Constructing Regular Self-complementary Uniform Hypergraphs

AMS Subject Classication Codes: 05C65, 05B05 05E20, 05C85.In this paper, we examine the possible orders of t-subset-regular self-complementary k-uniform hypergraphs, which form examples of large sets of two isomorphic t-designs. We reformulate Khosrovshahi and Tayfeh-Rezaie's necessary conditions on the order of these structures in terms of the binary representation of the rank k, and these conditions simplify to a more transparent relation between the order n and rank k in the case where k is a sum of consecutive powers of 2. Moreover, we present new constructions for 1-subset-regular self-complementary uniform hypergraphs, and prove that these necessary conditions are sufficient for all k, in the case where t = 1.https://onlinelibrary.wiley.com/doi/abs/10.1002/jcd.2028

Regular Two-Graphs and Equiangular Lines

Regular two-graphs are antipodal distance-regular double coverings of the complete graph, and they have many interesting combinatorial properties. We derive a construction for regular two-graphs containing cliques of specified order from their connection to large sets of equiangular lines in Euclidean space. It is shown that the existence of a regular two-graph with least eigenvalue τ containing a clique of order d depends on the existence of an incidence structure on d points with special properties. Quasi-symmetric designs provide examples of these incidence structures.University of WaterlooMaster of Mathematics in Combinatorics and Optimizatio

Self-Complementary Hypergraphs

In this thesis, we survey the current research into self-complementary hypergraphs, and present several new results. We characterize the cycle type of the permutations on n elements with order equal to a power of 2 which are k-complementing. The k-complementing permutations map the edges of a k-uniform hypergraph to the edges of its complement. This yields a test to determine whether a finite permutation is a k-complementing permutation, and an algorithm for generating all self-complementary k-uniform hypergraphs of order n, up to isomorphism, for feasible n. We also obtain an alternative description of the known necessary and sufficient conditions on the order of a self-complementary k-uniform hypergraph in terms of the binary representation of k. We examine the orders of t-subset-regular self-complementary uniform hyper- graphs. These form examples of large sets of two isomorphic t-designs. We restate the known necessary conditions on the order of these structures in terms of the binary representation of the rank k, and we construct 1-subset-regular self-complementary uniform hypergraphs to prove that these necessary conditions are sufficient for all ranks k in the case where t = 1. We construct vertex transitive self-complementary k-hypergraphs of order n for all integers n which satisfy the known necessary conditions due to Potocnik and Sajna, and consequently prove that these necessary conditions are also sufficient. We also generalize Potocnik and Sajna's necessary conditions on the order of a vertex transitive self-complementary uniform hypergraph for certain ranks k to give neces- sary conditions on the order of these structures when they are t-fold-transitive. In addition, we use Burnside's characterization of transitive groups of prime degree to determine the group of automorphisms and antimorphisms of certain vertex transitive self-complementary k-uniform hypergraphs of prime order, and we present an algorithm to generate all such hypergraphs. Finally, we examine the orders of self-complementary non-uniform hypergraphs, including the cases where these structures are t-subset-regular or t-fold-transitive. We find necessary conditions on the order of these structures, and we present constructions to show that in certain cases these necessary conditions are sufficient.University of OttawaDoctor of Philosophy in Mathematic

