Pub Date : 2025-10-09DOI: 10.1016/j.ejc.2025.104253
Renrong Mao, Jie Huang, Fan Yang
In 1988, Garvan made conjectures on inequalities satisfied by ranks and cranks modulo 5 and 7. We obtain improvements to two of these inequalities in this paper.
{"title":"A proof of some conjectures of Garvan on partitions rank and crank inequalities","authors":"Renrong Mao, Jie Huang, Fan Yang","doi":"10.1016/j.ejc.2025.104253","DOIUrl":"10.1016/j.ejc.2025.104253","url":null,"abstract":"<div><div>In 1988, Garvan made conjectures on inequalities satisfied by ranks and cranks modulo 5 and 7. We obtain improvements to two of these inequalities in this paper.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104253"},"PeriodicalIF":0.9,"publicationDate":"2025-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145248003","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-09DOI: 10.1016/j.ejc.2025.104250
Lubomíra Dvořáková , Savinien Kreczman , Edita Pelantová
We study a class of infinite words , , recently introduced by J. Shallit. This class includes the Thue–Morse sequence , the Fibonacci–Thue–Morse sequence , and the Allouche–Johnson sequence . Shallit stated and for proved two conjectures on properties of . The first conjecture concerns the factor complexity, the second one the critical exponent of these words. We confirm the validity of both conjectures for every .
{"title":"On two conjectures of Shallit about Thue–Morse-like sequences","authors":"Lubomíra Dvořáková , Savinien Kreczman , Edita Pelantová","doi":"10.1016/j.ejc.2025.104250","DOIUrl":"10.1016/j.ejc.2025.104250","url":null,"abstract":"<div><div>We study a class of infinite words <span><math><msub><mrow><mi>x</mi></mrow><mrow><mi>k</mi></mrow></msub></math></span>, <span><math><mrow><mi>k</mi><mo>∈</mo><mi>N</mi><mo>,</mo><mi>k</mi><mo>≥</mo><mn>1</mn></mrow></math></span>, recently introduced by J. Shallit. This class includes the Thue–Morse sequence <span><math><msub><mrow><mi>x</mi></mrow><mrow><mn>1</mn></mrow></msub></math></span>, the Fibonacci–Thue–Morse sequence <span><math><msub><mrow><mi>x</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span>, and the Allouche–Johnson sequence <span><math><msub><mrow><mi>x</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span>. Shallit stated and for <span><math><mrow><mi>k</mi><mo>=</mo><mn>3</mn></mrow></math></span> proved two conjectures on properties of <span><math><msub><mrow><mi>x</mi></mrow><mrow><mi>k</mi></mrow></msub></math></span>. The first conjecture concerns the factor complexity, the second one the critical exponent of these words. We confirm the validity of both conjectures for every <span><math><mi>k</mi></math></span>.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104250"},"PeriodicalIF":0.9,"publicationDate":"2025-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145247905","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-30DOI: 10.1016/j.ejc.2025.104246
Mohamed Omar, Justin M. Troyka
<div><div>Given a set <span><math><mrow><mi>I</mi><mo>⊆</mo><mi>N</mi></mrow></math></span>, consider the sequences <span><math><mrow><mrow><mo>{</mo><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>}</mo></mrow><mo>,</mo><mrow><mo>{</mo><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>}</mo></mrow></mrow></math></span> where for any <span><math><mi>n</mi></math></span>, <span><math><mrow><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow></mrow></math></span> and <span><math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow></mrow></math></span> respectively count the number of permutations in the symmetric group <span><math><msub><mrow><mi>S</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span> whose descent set (respectively peak set) is <span><math><mrow><mi>I</mi><mo>∩</mo><mrow><mo>[</mo><mi>n</mi><mo>−</mo><mn>1</mn><mo>]</mo></mrow></mrow></math></span>. We investigate the growth rates <span><math><mrow><mo>gr</mo><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>=</mo><msub><mrow><mo>lim</mo></mrow><mrow><mi>n</mi><mo>→</mo><mi>∞</mi></mrow></msub><msup><mrow><mfenced><mrow><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>/</mo><mi>n</mi><mo>!</mo></mrow></mfenced></mrow><mrow><mn>1</mn><mo>/</mo><mi>n</mi></mrow></msup></mrow></math></span> and <span><math><mrow><mo>gr</mo><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>=</mo><msub><mrow><mo>lim</mo></mrow><mrow><mi>n</mi><mo>→</mo><mi>∞</mi></mrow></msub><msup><mrow><mfenced><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>/</mo><mi>n</mi><mo>!