scholarly journals A Sharp Bound for the Product of Weights of Cross-Intersecting Families

10.37236/5782 ◽  
2016 ◽  
Vol 23 (4) ◽  
Author(s):  
Peter Borg
Keyword(s):  
General Setting ◽  
Sharp Bound ◽  
Integer Sequences ◽  
Sharp Bounds ◽  
The Family ◽  
Intersecting Families

Two families $\mathcal{A}$ and $\mathcal{B}$ of sets are said to be cross-intersecting if each set in $\mathcal{A}$ intersects each set in $\mathcal{B}$. For any two integers $n$ and $k$ with $1 \leq k \leq n$, let ${[n] \choose \leq k}$ denote the family of subsets of $\{1, \dots, n\}$ of size at most $k$, and let $\mathcal{S}_{n,k}$ denote the family of sets in ${[n] \choose \leq k}$ that contain $1$. The author recently showed that if $\mathcal{A} \subseteq {[m] \choose \leq r}$, $\mathcal{B} \subseteq {[n] \choose \leq s}$, and $\mathcal{A}$ and $\mathcal{B}$ are cross-intersecting, then $|\mathcal{A}||\mathcal{B}| \leq \mathcal{S}_{m,r}||\mathcal{S}_{n,s}|$. We prove a version of this result for the more general setting of \emph{weighted} sets. We show that if $g : {[m] \choose \leq r} \rightarrow \mathbb{R}^+$ and $h : {[n] \choose \leq s} \rightarrow \mathbb{R}^+$ are functions that obey certain conditions, $\mathcal{A} \subseteq {[m] \choose \leq r}$, $\mathcal{B} \subseteq {[n] \choose \leq s}$, and $\mathcal{A}$ and $\mathcal{B}$ are cross-intersecting, then $$\sum_{A \in \mathcal{A}} g(A) \sum_{B \in \mathcal{B}} h(B) \leq \sum_{C \in \mathcal{S}_{m,r}} g(C) \sum_{D \in \mathcal{S}_{n,s}} h(D).$$The bound is attained by taking $\mathcal{A} = \mathcal{S}_{m,r}$ and $\mathcal{B} = \mathcal{S}_{n,s}$. We also show that this result yields new sharp bounds for the product of sizes of cross-intersecting families of integer sequences and of cross-intersecting families of multisets.

scholarly journals Intersecting and Cross-Intersecting Families of Labeled Sets

10.37236/884 ◽  
2008 ◽  
Vol 15 (1) ◽  
Author(s):  
Peter Borg
Keyword(s):  
Sharp Bound ◽  
General Inequality ◽  
The Family ◽  
Intersecting Families ◽  
Special Case

A family ${\cal A}$ of sets is said to be intersecting if any two sets in ${\cal A}$ intersect. Families ${\cal A}_1, ..., {\cal A}_p$ are said to be cross-intersecting if, for any $i, j \in \{1, ..., p\}$ such that $i \neq j$, any set in ${\cal A}_i$ intersects any set in ${\cal A}_j$. For ${\bf k} = (k_1, ..., k_n) \in {\Bbb N}^n$, $2 \leq k_1 \leq ... \leq k_n$, let ${\cal L}_{\bf{k}}$ be the family of labeled $n$-sets given by ${\cal L}_{\bf{k}} := \{\{(1,l_1), ..., (n,l_n)\} \colon l_i \in \{1, ..., k_i\}, i = 1, ..., n\}$. We point out a relationship between intersecting families and cross-intersecting families of labeled sets, and we show that, if ${\cal A}_1, ..., {\cal A}_p$ are cross-intersecting sub-families of ${\cal L}_{\bf{k}}$, then $$ \sum_{j = 1}^p |{\cal A}_j| \leq \left\{ \matrix{ k_1k_2...k_n & \hbox{if $p \leq k_1$};\cr pk_2...k_n & \hbox{if $p \geq k_1$}.\cr } \right. $$ We also determine the cases of equality. We then obtain a more general inequality, a special case of which is a sharp bound for cross-intersecting families of permutations.

