ such that $p\le_P q$ implies $i(p)\subseteq i(q)$. A family $\mathcal F$ is $P$-free, if it does not contain a copy of $P$. In this paper we establish basic results on the maximum possible number of $k$-chains in a $P$-free family $\mathcal F\subseteq 2^{[n]}$. We prove that if the height of $P$, $h(P) > k$, then this number is of the order $Θ(\prod_{i=1}^{k+1}\binom{l_{i-1}}{l_i})$, where $l_0=n$ and $l_1\ge l_2\ge \dots \ge l_{k+1}$ are…

线性依赖与 Duke--Erd\H os 禁止向日葵问题中的多项式因子

Linear dependencies, polynomial factors in the Duke--Erd\H os forbidden sunflower problem

本文扩展了 Frankl 和 Füredi 关于禁止向日葵问题的结果，通过结合 spread 近似、Delta-系统和超收缩方法，将参数范围推广到多项式依赖，并应用于多个集合论领域。

在ℓ₂范数下的埃罗-科-拉多定理

The Erdős-Ko-Rado Theorem in $\ell_2$-Norm

本文在ℓ₂范数下证明了t-交家族的埃罗-科-拉多定理，并给出了相关广义Turán结果。

具有大边-上拉普拉斯谱间隙的三角形族

Triangle Families with Large Edge Up-Laplacian Spectral Gap

本文研究有限三角形族中边-三角形关联矩阵的谱间隙，证明大谱间隙迫使支撑图中每条边出现在至少⌈λ⌉-2个三角形中且最小度至少⌈λ⌉-1，并揭示了完全三系统是唯一的最小谱极值族，但谱间隙在连续团阈值之间呈孤立峰值。