Pub Date : 2025-12-04DOI: 10.1016/j.ejc.2025.104305
Dong Yeap Kang , Mihyun Kang , Jaehoon Kim , Sang-il Oum
We conduct a quantitative analysis of how many random edges need to be added to a base graph in order to significantly increase natural minor-monotone graph parameters of the resulting graph . Specifically, we show that if is obtained from a connected graph by adding only a few random edges, the tree-width, genus, and Hadwiger number of become very large, irrespective of the structure of .
{"title":"Fragile minor-monotone parameters under a random edge perturbation","authors":"Dong Yeap Kang , Mihyun Kang , Jaehoon Kim , Sang-il Oum","doi":"10.1016/j.ejc.2025.104305","DOIUrl":"10.1016/j.ejc.2025.104305","url":null,"abstract":"<div><div>We conduct a quantitative analysis of how many random edges need to be added to a base graph <span><math><mi>H</mi></math></span> in order to significantly increase natural minor-monotone graph parameters of the resulting graph <span><math><mi>R</mi></math></span>. Specifically, we show that if <span><math><mi>R</mi></math></span> is obtained from a connected graph <span><math><mi>H</mi></math></span> by adding only a few random edges, the tree-width, genus, and Hadwiger number of <span><math><mi>R</mi></math></span> become very large, irrespective of the structure of <span><math><mi>H</mi></math></span>.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"133 ","pages":"Article 104305"},"PeriodicalIF":0.9,"publicationDate":"2025-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145685593","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 : 2025-12-03DOI: 10.1016/j.ejc.2025.104302
Barna Schefler , Kevin Zhao , Qinghai Zhong
The separating Noether number of a finite group is the minimal positive integer such that for every finite dimensional -module there is a separating set consisting of invariant polynomials of degree at most . In this paper we use methods from additive combinatorics to investigate the separating Noether number for finite abelian groups. Among others, we obtain the exact value of , provided that is either a -group or has rank 2, 3 or 5.
{"title":"On the separating Noether number of finite abelian groups","authors":"Barna Schefler , Kevin Zhao , Qinghai Zhong","doi":"10.1016/j.ejc.2025.104302","DOIUrl":"10.1016/j.ejc.2025.104302","url":null,"abstract":"<div><div>The separating Noether number <span><math><mrow><msub><mrow><mi>β</mi></mrow><mrow><mi>sep</mi></mrow></msub><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow></mrow></math></span> of a finite group <span><math><mi>G</mi></math></span> is the minimal positive integer <span><math><mi>d</mi></math></span> such that for every finite dimensional <span><math><mi>G</mi></math></span>-module <span><math><mi>V</mi></math></span> there is a separating set consisting of invariant polynomials of degree at most <span><math><mi>d</mi></math></span>. In this paper we use methods from additive combinatorics to investigate the separating Noether number for finite abelian groups. Among others, we obtain the exact value of <span><math><mrow><msub><mrow><mi>β</mi></mrow><mrow><mi>sep</mi></mrow></msub><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow></mrow></math></span>, provided that <span><math><mi>G</mi></math></span> is either a <span><math><mi>p</mi></math></span>-group or has rank 2, 3 or 5.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"133 ","pages":"Article 104302"},"PeriodicalIF":0.9,"publicationDate":"2025-12-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145652037","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 : 2025-12-02DOI: 10.1016/j.ejc.2025.104303
Tomáš Hons
A theorem by Ding, Oporowski, Oxley, and Vertigan states that every sufficiently large bipartite graph without twins contains a matching, co-matching, or half-graph of any given size as an induced subgraph. We prove that this Ramsey statement has polynomial dependency assuming bounded VC-dimension of the initial graph, using the recent verification of the Erdős-Hajnal property for graphs of bounded VC-dimension. Since the theorem of Ding et al. plays a role in (finite) model theory, which studies even more restricted structures, we also comment on further refinements of the theorem within this context.
