Pub Date : 2024-11-16DOI: 10.1016/j.jcta.2024.105978
Takahiro Matsushita , Shun Wakatsuki
We show that the dominance complex of a graph G coincides with the combinatorial Alexander dual of the neighborhood complex of the complement of G. Using this, we obtain a relation between the chromatic number of G and the homology group of . We also obtain several known results related to dominance complexes from well-known facts of neighborhood complexes. After that, we suggest a new method for computing the homology groups of the dominance complexes, using independence complexes of simple graphs. We show that several known computations of homology groups of dominance complexes can be reduced to known computations of independence complexes. Finally, we determine the homology group of by determining the homotopy types of the independence complex of .
我们证明了图 G 的支配复数 D(G) 与 G 的补集的邻域复数 N(G‾) 的组合亚历山大对偶重合，并由此得到了 G 的色度数 χ(G) 与 D(G) 的同调群之间的关系。我们还从邻接复数的著名事实中得到了几个与支配复数有关的已知结果。之后，我们提出了一种利用简单图的独立复数计算支配复数同调群的新方法。我们证明了支配复数同调群的几种已知计算方法可以简化为独立复数的已知计算方法。最后，我们通过确定 Pn×P3×P2 独立复数的同调类型来确定 D(Pn×P3) 的同调群。
{"title":"Dominance complexes, neighborhood complexes and combinatorial Alexander duals","authors":"Takahiro Matsushita , Shun Wakatsuki","doi":"10.1016/j.jcta.2024.105978","DOIUrl":"10.1016/j.jcta.2024.105978","url":null,"abstract":"<div><div>We show that the dominance complex <span><math><mi>D</mi><mo>(</mo><mi>G</mi><mo>)</mo></math></span> of a graph <em>G</em> coincides with the combinatorial Alexander dual of the neighborhood complex <span><math><mi>N</mi><mo>(</mo><mover><mrow><mi>G</mi></mrow><mo>‾</mo></mover><mo>)</mo></math></span> of the complement of <em>G</em>. Using this, we obtain a relation between the chromatic number <span><math><mi>χ</mi><mo>(</mo><mi>G</mi><mo>)</mo></math></span> of <em>G</em> and the homology group of <span><math><mi>D</mi><mo>(</mo><mi>G</mi><mo>)</mo></math></span>. We also obtain several known results related to dominance complexes from well-known facts of neighborhood complexes. After that, we suggest a new method for computing the homology groups of the dominance complexes, using independence complexes of simple graphs. We show that several known computations of homology groups of dominance complexes can be reduced to known computations of independence complexes. Finally, we determine the homology group of <span><math><mi>D</mi><mo>(</mo><msub><mrow><mi>P</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>×</mo><msub><mrow><mi>P</mi></mrow><mrow><mn>3</mn></mrow></msub><mo>)</mo></math></span> by determining the homotopy types of the independence complex of <span><math><msub><mrow><mi>P</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>×</mo><msub><mrow><mi>P</mi></mrow><mrow><mn>3</mn></mrow></msub><mo>×</mo><msub><mrow><mi>P</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span>.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"211 ","pages":"Article 105978"},"PeriodicalIF":0.9,"publicationDate":"2024-11-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142652948","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-11-06DOI: 10.1016/j.jcta.2024.105968
Sam Mattheus, Geertrui Van de Voorde
We use techniques from algebraic and extremal combinatorics to derive upper bounds on the number of independent sets in several (hyper)graphs arising from finite geometry. In this way, we obtain asymptotically sharp upper bounds for partial ovoids and EKR-sets of flags in polar spaces, line spreads in and plane spreads in , and caps in . The latter result extends work due to Roche-Newton and Warren [21] and Bhowmick and Roche-Newton [6].
Finally, we investigate caps in p-random subsets of , which parallels recent work for arcs in projective planes by Bhowmick and Roche-Newton, and Roche-Newton and Warren [6], [21], and arcs in projective spaces by Chen, Liu, Nie and Zeng [8].