</mo></mrow></mfenced></mrow><mrow><mn>1</mn><mo>/</mo><mi>n</mi></mrow></msup></mrow></math></span> over all <span><math><mrow><mi>I</mi><mo>⊆</mo><mi>N</mi></mrow></math></span>. Our main contributions are two-fold. Firstly, we prove that the numbers <span><math><mrow><mo>gr</mo><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow></mrow></math></span> over all <span><math><mrow><mi>I</mi><mo>⊆</mo><mi>N</mi></mrow></math></span> are exactly the interval <span><math><mfenced><mrow><mn>0</mn><mo>,</mo><mn>2</mn><mo>/</mo><mi>π</mi></mrow></mfenced></math></span>. To do so, we construct an algorithm that explicitly builds <span><math><mi>I</mi></math></span> for any desired limit <span><math><mi>L</mi></math></span> in the interval. Secondly, we prove that the numbers <span><math><mrow><mo>gr</mo><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow></mrow></math></span> for periodic sets <span><math>
{"title":"Growth rates of permutations with given descent or peak set","authors":"Mohamed Omar, Justin M. Troyka","doi":"10.1016/j.ejc.2025.104246","DOIUrl":"10.1016/j.ejc.2025.104246","url":null,"abstract":"<div><div>Given a set <span><math><mrow><mi>I</mi><mo>⊆</mo><mi>N</mi></mrow></math></span>, consider the sequences <span><math><mrow><mrow><mo>{</mo><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>}</mo></mrow><mo>,</mo><mrow><mo>{</mo><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>}</mo></mrow></mrow></math></span> where for any <span><math><mi>n</mi></math></span>, <span><math><mrow><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow></mrow></math></span> and <span><math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow></mrow></math></span> respectively count the number of permutations in the symmetric group <span><math><msub><mrow><mi>S</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span> whose descent set (respectively peak set) is <span><math><mrow><mi>I</mi><mo>∩</mo><mrow><mo>[</mo><mi>n</mi><mo>−</mo><mn>1</mn><mo>]</mo></mrow></mrow></math></span>. We investigate the growth rates <span><math><mrow><mo>gr</mo><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>=</mo><msub><mrow><mo>lim</mo></mrow><mrow><mi>n</mi><mo>→</mo><mi>∞</mi></mrow></msub><msup><mrow><mfenced><mrow><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>/</mo><mi>n</mi><mo>!</mo></mrow></mfenced></mrow><mrow><mn>1</mn><mo>/</mo><mi>n</mi></mrow></msup></mrow></math></span> and <span><math><mrow><mo>gr</mo><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>=</mo><msub><mrow><mo>lim</mo></mrow><mrow><mi>n</mi><mo>→</mo><mi>∞</mi></mrow></msub><msup><mrow><mfenced><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow><mo>/</mo><mi>n</mi><mo>!</mo></mrow></mfenced></mrow><mrow><mn>1</mn><mo>/</mo><mi>n</mi></mrow></msup></mrow></math></span> over all <span><math><mrow><mi>I</mi><mo>⊆</mo><mi>N</mi></mrow></math></span>. Our main contributions are two-fold. Firstly, we prove that the numbers <span><math><mrow><mo>gr</mo><msub><mrow><mi>d</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow></mrow></math></span> over all <span><math><mrow><mi>I</mi><mo>⊆</mo><mi>N</mi></mrow></math></span> are exactly the interval <span><math><mfenced><mrow><mn>0</mn><mo>,</mo><mn>2</mn><mo>/</mo><mi>π</mi></mrow></mfenced></math></span>. To do so, we construct an algorithm that explicitly builds <span><math><mi>I</mi></math></span> for any desired limit <span><math><mi>L</mi></math></span> in the interval. Secondly, we prove that the numbers <span><math><mrow><mo>gr</mo><msub><mrow><mi>p</mi></mrow><mrow><mi>n</mi></mrow></msub><mrow><mo>(</mo><mi>I</mi><mo>)</mo></mrow></mrow></math></span> for periodic sets <span><math>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"131 ","pages":"Article 104246"},"PeriodicalIF":0.9,"publicationDate":"2025-09-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145220359","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-22DOI: 10.1016/j.ejc.2025.104245
Jiyou Li, Sicheng Zhao
Biases in integer partitions have been studied recently. For three disjoint subsets of positive integers, let be the number of partitions of with parts from and be the number of such partitions with a greater number of parts in than that in . In this paper, in the case that are finite, we obtain an explicit formula of the asymptotic ratio of to . The key technique for computing this ratio is to estimate a partition number at the volume of a certain polytope. A conjecture is proposed in the case that are certain infinite arithmetic progressions.