{"title":"A polynomial Ramsey statement for bounded VC-dimension","authors":"Tomáš Hons","doi":"10.1016/j.ejc.2025.104303","DOIUrl":"10.1016/j.ejc.2025.104303","url":null,"abstract":"<div><div>A theorem by Ding, Oporowski, Oxley, and Vertigan states that every sufficiently large bipartite graph without twins contains a matching, co-matching, or half-graph of any given size as an induced subgraph. We prove that this Ramsey statement has polynomial dependency assuming bounded VC-dimension of the initial graph, using the recent verification of the Erdős-Hajnal property for graphs of bounded VC-dimension. Since the theorem of Ding et al. plays a role in (finite) model theory, which studies even more restricted structures, we also comment on further refinements of the theorem within this context.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"133 ","pages":"Article 104303"},"PeriodicalIF":0.9,"publicationDate":"2025-12-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145652036","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 : 2025-11-25DOI: 10.1016/j.ejc.2025.104285
Nantel Bergeron , Vincent Pilaud
For a hypergraph on , the hypergraphic poset is the transitive closure of the oriented skeleton of the hypergraphic polytope (the Minkowski sum of the standard simplices for all ). Hypergraphic posets include the weak order for the permutahedron (when is the complete graph on ) and the Tamari lattice for the associahedron (when is the set of all intervals of ), which motivates the study of lattice properties of hypergraphic posets. In this paper, we focus on interval hypergraphs, where all hyperedges are intervals of . We characterize the interval hypergraphs for which is a lattice, a distributive lattice, a semidistributive lattice, and a lattice quotient of the weak order.
{"title":"Interval hypergraphic lattices","authors":"Nantel Bergeron , Vincent Pilaud","doi":"10.1016/j.ejc.2025.104285","DOIUrl":"10.1016/j.ejc.2025.104285","url":null,"abstract":"<div><div>For a hypergraph <span><math><mi>H</mi></math></span> on <span><math><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></math></span>, the hypergraphic poset <span><math><msub><mrow><mi>P</mi></mrow><mrow><mi>H</mi></mrow></msub></math></span> is the transitive closure of the oriented skeleton of the hypergraphic polytope <span><math><msub><mrow><mo>△</mo></mrow><mrow><mi>H</mi></mrow></msub></math></span> (the Minkowski sum of the standard simplices <span><math><msub><mrow><mo>△</mo></mrow><mrow><mi>H</mi></mrow></msub></math></span> for all <span><math><mrow><mi>H</mi><mo>∈</mo><mi>H</mi></mrow></math></span>). Hypergraphic posets include the weak order for the permutahedron (when <span><math><mi>H</mi></math></span> is the complete graph on <span><math><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></math></span>) and the Tamari lattice for the associahedron (when <span><math><mi>H</mi></math></span> is the set of all intervals of <span><math><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></math></span>), which motivates the study of lattice properties of hypergraphic posets. In this paper, we focus on interval hypergraphs, where all hyperedges are intervals of <span><math><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></math></span>. We characterize the interval hypergraphs <span><math><mi>I</mi></math></span> for which <span><math><msub><mrow><mi>P</mi></mrow><mrow><mi>I</mi></mrow></msub></math></span> is a lattice, a distributive lattice, a semidistributive lattice, and a lattice quotient of the weak order.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104285"},"PeriodicalIF":0.9,"publicationDate":"2025-11-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145623505","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 : 2025-11-24DOI: 10.1016/j.ejc.2025.104299
Imre Leader , Ta Sheng Tan
The ‘odd cover number’ of a complete graph is the smallest size of a family of complete bipartite graphs that covers each edge an odd number of times. For odd, Buchanan, Clifton, Culver, Nie, O’Neill, Rombach and Yin showed that the odd cover number of is equal to or , and they conjectured that it is always . We prove this conjecture.
For even, Babai and Frankl showed that the odd cover number of is always at least , and the above authors and Radhakrishnan, Sen and Vishwanathan gave some values of for which equality holds. We give some new examples.
Our constructions arise from some very symmetric constructions for the corresponding problem for complete hypergraphs. That is, the odd cover number of the complete 3-graph is the smallest number of complete 3-partite 3-graphs such that each 3-set is in an odd number of them. We show that the odd cover number of is exactly for even , and we show that for odd it is for infinitely many values of . We also show that for and , the odd cover number of is strictly less than the partition number, answering a question of Buchanan, Clifton, Culver, Nie, O’Neill, Rombach and Yin for those values of .