我们利用代数和极值组合学的技术，推导出有限几何中若干（超）图中独立集数的上界。通过这种方法，我们得到了极空间中部分敖包和旌旗的 EKR 集、PG(2r-1,q) 中的线展和 PG(5,q) 中的面展以及 PG(3,q) 中的盖的渐近尖锐上界。最后，我们研究了 PG(r,q) 的 p 个随机子集中的盖，这与 Bhowmick 和 Roche-Newton 以及 Roche-Newton 和 Warren [6], [21] 最近针对投影平面中的弧所做的工作，以及 Chen, Liu, Nie 和 Zeng [8] 最近针对投影空间中的弧所做的工作相似。
{"title":"Upper bounds for the number of substructures in finite geometries from the container method","authors":"Sam Mattheus, Geertrui Van de Voorde","doi":"10.1016/j.jcta.2024.105968","DOIUrl":"10.1016/j.jcta.2024.105968","url":null,"abstract":"<div><div>We use techniques from algebraic and extremal combinatorics to derive upper bounds on the number of independent sets in several (hyper)graphs arising from finite geometry. In this way, we obtain asymptotically sharp upper bounds for partial ovoids and EKR-sets of flags in polar spaces, line spreads in <span><math><mrow><mi>PG</mi></mrow><mo>(</mo><mn>2</mn><mi>r</mi><mo>−</mo><mn>1</mn><mo>,</mo><mi>q</mi><mo>)</mo></math></span> and plane spreads in <span><math><mrow><mi>PG</mi></mrow><mo>(</mo><mn>5</mn><mo>,</mo><mi>q</mi><mo>)</mo></math></span>, and caps in <span><math><mrow><mi>PG</mi></mrow><mo>(</mo><mn>3</mn><mo>,</mo><mi>q</mi><mo>)</mo></math></span>. The latter result extends work due to Roche-Newton and Warren <span><span>[21]</span></span> and Bhowmick and Roche-Newton <span><span>[6]</span></span>.</div><div>Finally, we investigate caps in <em>p</em>-random subsets of <span><math><mrow><mi>PG</mi></mrow><mo>(</mo><mi>r</mi><mo>,</mo><mi>q</mi><mo>)</mo></math></span>, which parallels recent work for arcs in projective planes by Bhowmick and Roche-Newton, and Roche-Newton and Warren <span><span>[6]</span></span>, <span><span>[21]</span></span>, and arcs in projective spaces by Chen, Liu, Nie and Zeng <span><span>[8]</span></span>.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"210 ","pages":"Article 105968"},"PeriodicalIF":0.9,"publicationDate":"2024-11-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142593647","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-11-06DOI: 10.1016/j.jcta.2024.105969
E. Ghorbani , S. Kamali , G.B. Khosrovshahi
Motivated by a classical result of Graver and Jurkat (1973) and Graham, Li, and Li (1980) in combinatorial design theory, which states that the permutations of t- minimal trades generate the vector space of all t- trades, we investigate the vector space spanned by permutations of an arbitrary trade. We prove that this vector space possesses a decomposition as a direct sum of subspaces formed in the same way by a specific family of so-called total trades. As an application, we demonstrate that for any t- design, its permutations can span the vector space generated by all t- designs for sufficiently large values of v. In other words, any t- design, or even any t-trade, can be expressed as a linear combination of permutations of a fixed t-design. This substantially extends a result by Ghodrati (2019), who proved the same result for Steiner designs.