The sharp bounds of the second and third Hankel determinants for the class 𝓢𝓛*

2020 ◽  
Vol 70 (4) ◽  
pp. 849-862
Author(s):  
Shagun Banga ◽  
S. Sivaprasad Kumar
Keyword(s):  
Triangle Inequality ◽  
Starlike Functions ◽  
Sharp Bound ◽  
The Novel ◽  
Sharp Bounds ◽  
Hankel Determinants ◽  
Lemniscate Of Bernoulli ◽  
Carathéodory Functions ◽  
The Right

AbstractIn this paper, we use the novel idea of incorporating the recently derived formula for the fourth coefficient of Carathéodory functions, in place of the routine triangle inequality to achieve the sharp bounds of the Hankel determinants H3(1) and H2(3) for the well known class 𝓢𝓛* of starlike functions associated with the right lemniscate of Bernoulli. Apart from that the sharp bound of the Zalcman functional: $\begin{array}{} |a_3^2-a_5| \end{array}$ for the class 𝓢𝓛* is also estimated. Further, a couple of interesting results of 𝓢𝓛* are also discussed.

scholarly journals On the union of intersecting families

2019 ◽  
Vol 28 (06) ◽  
pp. 826-839
Author(s):  
David Ellis ◽  
Noam Lifshitz
Keyword(s):  
Fixed Integer ◽  
Empty Intersection ◽  
Intersecting Family ◽  
The Family ◽  
Intersecting Families ◽  
Element Set

AbstractA family of sets is said to be intersecting if any two sets in the family have non-empty intersection. In 1973, Erdős raised the problem of determining the maximum possible size of a union of r different intersecting families of k-element subsets of an n-element set, for each triple of integers (n, k, r). We make progress on this problem, proving that for any fixed integer r ⩾ 2 and for any $$k \le ({1 \over 2} - o(1))n$$, if X is an n-element set, and $${\cal F} = {\cal F}_1 \cup {\cal F}_2 \cup \cdots \cup {\cal F}_r $$, where each $$ {\cal F}_i $$ is an intersecting family of k-element subsets of X, then $$|{\cal F}| \le \left( {\matrix{n \cr k \cr } } \right) - \left( {\matrix{{n - r} \cr k \cr } } \right)$$, with equality only if $${\cal F} = \{ S \subset X:|S| = k,\;S \cap R \ne \emptyset \} $$ for some R ⊂ X with |R| = r. This is best possible up to the size of the o(1) term, and improves a 1987 result of Frankl and Füredi, who obtained the same conclusion under the stronger hypothesis $$k < (3 - \sqrt 5 )n/2$$, in the case r = 2. Our proof utilizes an isoperimetric, influence-based method recently developed by Keller and the authors.

scholarly journals Extremal problems for a class of functions of positive real part and applications

1986 ◽  
Vol 41 (2) ◽  
pp. 152-164
Author(s):  
V. V. Anh ◽  
P. D. Tuan
Keyword(s):  
Lower Bound ◽  
Extremal Problems ◽  
Positive Real Part ◽  
Positive Real ◽  
Sharp Bounds ◽  
The Family ◽  
Radius Of Convexity ◽  
Class Of Functions

AbstractIn this paper we determine the lower bound on |z| = r < 1 for the functional Re{αp(z) + β zp′(z)/p(z)}, α ≧0, β ≧ 0, over the class Pk (A, B). By means of this result, sharp bounds for |F(z)|, |F',(z)| in the family and the radius of convexity for are obtained. Furthermore, we establish the radius of starlikness of order β, 0 ≦ β < 1, for the functions F(z) = λf(Z) + (1-λ) zf′ (Z), |z| < 1, where ∞ < λ <1, and .

Sharp coefficient bounds for starlike functions associated with the Bell numbers

2019 ◽  
Vol 69 (5) ◽  
pp. 1053-1064 ◽  
Author(s):  
Virendra Kumar ◽  
Nak Eun Cho ◽  
V. Ravichandran ◽  
H. M. Srivastava
Keyword(s):  
Starlike Functions ◽  
Higher Order ◽  
Positive Real Part ◽  
Bell Numbers ◽  
Positive Real ◽  
Sharp Bounds ◽  
Hankel Determinants ◽  
Coefficient Bounds ◽  
The Family ◽  
Schwarzian Derivatives

Abstract Let $\begin{array}{} \mathcal{S}^*_B \end{array}$ be the class of normalized starlike functions associated with a function related to the Bell numbers. By establishing bounds on some coefficient functionals for the family of functions with positive real part, we derive for functions in the class $\begin{array}{} \mathcal{S}^*_B \end{array}$ several sharp coefficient bounds on the first six coefficients and also further sharp bounds on the corresponding Hankel determinants. Bounds on the first three consecutive higher-order Schwarzian derivatives for functions in the class $\begin{array}{} \mathcal{S}^*_B \end{array}$ are investigated.