完全图的“奇覆盖数”是覆盖每条边奇数次的完全二部图族的最小尺寸。对于n个奇数,Buchanan, Clifton, Culver, Nie, O 'Neill, Rombach和Yin证明了Kn的奇数覆盖数等于(n+1)/2或(n+3)/2,并推测它总是(n+1)/2。我们证明了这个猜想。对于偶数n, Babai和Frankl证明了Kn的奇覆盖数总是至少为n/2,并且上述作者和Radhakrishnan, Sen和Vishwanathan给出了n的一些相等值。我们给出了一些新的例子。我们的构造是由完全超图对应问题的一些非常对称的构造而来的。即完全3-图Kn(3)的奇盖数是使每个3-集都在奇数个完全3-图中的最小数目。我们证明了对于偶数n, Kn(3)的奇覆盖数恰好是n/2,对于无限多个n值,我们证明了对于奇数n,它是(n−1)/2。我们还证明了对于r=3和r=4, Kn(r)的奇覆盖数严格小于分区数,回答了Buchanan, Clifton, Culver, Nie, O 'Neill, Rombach和Yin对于这些r值的问题。
{"title":"Odd covers of complete graphs and hypergraphs","authors":"Imre Leader , Ta Sheng Tan","doi":"10.1016/j.ejc.2025.104299","DOIUrl":"10.1016/j.ejc.2025.104299","url":null,"abstract":"<div><div>The ‘odd cover number’ of a complete graph is the smallest size of a family of complete bipartite graphs that covers each edge an odd number of times. For <span><math><mi>n</mi></math></span> odd, Buchanan, Clifton, Culver, Nie, O’Neill, Rombach and Yin showed that the odd cover number of <span><math><msub><mrow><mi>K</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span> is equal to <span><math><mrow><mrow><mo>(</mo><mi>n</mi><mo>+</mo><mn>1</mn><mo>)</mo></mrow><mo>/</mo><mn>2</mn></mrow></math></span> or <span><math><mrow><mrow><mo>(</mo><mi>n</mi><mo>+</mo><mn>3</mn><mo>)</mo></mrow><mo>/</mo><mn>2</mn></mrow></math></span>, and they conjectured that it is always <span><math><mrow><mrow><mo>(</mo><mi>n</mi><mo>+</mo><mn>1</mn><mo>)</mo></mrow><mo>/</mo><mn>2</mn></mrow></math></span>. We prove this conjecture.</div><div>For <span><math><mi>n</mi></math></span> even, Babai and Frankl showed that the odd cover number of <span><math><msub><mrow><mi>K</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span> is always at least <span><math><mrow><mi>n</mi><mo>/</mo><mn>2</mn></mrow></math></span>, and the above authors and Radhakrishnan, Sen and Vishwanathan gave some values of <span><math><mi>n</mi></math></span> for which equality holds. We give some new examples.</div><div>Our constructions arise from some very symmetric constructions for the corresponding problem for complete hypergraphs. That is, the odd cover number of the complete 3-graph <span><math><msubsup><mrow><mi>K</mi></mrow><mrow><mi>n</mi></mrow><mrow><mrow><mo>(</mo><mn>3</mn><mo>)</mo></mrow></mrow></msubsup></math></span> is the smallest number of complete 3-partite 3-graphs such that each 3-set is in an odd number of them. We show that the odd cover number of <span><math><msubsup><mrow><mi>K</mi></mrow><mrow><mi>n</mi></mrow><mrow><mrow><mo>(</mo><mn>3</mn><mo>)</mo></mrow></mrow></msubsup></math></span> is exactly <span><math><mrow><mi>n</mi><mo>/</mo><mn>2</mn></mrow></math></span> for even <span><math><mi>n</mi></math></span>, and we show that for odd <span><math><mi>n</mi></math></span> it is <span><math><mrow><mrow><mo>(</mo><mi>n</mi><mo>−</mo><mn>1</mn><mo>)</mo></mrow><mo>/</mo><mn>2</mn></mrow></math></span> for infinitely many values of <span><math><mi>n</mi></math></span>. We also show that for <span><math><mrow><mi>r</mi><mo>=</mo><mn>3</mn></mrow></math></span> and <span><math><mrow><mi>r</mi><mo>=</mo><mn>4</mn></mrow></math></span>, the odd cover number of <span><math><msubsup><mrow><mi>K</mi></mrow><mrow><mi>n</mi></mrow><mrow><mrow><mo>(</mo><mi>r</mi><mo>)</mo></mrow></mrow></msubsup></math></span> is strictly less than the partition number, answering a question of Buchanan, Clifton, Culver, Nie, O’Neill, Rombach and Yin for those values of <span><math><mi>r</mi></math></span>.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104299"},"PeriodicalIF":0.9,"publicationDate":"2025-11-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145623504","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 : 2025-11-21DOI: 10.1016/j.ejc.2025.104288
Andrey Kupavskii
In this paper, we address several Erdős–Ko–Rado type questions for families of partitions. Two partitions of are -intersecting if they share at least parts, and are partially-intersecting if some of their parts intersect in at least elements. The question of what is the largest family of pairwise -intersecting partitions was studied for several classes of partitions: Peter Erdős and Székely studied partitions of into parts of unrestricted size; Ku and Renshaw studied unrestricted partitions of ; Meagher and Moura, and then Godsil and Meagher studied partitions into parts of equal size. We improve and generalize the results proved by these authors.