受 Graver 和 Jurkat（1973 年）以及 Graham、Li 和 Li（1980 年）在组合设计理论中的一个经典结果（即 t-(v,k)最小交易的排列组合产生所有 t-(v,k)交易的向量空间）的启发，我们研究了任意交易的排列组合所跨越的向量空间。我们证明，这个向量空间可以分解为由特定的所谓总交易系列以相同方式形成的子空间的直接和。换句话说，任何 t-（v,k,λ）设计，甚至任何 t 交易，都可以表示为固定 t 设计的排列组合的线性组合。这大大扩展了 Ghodrati（2019）的一个结果，他为斯坦纳设计证明了同样的结果。
{"title":"The vector space generated by permutations of a trade or a design","authors":"E. Ghorbani , S. Kamali , G.B. Khosrovshahi","doi":"10.1016/j.jcta.2024.105969","DOIUrl":"10.1016/j.jcta.2024.105969","url":null,"abstract":"<div><div>Motivated by a classical result of Graver and Jurkat (1973) and Graham, Li, and Li (1980) in combinatorial design theory, which states that the permutations of <em>t</em>-<span><math><mo>(</mo><mi>v</mi><mo>,</mo><mi>k</mi><mo>)</mo></math></span> minimal trades generate the vector space of all <em>t</em>-<span><math><mo>(</mo><mi>v</mi><mo>,</mo><mi>k</mi><mo>)</mo></math></span> trades, we investigate the vector space spanned by permutations of an arbitrary trade. We prove that this vector space possesses a decomposition as a direct sum of subspaces formed in the same way by a specific family of so-called total trades. As an application, we demonstrate that for any <em>t</em>-<span><math><mo>(</mo><mi>v</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span> design, its permutations can span the vector space generated by all <em>t</em>-<span><math><mo>(</mo><mi>v</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span> designs for sufficiently large values of <em>v</em>. In other words, any <em>t</em>-<span><math><mo>(</mo><mi>v</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span> design, or even any <em>t</em>-trade, can be expressed as a linear combination of permutations of a fixed <em>t</em>-design. This substantially extends a result by Ghodrati (2019), who proved the same result for Steiner designs.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"210 ","pages":"Article 105969"},"PeriodicalIF":0.9,"publicationDate":"2024-11-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142593648","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-11-05DOI: 10.1016/j.jcta.2024.105967
Olivia X.M. Yao
In 2012, Andrews and Merca proved a truncated theorem on Euler's pentagonal number theorem. Since then, a number of results on truncated theta series have been proved. In this paper, we find the connections between truncated sums of certain partition functions and the minimal excludant statistic which has been found to exhibit connections with a handful of objects such as Dyson's crank. We present a uniform method to confirm five conjectures on truncated sums of certain partition functions given by Ballantine and Merca. In particular, we provide partition-theoretic interpretations for some truncated sums by using the minimal excludant in congruences classes.
{"title":"Some conjectures of Ballantine and Merca on truncated sums and the minimal excludant in congruences classes","authors":"Olivia X.M. Yao","doi":"10.1016/j.jcta.2024.105967","DOIUrl":"10.1016/j.jcta.2024.105967","url":null,"abstract":"<div><div>In 2012, Andrews and Merca proved a truncated theorem on Euler's pentagonal number theorem. Since then, a number of results on truncated theta series have been proved. In this paper, we find the connections between truncated sums of certain partition functions and the minimal excludant statistic which has been found to exhibit connections with a handful of objects such as Dyson's crank. We present a uniform method to confirm five conjectures on truncated sums of certain partition functions given by Ballantine and Merca. In particular, we provide partition-theoretic interpretations for some truncated sums by using the minimal excludant in congruences classes.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"210 ","pages":"Article 105967"},"PeriodicalIF":0.9,"publicationDate":"2024-11-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142586416","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-22DOI: 10.1016/j.jcta.2024.105966
Wenjie Zhong , Xiande Zhang
For a given n, what is the smallest number k such that every sequence of length n is determined by the multiset of all its k-subsequences? This is called the k-deck problem for sequence reconstruction, and has been generalized to the two-dimensional case – reconstruction of -matrices from submatrices. Previous works show that the smallest k is at most for sequences and at most for matrices. We study this k-deck problem for general dimension d and prove that, the smallest k is at most for reconstructing any d dimensional hypermatrix of order n from the multiset of all its subhypermatrices of order k.
