The Electronic Journal of Combinatorics
http://www.combinatorics.org/ojs/index.php/eljc
<p>The Electronic Journal of Combinatorics (E-JC) is a fully-refereed electronic journal with <em>very high standards,</em> publishing papers of substantial content and interest in all branches of discrete mathematics, including combinatorics, graph theory, and algorithms for combinatorial problems. The journal is <em>completely free</em> for both authors and readers. Authors retain the copyright of their articles. Articles published in E-JC are reviewed on MathSciNet and ZBMath, and are indexed by Web of Science. The latest papers are always available by clicking on the tab marked "Current" near the top of the page. You can also locate papers using the search facility on the right hand side of this page.</p><p>E-JC was founded in 1994 by Herbert S. Wilf and Neil Calkin, making it one of the oldest electronic journals.</p>en-US<p>The copyright of published papers remains with the authors. We only require your agreement that we publish it, as described in the following publication release agreement:</p><ol><li>This is an agreement between the Electronic Journal of Combinatorics (the "Journal"), and the copyright owner (the "Owner") of a work (the "Work") to be published in the Journal.</li><li>The Owner warrants that s/he has the full power and authority to enter into this Agreement and to grant the rights granted in this Agreement.</li><li>The Owner hereby grants to the Journal a worldwide, irrevocable, royalty free license to publish or distribute the Work, to enter into arrangements with others to publish or distribute the Work, and to archive the Work.</li><li>The Owner agrees that further publication of the Work, with the same or substantially the same content as appears in the Journal, will include an acknowledgement of prior publication in the Journal.</li></ol>akundgen@csusm.edu (André Kündgen)bdm@cs.anu.edu.au (Brendan McKay)Wed, 01 Jul 2015 14:19:19 +1000OJS 2.3.6.0http://blogs.law.harvard.edu/tech/rss60On Obstacle Numbers
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p1
<p>The obstacle number is a new graph parameter introduced by Alpert, Koch, and Laison (2010). Mukkamala et al. (2012) show that there exist graphs with $n$ vertices having obstacle number in $\Omega(n/\log n)$. In this note, we up this lower bound to $\Omega(n/(\log\log n)^2)$. Our proof makes use of an upper bound of Mukkamala et al. on the number of graphs having obstacle number at most $h$ in such a way that any subsequent improvements to their upper bound will improve our lower bound.</p>Vida Dujmović, Pat Morinhttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p1Wed, 01 Jul 2015 00:00:00 +1000On a Conjecture of Thomassen
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p2
<p>In 1989, Thomassen asked whether there is an integer-valued function $f(k)$ such that every $f(k)$-connected graph admits a spanning, bipartite $k$-connected subgraph. In this paper we take a first, humble approach, showing the conjecture is true up to a $\log n$ factor.</p>Michelle Delcourt, Asaf Ferberhttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p2Wed, 01 Jul 2015 00:00:00 +1000Distinct Parts Partitions without Sequences
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p3
<p>Partitions without sequences of consecutive integers as parts have been studied recently by many authors, including Andrews, Holroyd, Liggett, and Romik, among others. Their results include a description of combinatorial properties, hypergeometric representations for the generating functions, and asymptotic formulas for the enumeration functions. We complete a similar investigation of partitions into distinct parts without sequences, which are of particular interest due to their relationship with the Rogers-Ramanujan identities. Our main results include a double series representation for the generating function, an asymptotic formula for the enumeration function, and several combinatorial inequalities.</p>Kathrin Bringmann, Karl Mahlburg, Karthik Natarajhttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p3Wed, 01 Jul 2015 00:00:00 +1000Homomesy in Products of Two Chains
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p4
Many invertible actions $\tau$ on a set $\mathcal{S}$ of combinatorial objects, along with a natural statistic $f$ on $\mathcal{S}$, exhibit the following property which we dub <strong>homomesy</strong>: the average of $f$ over each $\tau$-orbit in $\mathcal{S}$ is the same as the average of $f$ over the whole set $\mathcal{S}$. This phenomenon was first noticed by Panyushev in 2007 in the context of the rowmotion action on the set of antichains of a root poset; Armstrong, Stump, and Thomas proved Panyushev's conjecture in 2011. We describe a theoretical framework for results of this kind that applies more broadly, giving examples in a variety of contexts. These include linear actions on vector spaces, sandpile dynamics, Suter's action on certain subposets of Young's Lattice, Lyness 5-cycles, promotion of rectangular semi-standard Young tableaux, and the rowmotion and promotion actions on certain posets. We give a detailed description of the latter situation for products of two chains. <br /><br /><br /><br />James Propp, Tom Robyhttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p4Wed, 01 Jul 2015 00:00:00 +1000Small Regular Graphs of Girth 7
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p5