Meagher and Moura, following the work of Erdős and Székely, introduced the notion of partially -intersecting partitions, and conjectured, what should be the largest partially -intersecting family of partitions into parts of equal size . The main result of this paper is the proof of their conjecture for all , provided is sufficiently large.
All our results are applications of the spread approximation technique, introduced by Zakharov and the author. In order to use it, we need to refine some of the theorems from the original paper. As a byproduct, this makes the present paper a self-contained presentation of the spread approximation technique for -intersecting problems.
{"title":"Erdős–Ko–Rado type results for partitions via spread approximations","authors":"Andrey Kupavskii","doi":"10.1016/j.ejc.2025.104288","DOIUrl":"10.1016/j.ejc.2025.104288","url":null,"abstract":"<div><div>In this paper, we address several Erdős–Ko–Rado type questions for families of partitions. Two partitions of <span><math><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></math></span> are <span><math><mi>t</mi></math></span><em>-intersecting</em> if they share at least <span><math><mi>t</mi></math></span> parts, and are <em>partially</em> <span><math><mi>t</mi></math></span><em>-intersecting</em> if some of their parts intersect in at least <span><math><mi>t</mi></math></span> elements. The question of what is the largest family of pairwise <span><math><mi>t</mi></math></span>-intersecting partitions was studied for several classes of partitions: Peter Erdős and Székely studied partitions of <span><math><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></math></span> into <span><math><mi>ℓ</mi></math></span> parts of unrestricted size; Ku and Renshaw studied unrestricted partitions of <span><math><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></math></span>; Meagher and Moura, and then Godsil and Meagher studied partitions into <span><math><mi>ℓ</mi></math></span> parts of equal size. We improve and generalize the results proved by these authors.</div><div>Meagher and Moura, following the work of Erdős and Székely, introduced the notion of partially <span><math><mi>t</mi></math></span>-intersecting partitions, and conjectured, what should be the largest partially <span><math><mi>t</mi></math></span>-intersecting family of partitions into <span><math><mi>ℓ</mi></math></span> parts of equal size <span><math><mi>k</mi></math></span>. The main result of this paper is the proof of their conjecture for all <span><math><mrow><mi>t</mi><mo>,</mo><mi>k</mi></mrow></math></span>, provided <span><math><mi>ℓ</mi></math></span> is sufficiently large.</div><div>All our results are applications of the spread approximation technique, introduced by Zakharov and the author. In order to use it, we need to refine some of the theorems from the original paper. As a byproduct, this makes the present paper a self-contained presentation of the spread approximation technique for <span><math><mi>t</mi></math></span>-intersecting problems.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104288"},"PeriodicalIF":0.9,"publicationDate":"2025-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145578993","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 : 2025-11-21DOI: 10.1016/j.ejc.2025.104286
P. Frankl
Let and . The complete -graph on vertices has edges, any two edges intersect in at least -vertices. Moreover, there is no -element set intersecting each edge in at least vertices. The main result shows that for any -graph with the above properties has at most edges and the complete -graph is the unique optimal family.
