{"title":"对(−1,0,1)-向量和s-交叉族的交集理论一文的修正","authors":"P. Frankl, A. Kupavskii","doi":"10.2140/moscow.2019.8.385","DOIUrl":null,"url":null,"abstract":"In this paper we study two directions of extending the classical Erd\\H os-Ko-Rado theorem which states that any family of $k$-element subsets of the set $[n] = \\{1,\\ldots,n\\}$ in which any two sets intersect, has cardinality at most ${n-1\\choose k-1}$. \nIn the first part of the paper we study the families of $\\{0,\\pm 1\\}$-vectors. Denote by $\\mathcal L_k$ the family of all vectors $\\mathbf v$ from $\\{0,\\pm 1\\}^n$ such that $\\langle\\mathbf v,\\mathbf v\\rangle = k$. For any $k$ and $l$ and sufficiently large $n$ we determine the maximal size of the family $\\mathcal V\\subset \\mathcal L_k$ such that for any $\\mathbf v,\\mathbf w\\in \\mathcal V$ we have $\\langle \\mathbf v,\\mathbf w\\rangle\\ge l$. We find some exact values of this function for all $n$ for small values of $k$. \nIn the second part of the paper we study cross-intersecting pairs of families. We say that two families are $\\mathcal A, \\mathcal B$ are \\textit{$s$-cross-intersecting}, if for any $A\\in\\mathcal A,B\\in \\mathcal B$ we have $|A\\cap B|\\ge s$. We also say that a set family $\\mathcal A$ is \\textit{$t$-intersecting}, if for any $A_1,A_2\\in \\mathcal A$ we have $|A_1\\cap A_2|\\ge t$. For a pair of nonempty $s$-cross-intersecting $t$-intersecting families $\\mathcal A,\\mathcal B$ of $k$-sets, we determine the maximal value of $|\\mathcal A|+|\\mathcal B|$ for $n$ sufficiently large. \nIf the nonempty families $\\mathcal A,\\mathcal B$ are $s$-cross-intersecting (and the $t$-intersecting condition is omitted), then we determine the maximum of $|\\mathcal A|+|\\mathcal B|$ for all $n$. This generalizes a result of Hilton and Milner, who determined the maximum of $|\\mathcal A|+|\\mathcal B|$ for nonempty $1$-cross-intersecting families.","PeriodicalId":36590,"journal":{"name":"Moscow Journal of Combinatorics and Number Theory","volume":" ","pages":""},"PeriodicalIF":0.0000,"publicationDate":"2019-10-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2140/moscow.2019.8.385","citationCount":"1","resultStr":"{\"title\":\"Correction to the article Intersection theorems\\nfor (−1,0,1)-vectors and s-cross-intersecting families\",\"authors\":\"P. Frankl, A. Kupavskii\",\"doi\":\"10.2140/moscow.2019.8.385\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"In this paper we study two directions of extending the classical Erd\\\\H os-Ko-Rado theorem which states that any family of $k$-element subsets of the set $[n] = \\\\{1,\\\\ldots,n\\\\}$ in which any two sets intersect, has cardinality at most ${n-1\\\\choose k-1}$. \\nIn the first part of the paper we study the families of $\\\\{0,\\\\pm 1\\\\}$-vectors. Denote by $\\\\mathcal L_k$ the family of all vectors $\\\\mathbf v$ from $\\\\{0,\\\\pm 1\\\\}^n$ such that $\\\\langle\\\\mathbf v,\\\\mathbf v\\\\rangle = k$. For any $k$ and $l$ and sufficiently large $n$ we determine the maximal size of the family $\\\\mathcal V\\\\subset \\\\mathcal L_k$ such that for any $\\\\mathbf v,\\\\mathbf w\\\\in \\\\mathcal V$ we have $\\\\langle \\\\mathbf v,\\\\mathbf w\\\\rangle\\\\ge l$. We find some exact values of this function for all $n$ for small values of $k$. \\nIn the second part of the paper we study cross-intersecting pairs of families. We say that two families are $\\\\mathcal A, \\\\mathcal B$ are \\\\textit{$s$-cross-intersecting}, if for any $A\\\\in\\\\mathcal A,B\\\\in \\\\mathcal B$ we have $|A\\\\cap B|\\\\ge s$. We also say that a set family $\\\\mathcal A$ is \\\\textit{$t$-intersecting}, if for any $A_1,A_2\\\\in \\\\mathcal A$ we have $|A_1\\\\cap A_2|\\\\ge t$. For a pair of nonempty $s$-cross-intersecting $t$-intersecting families $\\\\mathcal A,\\\\mathcal B$ of $k$-sets, we determine the maximal value of $|\\\\mathcal A|+|\\\\mathcal B|$ for $n$ sufficiently large. \\nIf the nonempty families $\\\\mathcal A,\\\\mathcal B$ are $s$-cross-intersecting (and the $t$-intersecting condition is omitted), then we determine the maximum of $|\\\\mathcal A|+|\\\\mathcal B|$ for all $n$. This generalizes a result of Hilton and Milner, who determined the maximum of $|\\\\mathcal A|+|\\\\mathcal B|$ for nonempty $1$-cross-intersecting families.\",\"PeriodicalId\":36590,\"journal\":{\"name\":\"Moscow Journal of Combinatorics and Number Theory\",\"volume\":\" \",\"pages\":\"\"},\"PeriodicalIF\":0.0000,\"publicationDate\":\"2019-10-11\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"https://sci-hub-pdf.com/10.2140/moscow.2019.8.385\",\"citationCount\":\"1\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Moscow Journal of Combinatorics and Number Theory\",\"FirstCategoryId\":\"1085\",\"ListUrlMain\":\"https://doi.org/10.2140/moscow.2019.8.385\",\"RegionNum\":0,\"RegionCategory\":null,\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q4\",\"JCRName\":\"Mathematics\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Moscow Journal of Combinatorics and Number Theory","FirstCategoryId":"1085","ListUrlMain":"https://doi.org/10.2140/moscow.2019.8.385","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q4","JCRName":"Mathematics","Score":null,"Total":0}
Correction to the article Intersection theorems
for (−1,0,1)-vectors and s-cross-intersecting families
In this paper we study two directions of extending the classical Erd\H os-Ko-Rado theorem which states that any family of $k$-element subsets of the set $[n] = \{1,\ldots,n\}$ in which any two sets intersect, has cardinality at most ${n-1\choose k-1}$.
In the first part of the paper we study the families of $\{0,\pm 1\}$-vectors. Denote by $\mathcal L_k$ the family of all vectors $\mathbf v$ from $\{0,\pm 1\}^n$ such that $\langle\mathbf v,\mathbf v\rangle = k$. For any $k$ and $l$ and sufficiently large $n$ we determine the maximal size of the family $\mathcal V\subset \mathcal L_k$ such that for any $\mathbf v,\mathbf w\in \mathcal V$ we have $\langle \mathbf v,\mathbf w\rangle\ge l$. We find some exact values of this function for all $n$ for small values of $k$.
In the second part of the paper we study cross-intersecting pairs of families. We say that two families are $\mathcal A, \mathcal B$ are \textit{$s$-cross-intersecting}, if for any $A\in\mathcal A,B\in \mathcal B$ we have $|A\cap B|\ge s$. We also say that a set family $\mathcal A$ is \textit{$t$-intersecting}, if for any $A_1,A_2\in \mathcal A$ we have $|A_1\cap A_2|\ge t$. For a pair of nonempty $s$-cross-intersecting $t$-intersecting families $\mathcal A,\mathcal B$ of $k$-sets, we determine the maximal value of $|\mathcal A|+|\mathcal B|$ for $n$ sufficiently large.
If the nonempty families $\mathcal A,\mathcal B$ are $s$-cross-intersecting (and the $t$-intersecting condition is omitted), then we determine the maximum of $|\mathcal A|+|\mathcal B|$ for all $n$. This generalizes a result of Hilton and Milner, who determined the maximum of $|\mathcal A|+|\mathcal B|$ for nonempty $1$-cross-intersecting families.