On null 3-hypergraphs

International audienceGiven a 3-uniform hypergraph H consisting of a set V of vertices, and T ⊆ V 3 triples, a null labelling is an assignment of ±1 to the triples such that each vertex is contained in an equal number of triples labelled +1 and −1. Thus, the signed degree of each vertex is zero. A necessary condition for a null labelling is that the degree of every vertex of H is even. The Null Labelling Problem is to determine whether H has a null labelling. It is proved that this problem is NP-complete. Computer enumerations suggest that most hypergraphs which satisfy the necessary condition do have a null labelling. Some constructions are given which produce hypergraphs satisfying the necessary condition, but which do not have a null labelling. A self complementary 3-hypergraph with this property is also constructed

Graphs, Groups and Pseudo-Similar Vertices

Vertices u, u.u of a graph X are mutually pseudo-similar if X — u = X — u≃. • = X — u_ but no two of the vertices are related by an automorphism of X, We describe a method for constructing graphs with a set of k ≥2 mutually pseudo-similar vertices, using a group with a special subgroup. We show that in all graphs with pseudo-similar vertices, the vertices are pseudo-similar due to the action of a group on the cosets of some subgroup

Canonical Ordering for Triangulations on the Cylinder, with Applications to Periodic Straight-line Drawings

We extend the notion of canonical orderings to cylindric triangulations. This allows us to extend the incremental straight-line drawing algorithm of de Fraysseix, Pach and Pollack to this setting. Our algorithm yields in linear time a crossing-free straight-line drawing of a cylindric triangulation G with n vertices on a regular grid Z/wZ × [0..h], with w ≤ 2n and h ≤ n(2d + 1), where d is the (graph-) distance between the two boundaries. As a by-product, we can also obtain in linear time a crossing-free straight-line drawing of a toroidal triangulation with n vertices on a periodic regular grid Z/wZ × Z/hZ, with w ≤ 2n and h ≤ 1 + n(2c + 1), where c is the length of a shortest noncontractible cycle. Since c ≤ √ 2n, the grid area is O(n 5/2). Our algorithms apply to any triangulation (whether on the cylinder or on the torus) with no loops nor multiple edges in the periodic representation