{"title":"Critically intersecting hypergraphs","authors":"P. Frankl","doi":"10.1016/j.ejc.2025.104286","DOIUrl":"10.1016/j.ejc.2025.104286","url":null,"abstract":"<div><div>Let <span><math><mrow><mi>k</mi><mo>></mo><mi>t</mi><mo>≥</mo><mn>1</mn></mrow></math></span> and <span><math><mrow><mi>d</mi><mo>=</mo><mi>k</mi><mo>−</mo><mi>t</mi></mrow></math></span>. The complete <span><math><mi>k</mi></math></span>-graph on <span><math><mrow><mi>k</mi><mo>+</mo><mi>d</mi></mrow></math></span> vertices has <span><math><mfenced><mfrac><mrow><mi>k</mi><mo>+</mo><mi>d</mi></mrow><mrow><mi>d</mi></mrow></mfrac></mfenced></math></span> edges, any two edges intersect in at least <span><math><mi>t</mi></math></span>-vertices. Moreover, there is no <span><math><mrow><mo>(</mo><mi>k</mi><mo>−</mo><mn>1</mn><mo>)</mo></mrow></math></span>-element set intersecting each edge in at least <span><math><mi>t</mi></math></span> vertices. The main result shows that for <span><math><mrow><mi>k</mi><mo>≥</mo><msup><mrow><mi>d</mi></mrow><mrow><mn>4</mn></mrow></msup></mrow></math></span> any <span><math><mi>k</mi></math></span>-graph with the above properties has at most <span><math><mfenced><mfrac><mrow><mi>k</mi><mo>+</mo><mi>d</mi></mrow><mrow><mi>d</mi></mrow></mfrac></mfenced></math></span> edges and the complete <span><math><mi>k</mi></math></span>-graph is the unique optimal family.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104286"},"PeriodicalIF":0.9,"publicationDate":"2025-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145578995","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 : 2025-11-18DOI: 10.1016/j.ejc.2025.104287
Guilherme Simon Torres , Vilmar Trevisan
For a graph having Laplacian spectrum , Brouwer’s Conjecture states that . We prove that for each graph , there is a number , called Brouwer Critical Index - BCI, so that if , then satisfies Brouwer’s Conjecture. We also explore this graph invariant as a spectral parameter, obtaining natural properties. As an application of the BCI, we show that a class of bipartite graphs satisfies Brouwer’s Conjecture. Additionally, we prove that the corona product of graphs preserves Brouwer’s Conjecture.
{"title":"The Critical Index of Brouwer’s conjecture","authors":"Guilherme Simon Torres , Vilmar Trevisan","doi":"10.1016/j.ejc.2025.104287","DOIUrl":"10.1016/j.ejc.2025.104287","url":null,"abstract":"<div><div>For a graph <span><math><mi>G</mi></math></span> having Laplacian spectrum <span><math><mrow><msub><mrow><mi>μ</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>≥</mo><msub><mrow><mi>μ</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>≥</mo><mo>⋯</mo><mo>≥</mo><msub><mrow><mi>μ</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>=</mo><mn>0</mn></mrow></math></span>, Brouwer’s Conjecture states that <span><math><mrow><msub><mrow><mi>S</mi></mrow><mrow><mi>k</mi></mrow></msub><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow><mo>=</mo><msubsup><mrow><mo>∑</mo></mrow><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mrow><mi>k</mi></mrow></msubsup><msub><mrow><mi>μ</mi></mrow><mrow><mi>i</mi></mrow></msub><mo>≤</mo><mi>m</mi><mo>+</mo><mfenced><mfrac><mrow><mi>k</mi><mo>+</mo><mn>1</mn></mrow><mrow><mn>2</mn></mrow></mfrac></mfenced><mo>,</mo><mspace></mspace><mtext>for any</mtext><mspace></mspace><mn>1</mn><mo>≤</mo><mi>k</mi><mo>≤</mo><mi>n</mi></mrow></math></span>. We prove that for each graph <span><math><mi>G</mi></math></span>, there is a number <span><math><mrow><mi>h</mi><mo>∈</mo><mrow><mo>{</mo><mn>1</mn><mo>,</mo><mo>…</mo><mo>,</mo><mi>n</mi><mo>}</mo></mrow></mrow></math></span>, called Brouwer Critical Index - BCI, so that if <span><math><mrow><msub><mrow><mi>S</mi></mrow><mrow><mi>h</mi></mrow></msub><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow><mo>≤</mo><mi>m</mi><mo>+</mo><mfenced><mfrac><mrow><mi>h</mi><mo>+</mo><mn>1</mn></mrow><mrow><mn>2</mn></mrow></mfrac></mfenced></mrow></math></span>, then <span><math><mi>G</mi></math></span> satisfies Brouwer’s Conjecture. We also explore this graph invariant as a spectral parameter, obtaining natural properties. As an application of the BCI, we show that a class of bipartite graphs satisfies Brouwer’s Conjecture. Additionally, we prove that the corona product of graphs preserves Brouwer’s Conjecture.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104287"},"PeriodicalIF":0.9,"publicationDate":"2025-11-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145578994","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 : 2025-11-18DOI: 10.1016/j.ejc.2025.104289
Tomasz Przybyłowski
We construct a simple acyclic directed graph for which the Bunkbed Conjecture is false, thereby resolving conjectures posed by Leander and by Hollom.