scholarly journals Cross-Intersecting Families of Labeled Sets

10.37236/3047 ◽  
2013 ◽  
Vol 20 (1) ◽  
Author(s):  
Huajun Zhang
Keyword(s):  
The Family ◽  
Intersecting Families ◽  
Positive Integers

For two positive integers $n$ and $p$, let $\mathcal{L}_{p}$ be the family of labeled $n$-sets given by $$\mathcal{L}_{p}=\big\{\{(1,\ell_1),(2,\ell_2),\ldots,(n,\ell_n)\}: \ell_i\in[p], i=1,2\ldots,n\big\}.$$ Families $\mathcal{A}$ and $\mathcal{B}$ are said to be cross-intersecting if $A\cap B\neq\emptyset$ for all $A\in \mathcal{A}$ and $B\in\mathcal{B}$. In this paper, we will prove that for $p\geq 4$,  if $\mathcal{A}$ and $\mathcal{B}$ are cross-intersecting subfamilies of $\mathcal{L}_{\mathfrak{p}}$, then $|\mathcal{A}||\mathcal{B}|\leq p^{2n-2}$, and equality holds if and only if $\mathcal{A}$ and $\mathcal{B}$ are an identical largest intersecting subfamily of $\mathcal{L}_{p}$.

scholarly journals Toeplitz Matrices Whose Elements Are the Coefficients of Functions with Bounded Boundary Rotation

2016 ◽  
Vol 2016 ◽  
pp. 1-4 ◽  
Author(s):  
V. Radhika ◽  
S. Sivasubramanian ◽  
G. Murugusundaramoorthy ◽  
Jay M. Jahangiri
Keyword(s):  
Unit Disk ◽  
Toeplitz Matrices ◽  
Open Unit ◽  
Open Unit Disk ◽  
Toeplitz Determinants ◽  
Sharp Bounds ◽  
The Family ◽  
Bounded Boundary Rotation ◽  
Boundary Rotation

Let R denote the family of functions f(z)=z+∑n=2∞anzn of bounded boundary rotation so that Ref′(z)>0 in the open unit disk U={z:z<1}. We obtain sharp bounds for Toeplitz determinants whose elements are the coefficients of functions f∈R.

scholarly journals Cross t-Intersecting Integer Sequences from Weighted Erdős–Ko–Rado

2013 ◽  
Vol 22 (4) ◽  
pp. 622-637 ◽  
Author(s):  
NORIHIDE TOKUSHIGE
Keyword(s):  
Integer Sequences ◽  
Weighted Version ◽  
Letter Alphabet ◽  
Eigenvalue Method ◽  
Intersecting Families ◽  
Positive Integers ◽  
Families Of Subsets

Let m,n and t be positive integers. Consider [m]n as the set of sequences of length n on an m-letter alphabet. We say that two subsets A⊂[m]n and B⊂[m]n cross t-intersect if any two sequences a∈A and b∈B match in at least t positions. In this case it is shown that if $m > (1-\frac 1{\sqrt[t]2})^{-1}$ then |A||B|≤(mn−t)2. We derive this result from a weighted version of the Erdős–Ko–Rado theorem concerning cross t-intersecting families of subsets, and we also include the corresponding stability statement. One of our main tools is the eigenvalue method for intersection matrices due to Friedgut [10].

Quantifier elimination for neocompact sets

1998 ◽  
Vol 63 (4) ◽  
pp. 1442-1472 ◽  
Author(s):  
H. Jerome Keisler
Keyword(s):  
Probability Theory ◽  
Existence Theorems ◽  
Quantifier Elimination ◽  
General Setting ◽  
Random Variable ◽  
Compact Sets ◽  
The Family ◽  
Countably Compact ◽  
Universal Quantifiers

AbstractWe shall prove quantifier elimination theorems for neocompact formulas, which define neocompact sets and are built from atomic formulas using finite disjunctions, infinite conjunctions, existential quantifiers, and bounded universal quantifiers. The neocompact sets were first introduced to provide an easy alternative to nonstandard methods of proving existence theorems in probability theory, where they behave like compact sets. The quantifier elimination theorems in this paper can be applied in a general setting to show that the family of neocompact sets is countably compact. To provide the necessary setting we introduce the notion of a law structure. This notion was motivated by the probability law of a random variable. However, in this paper we discuss a variety of model theoretic examples of the notion in the light of our quantifier elimination results.

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