The metric dimension of Cayley digraphs

AbstractA vertex x in a digraph D is said to resolve a pair u, v of vertices of D if the distance from u to x does not equal the distance from v to x. A set S of vertices of D is a resolving set for D if every pair of vertices of D is resolved by some vertex of S. The smallest cardinality of a resolving set for D, denoted by dim(D), is called the metric dimension for D. Sharp upper and lower bounds for the metric dimension of the Cayley digraphs Cay(Δ:Γ), where Γ is the group Zn1⊕Zn2⊕⋯⊕Znm and Δ is the canonical set of generators, are established. The exact value for the metric dimension of Cay({(0,1),(1,0)}:Zn⊕Zm) is found. Moreover, the metric dimension of the Cayley digraph of the dihedral group Dn of order 2n with a minimum set of generators is established. The metric dimension of a (di)graph is formulated as an integer programme. The corresponding linear programming formulation naturally gives rise to a fractional version of the metric dimension of a (di)graph. The fractional dual implies an integer dual for the metric dimension of a (di)graph which is referred to as the metric independence of the (di)graph. The metric independence of a (di)graph is the maximum number of pairs of vertices such that no two pairs are resolved by the same vertex. The metric independence of the n-cube and the Cayley digraph Cay(Δ:Dn), where Δ is a minimum set of generators for Dn, are established

Generating self-complementary uniform hypergraphs

In 2007, Szymanski and Wojda proved that for positive integers n; k with k less than n, a self-complementary k-uniform hypergraph of order n exists if and only if n/k is even. In this paper, we characterize the cycle type of a k-complementing permutation in Sym.n/ which has order equal to a power of 2. This yields a test for determining whether a finite permutation is a k-complementing permutation, and an algorithm for generating all self-complementary k-hypergraphs of order n, up to isomorphism, for feasible n.We also obtain an alternative description of the necessary and sufficient conditions on the order of a self-complementary k-uniform hypergraph, in terms of the binary representation of k. This extends previous results for the cases k D 2; 3; 4 due to Ringel, Sachs, Suprunenko, Kocay and Szymanski.University of Winnipe

Cyclically t-complementary uniform hypergraphs

A cyclically t-complementary k-hypergraph is a k-uniform hypergraph with vertex set V and edge set E for which there exists a permutation 2 Sym.V/ such that the sets E; E ; E 2; : : : ; E t1 partition the set of all k-subsets of V. Such a permutation is called a .t; k/-complementing permutation. The cyclically t-complementary k-hypergraphs are a natural and useful generalization of the self-complementary graphs, which have been studied extensively in the past due to their important connection to the graph isomorphism problem. For a prime p, we characterize the cycle type of the .pr ; k/- complementing permutations 2 Sym.V/ which have order a power of p. This yields a test for determining whether a permutation in Sym.V/ is a .pr ; k/-complementing permutation, and an algorithm for generating all of the cyclically pr-complementing k- hypergraphs of order n, for feasible n, up to isomorphism. We also obtain some necessary and sufficient conditions on the order of these structures. This generalizes previous results due to Ringel, Sachs, Adamus, Orchel, Szymański, Wojda, Zwonek, and Bernaldez.University of Winnipeghttps://www.sciencedirect.com/science/article/pii/S0195669810000594?via%3Dihubhttps://www.sciencedirect.com/science/article/pii/S0195669810000594?via%3Dihu

Vertex-transitive self-complementary uniform hypergraphs of prime order

For an integer n and a prime p, let n(p)=max{i:pidividesn}. In this paper, we present a construction for vertex-transitive self-complementary k-uniform hypergraphs of order n for each integer n such that pn(p)≡1(mod2ℓ+1) for every prime p, where ℓ=max{k(2),(k−1)(2)}, and consequently we prove that the necessary conditions on the order of vertex-transitive self-complementary uniform hypergraphs of rank k=2ℓ or k=2ℓ+1 due to Potoňick and Šajna are sufficient. In addition, we use Burnside’s characterization of transitive groups of prime degree to characterize the structure of vertex-transitive self-complementary k-hypergraphs which have prime order p in the case where k=2ℓ or k=2ℓ+1 and p≡1(mod2ℓ+1), and we present an algorithm to generate all of these structures. We obtain a bound on the number of distinct vertex-transitive self-complementary graphs of prime order p≡1(mod4), up to isomorphism.University of Winnipeghttps://www.sciencedirect.com/science/article/pii/S0012365X09004051?via%3DihubThis is an author-produced, peer-reviewed article that has been accepted for publication in Discrete Mathematics, but has not been copy-edited