{"title":"On the biases and asymptotics of partitions with finite choices of parts","authors":"Jiyou Li, Sicheng Zhao","doi":"10.1016/j.ejc.2025.104245","DOIUrl":"10.1016/j.ejc.2025.104245","url":null,"abstract":"<div><div>Biases in integer partitions have been studied recently. For three disjoint subsets <span><math><mrow><mi>R</mi><mo>,</mo><mi>S</mi><mo>,</mo><mi>I</mi></mrow></math></span> of positive integers, let <span><math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>R</mi><mi>S</mi><mi>I</mi></mrow></msub><mrow><mo>(</mo><mi>n</mi><mo>)</mo></mrow></mrow></math></span> be the number of partitions of <span><math><mi>n</mi></math></span> with parts from <span><math><mrow><mi>R</mi><mo>∪</mo><mi>S</mi><mo>∪</mo><mi>I</mi></mrow></math></span> and <span><math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>R</mi><mo>></mo><mi>S</mi><mo>,</mo><mi>I</mi></mrow></msub><mrow><mo>(</mo><mi>n</mi><mo>)</mo></mrow></mrow></math></span> be the number of such partitions with a greater number of parts in <span><math><mi>R</mi></math></span> than that in <span><math><mi>S</mi></math></span>. In this paper, in the case that <span><math><mrow><mi>R</mi><mo>,</mo><mi>S</mi><mo>,</mo><mi>I</mi></mrow></math></span> are finite, we obtain an explicit formula of the asymptotic ratio of <span><math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>R</mi><mo>></mo><mi>S</mi><mo>,</mo><mi>I</mi></mrow></msub><mrow><mo>(</mo><mi>n</mi><mo>)</mo></mrow></mrow></math></span> to <span><math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>R</mi><mi>S</mi><mi>I</mi></mrow></msub><mrow><mo>(</mo><mi>n</mi><mo>)</mo></mrow></mrow></math></span>. The key technique for computing this ratio is to estimate a partition number at the volume of a certain polytope. A conjecture is proposed in the case that <span><math><mrow><mi>R</mi><mo>,</mo><mi>S</mi></mrow></math></span> are certain infinite arithmetic progressions.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"131 ","pages":"Article 104245"},"PeriodicalIF":0.9,"publicationDate":"2025-09-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145118229","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-20DOI: 10.1016/j.ejc.2025.104244
Amanda Burcroff , Nicholas Ovenhouse , Ralf Schiffler , Sylvester W. Zhang
We introduce a -analog of the higher continued fractions introduced by the last three authors in a previous work (together with Gregg Musiker), which are simultaneously a generalization of the -rational numbers of Morier-Genoud and Ovsienko. They are defined as ratios of generating functions for -partitions on certain posets. We give matrix formulas for computing them, which generalize previous results in the case. We also show that certain properties enjoyed by the -rationals are also satisfied by our higher versions.