In this paper, we construct new infinite families of regular graphs of girth 7 of smallest order known so far. Our constructions are based on combinatorial and geometric properties of $(q+1,8)$-cages, for $q$ a prime power. We remove vertices from such cages and add matchings among the vertices of minimum degree to achieve regularity in the new graphs. We obtain $(q+1)$-regular graphs of girth 7 and order $2q^3+q^2+2q$ for each even prime power $q \ge 4$, and of order $2q^3+2q^2-q+1$ for each odd prime power $q\ge 5$.M. Abreu, G. Araujo-Pardo, C. Balbuena, D. Labbate, J. Salashttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p5Wed, 01 Jul 2015 00:00:00 +1000Completing Partial Proper Colorings using Hall's Condition
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p6
<pre><!--StartFragment-->In the context of list-coloring the vertices of a graph, Hall's condition is a generalization of Hall's Marriage Theorem and is necessary (but not sufficient) for a graph to admit a proper list-coloring. The graph <span>$G$</span> with list assignment <span>$L$</span> satisfies <em>Hall's condition</em> if for each subgraph <span>$H$</span> of <span>$G$</span>, the inequality <span>$|V(H)| \leq \sum_{\sigma \in \mathcal{C}} \alpha(H(\sigma, L))$ </span>is satisfied, where $\mathcal{C}$ is the set of colors and $\alpha(H(\sigma, L))$ is the independence number of the subgraph of $H$ induced on the set of vertices having color $\sigma$ in their lists. A list assignment $L$ to a graph $G$ is called <em>Hall</em> if $(G,L)$ satisfies Hall's condition. A graph $G$ is <em>Hall</em> $m$-c<em>ompletable</em> for some $m \geq \chi(G)$ if every partial proper $m$-coloring of $G$ whose corresponding list assignment is Hall can be extended to a proper coloring of $G$. In 2011, Bobga et al. posed the following questions: (1) Are there examples of graphs that are Hall $m$-completable, but not Hall $(m+1)$-completable for some $m \geq 3$? (2) If $G$ is neither complete nor an odd cycle, is $G$ Hall $\Delta(G)$-completable? This paper establishes that for every $m \geq 3$, there exists a graph that is Hall $m$-completable but not Hall $(m+1)$-completable and also that every bipartite planar graph $G$ is Hall $\Delta(G)$-completable. </pre>Sarah Holliday, Jennifer Vandenbussche, Erik E Westlundhttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p6Wed, 01 Jul 2015 00:00:00 +1000Acyclic Subgraphs of Planar Digraphs
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p7
An acyclic set in a digraph is a set of vertices that induces an acyclic subgraph. In 2011, Harutyunyan conjectured that every planar digraph on $n$ vertices without directed 2-cycles possesses an acyclic set of size at least $3n/5$. We prove this conjecture for digraphs where every directed cycle has length at least 8. More generally, if $g$ is the length of the shortest directed cycle, we show that there exists an acyclic set of size at least $(1 - 3/g)n$.Noah Golowich, David Rolnickhttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p7Wed, 01 Jul 2015 00:00:00 +1000Arithmetic Properties of a Restricted Bipartition Function
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p8
<p>A bipartition of $n$ is an ordered pair of partitions $(\lambda,\mu)$ such that the sum of all of the parts equals $n$. In this article, we concentrate on the function $c_5(n)$, which counts the number of bipartitions $(\lambda,\mu)$ of $n$ subject to the restriction that each part of $\mu$ is divisible by $5$. We explicitly establish four Ramanujan type congruences and several infinite families of congruences for $c_5(n)$ modulo $3$.</p>Jian Liu, Andrew Y.Z. Wanghttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p8Wed, 01 Jul 2015 00:00:00 +1000Strong Turán Stability
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p9
We study maximal $K_{r+1}$-free graphs $G$ of almost extremal size<span>—</span>typically, $e(G)=\operatorname{ex}(n,K_{r+1})-O(n)$. We show that any such graph $G$ must have a large amount of `symmetry': in particular, all but very few vertices of $G$ must have twins. (Two vertices $u$ and $v$ are <em>twins</em> if they have the same neighbourhood.) As a corollary, we obtain a new, short proof of a theorem of Simonovits on the structure of extremal $K_{r+1}$-free graphs of chromatic number at least $k$ for all fixed $k \geq r \geq 2$.Mykhaylo Tyomkyn, Andrew J. Uzzellhttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p9Wed, 01 Jul 2015 00:00:00 +1000Mutually Unbiased Bush-type Hadamard Matrices and Association Schemes
http://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p10
<p>It was shown by LeCompte, Martin, and Owens in 2010 that the existence of mutually unbiased Hadamard matrices and the identity matrix, which coincide with mutually unbiased bases, is equivalent to that of a $Q$-polynomial association scheme of class four which is both $Q$-antipodal and $Q$-bipartite. We prove that the existence of a set of mutually unbiased Bush-type Hadamard matrices is equivalent to that of an association scheme of class five. As an application of this equivalence, we obtain an upper bound of the number of mutually unbiased Bush-type Hadamard matrices of order $4n^2$ to be $2n-1$. This is in contrast to the fact that the best general upper bound for the mutually unbiased Hadamard matrices of order $4n^2$ is $2n^2$. We also discuss a relation of our scheme to some fusion schemes which are $Q$-antipodal and $Q$-bipartite $Q$-polynomial of class $4$.</p>Hadi Kharaghani, Sara Sasani, Sho Sudahttp://www.combinatorics.org/ojs/index.php/eljc/article/view/v22i3p10Wed, 01 Jul 2015 00:00:00 +1000