对于给定的 n，使得长度为 n 的每个序列都由其所有 k 个子序列的多集决定的最小数 k 是多少？这被称为序列重构的 k 层问题，并已被推广到二维情况--从子矩阵重构 n×n 矩阵。之前的研究表明，对于序列，最小的 k 至多为 O(n12)，而对于矩阵，则至多为 O(n23)。我们研究了一般维数为 d 的 k 层问题，并证明了从其所有阶数为 k 的子超矩阵的多集重构任何阶数为 n 的 d 维超矩阵时，最小 k 至多为 O(ndd+1)。
{"title":"Reconstruction of hypermatrices from subhypermatrices","authors":"Wenjie Zhong , Xiande Zhang","doi":"10.1016/j.jcta.2024.105966","DOIUrl":"10.1016/j.jcta.2024.105966","url":null,"abstract":"<div><div>For a given <em>n</em>, what is the smallest number <em>k</em> such that every sequence of length <em>n</em> is determined by the multiset of all its <em>k</em>-subsequences? This is called the <em>k</em>-deck problem for sequence reconstruction, and has been generalized to the two-dimensional case – reconstruction of <span><math><mi>n</mi><mo>×</mo><mi>n</mi></math></span>-matrices from submatrices. Previous works show that the smallest <em>k</em> is at most <span><math><mi>O</mi><mo>(</mo><msup><mrow><mi>n</mi></mrow><mrow><mfrac><mrow><mn>1</mn></mrow><mrow><mn>2</mn></mrow></mfrac></mrow></msup><mo>)</mo></math></span> for sequences and at most <span><math><mi>O</mi><mo>(</mo><msup><mrow><mi>n</mi></mrow><mrow><mfrac><mrow><mn>2</mn></mrow><mrow><mn>3</mn></mrow></mfrac></mrow></msup><mo>)</mo></math></span> for matrices. We study this <em>k</em>-deck problem for general dimension <em>d</em> and prove that, the smallest <em>k</em> is at most <span><math><mi>O</mi><mo>(</mo><msup><mrow><mi>n</mi></mrow><mrow><mfrac><mrow><mi>d</mi></mrow><mrow><mi>d</mi><mo>+</mo><mn>1</mn></mrow></mfrac></mrow></msup><mo>)</mo></math></span> for reconstructing any <em>d</em> dimensional hypermatrix of order <em>n</em> from the multiset of all its subhypermatrices of order <em>k</em>.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"209 ","pages":"Article 105966"},"PeriodicalIF":0.9,"publicationDate":"2024-10-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142530619","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-18DOI: 10.1016/j.jcta.2024.105965
Jingjun Bao , Lijun Ji , Juanjuan Xu
Strong orthogonal arrays have better space-filling properties than ordinary orthogonal arrays for computer experiments. Strong orthogonal arrays of strengths two plus, two star and three minus can improve the space-filling properties in low dimensions and column orthogonality plays a vital role in computer experiments. In this paper, we use difference matrices and generator matrices of linear codes to present several constructions of column-orthogonal strong orthogonal arrays of strengths two plus, two star, three minus and t. Our constructions can provide larger numbers of factors of column-orthogonal strong orthogonal arrays of strengths two plus, two star, three minus and t than those in the existing literature, enjoy flexible run sizes. These constructions are convenient, and the resulting designs are good choices for computer experiments.
{"title":"Direct constructions of column-orthogonal strong orthogonal arrays","authors":"Jingjun Bao , Lijun Ji , Juanjuan Xu","doi":"10.1016/j.jcta.2024.105965","DOIUrl":"10.1016/j.jcta.2024.105965","url":null,"abstract":"<div><div>Strong orthogonal arrays have better space-filling properties than ordinary orthogonal arrays for computer experiments. Strong orthogonal arrays of strengths two plus, two star and three minus can improve the space-filling properties in low dimensions and column orthogonality plays a vital role in computer experiments. In this paper, we use difference matrices and generator matrices of linear codes to present several constructions of column-orthogonal strong orthogonal arrays of strengths two plus, two star, three minus and <em>t</em>. Our constructions can provide larger numbers of factors of column-orthogonal strong orthogonal arrays of strengths two plus, two star, three minus and <em>t</em> than those in the existing literature, enjoy flexible run sizes. These constructions are convenient, and the resulting designs are good choices for computer experiments.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"209 ","pages":"Article 105965"},"PeriodicalIF":0.9,"publicationDate":"2024-10-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142530618","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-15DOI: 10.1016/j.jcta.2024.105964
Michael Fisher , Neil A. McKay , Rebecca Milley , Richard J. Nowakowski , Carlos P. Santos
In Combinatorial Game Theory, short game forms are defined recursively over all the positions the two players are allowed to move to. A form is decomposable if it can be expressed as a disjunctive sum of two forms with smaller birthday. If there are no such summands, then the form is indecomposable. The main contribution of this document is the characterization of the indecomposable nimbers and the characterization of the indecomposable numbers. More precisely, a nimber is indecomposable if and only if its size is a power of two, and a number is indecomposable if and only if its absolute value is less than or equal to one.