{"title":"Higher q-continued fractions","authors":"Amanda Burcroff , Nicholas Ovenhouse , Ralf Schiffler , Sylvester W. Zhang","doi":"10.1016/j.ejc.2025.104244","DOIUrl":"10.1016/j.ejc.2025.104244","url":null,"abstract":"<div><div>We introduce a <span><math><mi>q</mi></math></span>-analog of the higher continued fractions introduced by the last three authors in a previous work (together with Gregg Musiker), which are simultaneously a generalization of the <span><math><mi>q</mi></math></span>-rational numbers of Morier-Genoud and Ovsienko. They are defined as ratios of generating functions for <span><math><mi>P</mi></math></span>-partitions on certain posets. We give matrix formulas for computing them, which generalize previous results in the <span><math><mrow><mi>q</mi><mo>=</mo><mn>1</mn></mrow></math></span> case. We also show that certain properties enjoyed by the <span><math><mi>q</mi></math></span>-rationals are also satisfied by our higher versions.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"131 ","pages":"Article 104244"},"PeriodicalIF":0.9,"publicationDate":"2025-09-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145096502","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-17DOI: 10.1016/j.ejc.2025.104241
Colleen Robichaux
We give a minimal counterexample for a conjecture of Ross and Yong (2015) which proposes a K-Kohnert rule for Grothendieck polynomials. We conjecture a revised version of this rule. We then prove both rules hold in the 321-avoiding case.
{"title":"A counterexample to the Ross–Yong conjecture for Grothendieck polynomials","authors":"Colleen Robichaux","doi":"10.1016/j.ejc.2025.104241","DOIUrl":"10.1016/j.ejc.2025.104241","url":null,"abstract":"<div><div>We give a minimal counterexample for a conjecture of Ross and Yong (2015) which proposes a K-Kohnert rule for Grothendieck polynomials. We conjecture a revised version of this rule. We then prove both rules hold in the 321-avoiding case.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"131 ","pages":"Article 104241"},"PeriodicalIF":0.9,"publicationDate":"2025-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145096538","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-17DOI: 10.1016/j.ejc.2025.104242
Avichai Cohen, Shaul Zemel
Given a partition of a number , it is known that by adding a long line of length , the dimension of the associated representation of is an integer-valued polynomial of degree in . We show that its expansion in the binomial basis is bounded by the length of , and that the resulting coefficient of index , with alternating signs, counts the standard Young tableaux of shape in which a given collection of consecutive numbers lie in increasing rows. We also construct bijections in order to demonstrate explicitly that this number is indeed independent of the set of consecutive numbers used.
{"title":"Polynomial expressions for the dimensions of the representations of symmetric groups and restricted standard Young tableaux","authors":"Avichai Cohen, Shaul Zemel","doi":"10.1016/j.ejc.2025.104242","DOIUrl":"10.1016/j.ejc.2025.104242","url":null,"abstract":"<div><div>Given a partition <span><math><mi>λ</mi></math></span> of a number <span><math><mi>k</mi></math></span>, it is known that by adding a long line of length <span><math><mrow><mi>n</mi><mo>−</mo><mi>k</mi></mrow></math></span>, the dimension of the associated representation of <span><math><msub><mrow><mi>S</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span> is an integer-valued polynomial of degree <span><math><mi>k</mi></math></span> in <span><math><mi>n</mi></math></span>. We show that its expansion in the binomial basis is bounded by the length of <span><math><mi>λ</mi></math></span>, and that the resulting coefficient of index <span><math><mi>h</mi></math></span>, with alternating signs, counts the standard Young tableaux of shape <span><math><mi>λ</mi></math></span> in which a given collection of consecutive <span><math><mi>h</mi></math></span> numbers lie in increasing rows. We also construct bijections in order to demonstrate explicitly that this number is indeed independent of the set of consecutive <span><math><mi>h</mi></math></span> numbers used.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"131 ","pages":"Article 104242"},"PeriodicalIF":0.9,"publicationDate":"2025-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145096500","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-16DOI: 10.1016/j.ejc.2025.104236
Gábor Damásdi , Zichao Dong , Manfred Scheucher , Ji Zeng
In this paper, we consider saturation problems related to the celebrated Erdős–Szekeres convex polygon problem. For each , we construct a planar point set of size which is saturated for convex -gons. That is, the set contains no points in convex position while the addition of any new point creates such a configuration. This demonstrates that the saturation number is smaller than the Ramsey number for the Erdős–Szekeres problem. The proof also shows that the original Erdős–Szekeres construction is indeed saturated. Our construction is based on a similar improvement for the saturation version of the cups-versus-caps theorem. Moreover, we consider the generalization of the cups-versus-caps theorem to monotone paths in ordered hypergraphs. In contrast to the geometric setting, we show that this abstract saturation number is always equal to the corresponding Ramsey number.
