{"title":"On Finding Constrained Independent Sets in Cycles","authors":"I. Haviv","doi":"10.4230/LIPIcs.ICALP.2023.73","DOIUrl":null,"url":null,"abstract":"A subset of $[n] = \\{1,2,\\ldots,n\\}$ is called stable if it forms an independent set in the cycle on the vertex set $[n]$. In 1978, Schrijver proved via a topological argument that for all integers $n$ and $k$ with $n \\geq 2k$, the family of stable $k$-subsets of $[n]$ cannot be covered by $n-2k+1$ intersecting families. We study two total search problems whose totality relies on this result. In the first problem, denoted by $\\mathsf{Schrijve}r(n,k,m)$, we are given an access to a coloring of the stable $k$-subsets of $[n]$ with $m = m(n,k)$ colors, where $m \\leq n-2k+1$, and the goal is to find a pair of disjoint subsets that are assigned the same color. While for $m = n-2k+1$ the problem is known to be $\\mathsf{PPA}$-complete, we prove that for $m<d \\cdot \\lfloor \\frac{n}{2k+d-2} \\rfloor$, with $d$ being any fixed constant, the problem admits an efficient algorithm. For $m = \\lfloor n/2 \\rfloor-2k+1$, we prove that the problem is efficiently reducible to the $\\mathsf{Kneser}$ problem. Motivated by the relation between the problems, we investigate the family of unstable $k$-subsets of $[n]$, which might be of independent interest. In the second problem, called Unfair Independent Set in Cycle, we are given $\\ell$ subsets $V_1, \\ldots, V_\\ell$ of $[n]$, where $\\ell \\leq n-2k+1$ and $|V_i| \\geq 2$ for all $i \\in [\\ell]$, and the goal is to find a stable $k$-subset $S$ of $[n]$ satisfying the constraints $|S \\cap V_i| \\leq |V_i|/2$ for $i \\in [\\ell]$. We prove that the problem is $\\mathsf{PPA}$-complete and that its restriction to instances with $n=3k$ is at least as hard as the Cycle plus Triangles problem, for which no efficient algorithm is known. On the contrary, we prove that there exists a constant $c$ for which the restriction of the problem to instances with $n \\geq c \\cdot k$ can be solved in polynomial time.","PeriodicalId":266158,"journal":{"name":"International Colloquium on Automata, Languages and Programming","volume":"22 1","pages":"0"},"PeriodicalIF":0.0000,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"1","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"International Colloquium on Automata, Languages and Programming","FirstCategoryId":"1085","ListUrlMain":"https://doi.org/10.4230/LIPIcs.ICALP.2023.73","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"","JCRName":"","Score":null,"Total":0}
引用次数: 1
Abstract
A subset of $[n] = \{1,2,\ldots,n\}$ is called stable if it forms an independent set in the cycle on the vertex set $[n]$. In 1978, Schrijver proved via a topological argument that for all integers $n$ and $k$ with $n \geq 2k$, the family of stable $k$-subsets of $[n]$ cannot be covered by $n-2k+1$ intersecting families. We study two total search problems whose totality relies on this result. In the first problem, denoted by $\mathsf{Schrijve}r(n,k,m)$, we are given an access to a coloring of the stable $k$-subsets of $[n]$ with $m = m(n,k)$ colors, where $m \leq n-2k+1$, and the goal is to find a pair of disjoint subsets that are assigned the same color. While for $m = n-2k+1$ the problem is known to be $\mathsf{PPA}$-complete, we prove that for $m