我们构造了一个简单的无环有向图,它的铺床猜想是假的,从而解决了Leander和Hollom的猜想。
{"title":"The acyclic directed bunkbed conjecture is false","authors":"Tomasz Przybyłowski","doi":"10.1016/j.ejc.2025.104289","DOIUrl":"10.1016/j.ejc.2025.104289","url":null,"abstract":"<div><div>We construct a simple acyclic directed graph for which the Bunkbed Conjecture is false, thereby resolving conjectures posed by Leander and by Hollom.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104289"},"PeriodicalIF":0.9,"publicationDate":"2025-11-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145579011","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 : 2025-11-13DOI: 10.1016/j.ejc.2025.104283
Tongyuan Zhao , Zhicong Lin , Yongchun Zang
We prove a combinatorial identity relating Catalan numbers to tangent numbers arising from the study of peak algebra that was conjectured by Aliniaeifard and Li. This identity leads to the discovery of the intriguing identity where denote the tangent numbers. Interestingly, the latter identity can be applied to prove that is divisible by and the quotient is an odd number, a fact whose traditional proofs require significant calculations. Moreover, we find a natural -analog of the latter identity with a combinatorial proof. This -identity can be applied to prove Foata’s divisibility property of the -tangent numbers, which responds to a problem raised by Schützenberger.
{"title":"An identity relating Catalan numbers to tangent numbers with arithmetic applications","authors":"Tongyuan Zhao , Zhicong Lin , Yongchun Zang","doi":"10.1016/j.ejc.2025.104283","DOIUrl":"10.1016/j.ejc.2025.104283","url":null,"abstract":"<div><div>We prove a combinatorial identity relating Catalan numbers to tangent numbers arising from the study of peak algebra that was conjectured by Aliniaeifard and Li. This identity leads to the discovery of the intriguing identity <span><math><mrow><munderover><mrow><mo>∑</mo></mrow><mrow><mi>k</mi><mo>=</mo><mn>0</mn></mrow><mrow><mi>n</mi><mo>−</mo><mn>1</mn></mrow></munderover><mfenced><mfrac><mrow><mn>2</mn><mi>n</mi></mrow><mrow><mn>2</mn><mi>k</mi><mo>+</mo><mn>1</mn></mrow></mfrac></mfenced><msup><mrow><mn>2</mn></mrow><mrow><mn>2</mn><mi>n</mi><mo>−</mo><mn>2</mn><mi>k</mi></mrow></msup><msup><mrow><mrow><mo>(</mo><mo>−</mo><mn>1</mn><mo>)</mo></mrow></mrow><mrow><mi>k</mi></mrow></msup><msub><mrow><mi>E</mi></mrow><mrow><mn>2</mn><mi>k</mi><mo>+</mo><mn>1</mn></mrow></msub><mo>=</mo><msup><mrow><mn>2</mn></mrow><mrow><mn>2</mn><mi>n</mi><mo>+</mo><mn>1</mn></mrow></msup><mo>,</mo></mrow></math></span> where <span><math><msub><mrow><mi>E</mi></mrow><mrow><mn>2</mn><mi>k</mi><mo>+</mo><mn>1</mn></mrow></msub></math></span> denote the tangent numbers. Interestingly, the latter identity can be applied to prove that <span><math><mrow><mrow><mo>(</mo><mi>n</mi><mo>+</mo><mn>1</mn><mo>)</mo></mrow><msub><mrow><mi>E</mi></mrow><mrow><mn>2</mn><mi>n</mi><mo>+</mo><mn>1</mn></mrow></msub></mrow></math></span> is divisible by <span><math><msup><mrow><mn>2</mn></mrow><mrow><mn>2</mn><mi>n</mi></mrow></msup></math></span> and the quotient is an odd number, a fact whose traditional proofs require significant calculations. Moreover, we find a natural <span><math><mi>q</mi></math></span>-analog of the latter identity with a combinatorial proof. This <span><math><mi>q</mi></math></span>-identity can be applied to prove Foata’s divisibility property of the <span><math><mi>q</mi></math></span>-tangent numbers, which responds to a problem raised by Schützenberger.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":"132 ","pages":"Article 104283"},"PeriodicalIF":0.9,"publicationDate":"2025-11-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145528891","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}