本文考虑与著名的Erdős-Szekeres凸多边形问题相关的饱和问题。对于每个n≥7,我们构造一个大小为(7/8)·2n−2的平面点集,该点集对于凸n-gon是饱和的。也就是说,该集合不包含n个处于凸位置的点,而添加任何新点都会创建这样一个构型。这表明饱和数小于Erdős-Szekeres问题的Ramsey数。证明还表明,原来的Erdős-Szekeres结构确实是饱和的。我们的构造是基于杯子对帽子定理的饱和版本的类似改进。此外,我们考虑了cups- vs -caps定理在有序超图单调路径上的推广。与几何设置相反,我们证明了这个抽象饱和数总是等于相应的拉姆齐数。
{"title":"Saturation results around the Erdős–Szekeres problem","authors":"Gábor Damásdi , Zichao Dong , Manfred Scheucher , Ji Zeng","doi":"10.1016/j.ejc.2025.104236","DOIUrl":"10.1016/j.ejc.2025.104236","url":null,"abstract":"<div><div>In this paper, we consider saturation problems related to the celebrated Erdős–Szekeres convex polygon problem. For each <span><math><mrow><mi>n</mi><mo>≥</mo><mn>7</mn></mrow></math></span>, we construct a planar point set of size <span><math><mrow><mrow><mo>(</mo><mn>7</mn><mo>/</mo><mn>8</mn><mo>)</mo></mrow><mi>⋅</mi><msup><mrow><mn>2</mn></mrow><mrow><mi>n</mi><mo>−</mo><mn>2</mn></mrow></msup></mrow></math></span> which is saturated for convex <span><math><mi>n</mi></math></span>-gons. That is, the set contains no <span><math><mi>n</mi></math></span> points in convex position while the addition of any new point creates such a configuration. This demonstrates that the saturation number is smaller than the Ramsey number for the Erdős–Szekeres problem. The proof also shows that the original Erdős–Szekeres construction is indeed saturated. Our construction is based on a similar improvement for the saturation version of the cups-versus-caps theorem. Moreover, we consider the generalization of the cups-versus-caps theorem to monotone paths in ordered hypergraphs. In contrast to the geometric setting, we show that this abstract saturation number is always equal to the corresponding Ramsey number.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"131 ","pages":"Article 104236"},"PeriodicalIF":0.9,"publicationDate":"2025-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145096501","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-13DOI: 10.1016/j.ejc.2025.104235
Simona Boyadzhiyska , Shagnik Das , Thomas Lesgourgues , Kalina Petrova
In his study of graph codes, Alon introduced the concept of the odd-Ramsey number of a family of graphs in , defined as the minimum number of colours needed to colour the edges of so that every copy of a graph intersects some colour class in an odd number of edges. In this paper, we focus on complete bipartite graphs. First, we completely resolve the problem when is the family of all spanning complete bipartite graphs on vertices. We then focus on its subfamilies, that is, for a fixed set of integers . We prove that the odd-Ramsey problem is equivalent to determining the maximum dimension of a linear binary code avoiding codewords of given weights, and leverage known results from coding theory to deduce asymptotically tight bounds in our setting. We conclude with bounds for the odd-Ramsey numbers of fixed (that is, non-spanning) complete bipartite subgraphs.