{"title":"Indecomposable combinatorial games","authors":"Michael Fisher , Neil A. McKay , Rebecca Milley , Richard J. Nowakowski , Carlos P. Santos","doi":"10.1016/j.jcta.2024.105964","DOIUrl":"10.1016/j.jcta.2024.105964","url":null,"abstract":"<div><div>In Combinatorial Game Theory, short game forms are defined recursively over all the positions the two players are allowed to move to. A form is decomposable if it can be expressed as a disjunctive sum of two forms with smaller birthday. If there are no such summands, then the form is indecomposable. The main contribution of this document is the characterization of the indecomposable nimbers and the characterization of the indecomposable numbers. More precisely, a nimber is indecomposable if and only if its size is a power of two, and a number is indecomposable if and only if its absolute value is less than or equal to one.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"209 ","pages":"Article 105964"},"PeriodicalIF":0.9,"publicationDate":"2024-10-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142437901","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-04DOI: 10.1016/j.jcta.2024.105962
Mark Pankov , Krzysztof Petelczyc , Mariusz Żynel
Point-line geometries whose singular subspaces correspond to binary equidistant codes are investigated. The main result is a description of automorphisms of these geometries. In some important cases, automorphisms induced by non-monomial linear automorphisms surprisingly arise.
{"title":"Point-line geometries related to binary equidistant codes","authors":"Mark Pankov , Krzysztof Petelczyc , Mariusz Żynel","doi":"10.1016/j.jcta.2024.105962","DOIUrl":"10.1016/j.jcta.2024.105962","url":null,"abstract":"<div><div>Point-line geometries whose singular subspaces correspond to binary equidistant codes are investigated. The main result is a description of automorphisms of these geometries. In some important cases, automorphisms induced by non-monomial linear automorphisms surprisingly arise.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"210 ","pages":"Article 105962"},"PeriodicalIF":0.9,"publicationDate":"2024-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142425591","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-04DOI: 10.1016/j.jcta.2024.105963
Pooneh Afsharijoo , Hussein Mourtada
We prove a family of partition identities which is “dual” to the family of Andrews-Gordon's identities. These identities are inspired by a correspondence between a special type of partitions and “hypergraphs” and their proof uses combinatorial commutative algebra.
{"title":"Neighborly partitions, hypergraphs and Gordon's identities","authors":"Pooneh Afsharijoo , Hussein Mourtada","doi":"10.1016/j.jcta.2024.105963","DOIUrl":"10.1016/j.jcta.2024.105963","url":null,"abstract":"<div><div>We prove a family of partition identities which is “dual” to the family of Andrews-Gordon's identities. These identities are inspired by a correspondence between a special type of partitions and “hypergraphs” and their proof uses combinatorial commutative algebra.</div></div>","PeriodicalId":50230,"journal":{"name":"Journal of Combinatorial Theory Series A","volume":"210 ","pages":"Article 105963"},"PeriodicalIF":0.9,"publicationDate":"2024-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142425592","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-26DOI: 10.1016/j.jcta.2024.105957
Carmen Amarra , Wei Jin , Cheryl E. Praeger
We investigate locally grid graphs, that is, graphs in which the neighbourhood of any vertex is the Cartesian product of two complete graphs on n vertices. We consider the subclass of these graphs for which each pair of vertices at distance two is joined by sufficiently many paths of length 2. The number of such paths is known to be at most 2n by previous work of Blokhuis and Brouwer. We show that if each pair is joined by at least such paths then the diameter is at most 3 and we give a tight upper bound on the order of the graphs. We show that graphs meeting this upper bound are distance-regular antipodal covers of complete graphs. We exhibit an infinite family of such graphs which are locally grid for odd prime powers n, and apply these results to locally grid graphs to obtain a classification for the case where either all μ-graphs (induced subgraphs on the set of common neighbours of two vertices at distance two) have order at least 8 or all μ-graphs have order c for some constant c.
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