The de Bruijn-Erdős theorem for hypergraphs

Lines in hypergraphs

One of the De Bruijn Erdős theorems deals with finite hypergraphs where every two vertices belong to precisely one hyperedge. It asserts that, except in the perverse case where a single hyperedge equals the whole vertex set, the number of hyperedges is at least the number of vertices and the two numbers are equal if and only if the hypergraph belongs to one of simply described families, nearpen...

A de Bruijn - Erdős theorem and metric spaces

De Bruijn and Erdős proved that every noncollinear set of n points in the plane determines at least n distinct lines. Chen and Chvátal suggested a possible generalization of this theorem in the framework of metric spaces. We provide partial results in this direction.

De Bruijn Covering Codes for Rooted Hypergraphs

What is the length of the shortest sequence S of reals so that the set of consecutive n-words in S form a covering code for permutations on {1, 2, . . . , n} of radius R ? (The distance between two n-words is the number of transpositions needed to have the same order type.) The above problem can be viewed as a special case of finding a De Bruijn covering code for a rooted hypergraph. Each edge ...

Eulerian and Hamiltonian dicycles in Directed hypergraphs

In this article, we generalize the concepts of Eulerian and Hamiltonian digraphs to directed hypergraphs. A dihypergraph H is a pair (V(H), E(H)), where V(H) is a non-empty set of elements, called vertices, and E(H) is a collection of ordered pairs of subsets of V(H), called hyperarcs. It is Eulerian (resp. Hamiltonian) if there is a dicycle containing each hyperarc (resp. each vertex) exactly ...

Biological applications for de Bruijn subgraphs and interval group testing

A De Bruijn-Erdős Theorem for Chordal Graphs

A special case of a combinatorial theorem of De Bruijn and Erdős asserts that every noncollinear set of n points in the plane determines at least n distinct lines. Chen and Chvátal suggested a possible generalization of this assertion in metric spaces with appropriately defined lines. We prove this generalization in all metric spaces induced by connected chordal graphs.