{"title":"Odd-Ramsey numbers of complete bipartite graphs","authors":"Simona Boyadzhiyska , Shagnik Das , Thomas Lesgourgues , Kalina Petrova","doi":"10.1016/j.ejc.2025.104235","DOIUrl":"10.1016/j.ejc.2025.104235","url":null,"abstract":"<div><div>In his study of graph codes, Alon introduced the concept of the <em>odd-Ramsey</em> number of a family of graphs <span><math><mi>H</mi></math></span> in <span><math><msub><mrow><mi>K</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span>, defined as the minimum number of colours needed to colour the edges of <span><math><msub><mrow><mi>K</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span> so that every copy of a graph <span><math><mrow><mi>H</mi><mo>∈</mo><mi>H</mi></mrow></math></span> intersects some colour class in an odd number of edges. In this paper, we focus on complete bipartite graphs. First, we completely resolve the problem when <span><math><mi>H</mi></math></span> is the family of all spanning complete bipartite graphs on <span><math><mi>n</mi></math></span> vertices. We then focus on its subfamilies, that is, <span><math><mrow><mo>{</mo><msub><mrow><mi>K</mi></mrow><mrow><mi>t</mi><mo>,</mo><mi>n</mi><mo>−</mo><mi>t</mi></mrow></msub><mo>:</mo><mi>t</mi><mo>∈</mo><mi>T</mi><mo>}</mo></mrow></math></span> for a fixed set of integers <span><math><mrow><mi>T</mi><mo>⊆</mo><mrow><mo>[</mo><mrow><mo>⌊</mo><mi>n</mi><mo>/</mo><mn>2</mn><mo>⌋</mo></mrow><mo>]</mo></mrow></mrow></math></span>. We prove that the odd-Ramsey problem is equivalent to determining the maximum dimension of a linear binary code avoiding codewords of given weights, and leverage known results from coding theory to deduce asymptotically tight bounds in our setting. We conclude with bounds for the odd-Ramsey numbers of fixed (that is, non-spanning) complete bipartite subgraphs.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"131 ","pages":"Article 104235"},"PeriodicalIF":0.9,"publicationDate":"2025-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145049185","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-12DOI: 10.1016/j.ejc.2025.104238
Soichiro Fujii , Kei Kimura , Yuta Nozaki
Given finite simple graphs and , the Hom complex is a polyhedral complex having the graph homomorphisms as the vertices. We determine the homotopy type of each connected component of when is square-free, meaning that it does not contain the 4-cycle graph as a subgraph. Specifically, for a connected and a square-free , we show that each connected component of is homotopy equivalent to a wedge sum of circles. We further show that, given any graph homomorphism to a square-free , one can determine the homotopy type of the connected component of containing algorithmically.
{"title":"Homotopy types of Hom complexes of graph homomorphisms whose codomains are square-free","authors":"Soichiro Fujii , Kei Kimura , Yuta Nozaki","doi":"10.1016/j.ejc.2025.104238","DOIUrl":"10.1016/j.ejc.2025.104238","url":null,"abstract":"<div><div>Given finite simple graphs <span><math><mi>G</mi></math></span> and <span><math><mi>H</mi></math></span>, the Hom complex <span><math><mrow><mi>Hom</mi><mrow><mo>(</mo><mi>G</mi><mo>,</mo><mi>H</mi><mo>)</mo></mrow></mrow></math></span> is a polyhedral complex having the graph homomorphisms <span><math><mrow><mi>G</mi><mo>→</mo><mi>H</mi></mrow></math></span> as the vertices. We determine the homotopy type of each connected component of <span><math><mrow><mi>Hom</mi><mrow><mo>(</mo><mi>G</mi><mo>,</mo><mi>H</mi><mo>)</mo></mrow></mrow></math></span> when <span><math><mi>H</mi></math></span> is square-free, meaning that it does not contain the 4-cycle graph <span><math><msub><mrow><mi>C</mi></mrow><mrow><mn>4</mn></mrow></msub></math></span> as a subgraph. Specifically, for a connected <span><math><mi>G</mi></math></span> and a square-free <span><math><mi>H</mi></math></span>, we show that each connected component of <span><math><mrow><mi>Hom</mi><mrow><mo>(</mo><mi>G</mi><mo>,</mo><mi>H</mi><mo>)</mo></mrow></mrow></math></span> is homotopy equivalent to a wedge sum of circles. We further show that, given any graph homomorphism <span><math><mrow><mi>f</mi><mo>:</mo><mi>G</mi><mo>→</mo><mi>H</mi></mrow></math></span> to a square-free <span><math><mi>H</mi></math></span>, one can determine the homotopy type of the connected component of <span><math><mrow><mi>Hom</mi><mrow><mo>(</mo><mi>G</mi><mo>,</mo><mi>H</mi><mo>)</mo></mrow></mrow></math></span> containing <span><math><mi>f</mi></math></span> algorithmically.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"131 ","pages":"Article 104238"},"PeriodicalIF":0.9,"publicationDate":"2025-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145049184","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}