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The 1-2-3 conjecture states that every simple graph (with no isolated edges) has an edge weigthing by numbers 1, 2, 3 such that the resulting weighted vertex degrees form a proper coloring of the graph. We study a similar problem for oriented hypergraphs. We prove that every oriented hypergraph has an edge weighting satisfying a similar condition, even if the weights are to be chosen from arbitrary lists of size two. The proof is based on the combinatorial nullstellensatz and a theorem of I. Schur Math. Z. 1, 184--207 (1918) for permanents of positive semi-definite matrices. We derive several consequences of the main result for uniform hypergraphs. We also point on possible applications of our results to problems of 1-2-3 type for non-oriented hypergraphs. 1-2-3 domneva izjavlja, da za vsak enostaven graf (brez izoliranih povezav) obstaja obtežitev povezav s števili 1, 2, 3, tako da rezultirajoče utežene stopnje vozlišč tvorijo pravilno barvanje grafa. Raziskujemo podoben problem za orientirane hipergrafe. Dokažemo, da ima vsak orientiran hipergraf obtežitev povezav, ki zadošča podobnemu pogoju, tudi če je uteži treba izbirati iz poljubnega seznama dolžine dve. Dokaz je osnovan na kombinatoričnem izreku o položajih ničel in Schurovem izreku za permanente pozitivnih semi-definitnih matrik. Izpeljemo več posledic glavnega rezultata za uniformne hipergrafe. Opozorimo tudi na možne uporabe naših rezultatov pri problemih tipa 1-2-3 za neorientirane hipergrafe.

We introduce sequence hypergraphs by extending the concept of a directed edge (from simple directed graphs) to hypergraphs. Specifically, every hyperedge of a sequence hypergraph is defined as a sequence of vertices (imagine it as a directed path). Note that this differs substantially from the standard definition of directed hypergraphs. Sequence hypergraphs are motivated by problems in public transportation networks, as they conveniently represent transportation lines. We study the complexity of some classic algorithmic problems, arising (not only) in transportation, in the setting of sequence hypergraphs. In particular, we consider the problem of finding a shortest st-hyperpath: a minimum set of hyperedges that "connects" (allows to travel to) t from s; finding a minimum st-hypercut: a minimum set of hyperedges whose removal "disconnects" t from s; or finding a maximum st-hyperflow: a maximum number of hyperedge-disjoint st-hyperpaths. We show that many of these problems are APX-hard, even in acyclic sequence hypergraphs or with hyperedges of constant length. However, if all the hyperedges are of length at most 2, we show, these problems become polynomially solvable. We also study the special setting in which for every hyperedge there also is a hyperedge with the same sequence, but in the reverse order. Finally, we briefly discuss other algorithmic problems (e.g., finding a minimum spanning tree, or connected components).

The family $D(k,m)$ of graphs having an orientation such that for every vertex $v \in V(G)$ either (outdegree) $\deg^+(v) \le k$ or (indegree) $\deg^-(v) \le m$ have been investigated recently in several papers because of the role $D(k,m)$ plays in the efforts to estimate the maximum directed cut in digraphs and the minimum cover of digraphs by directed cuts. Results concerning the chromatic number of graphs in the family $D(k,m)$ have been obtained via the notion of $d$-degeneracy of graphs. In this paper we consider a far reaching generalization of the family $D(k,m)$, in a complementary form, into the context of $r$-uniform hypergraphs, using a generalization of Hakimi's theorem to $r$-uniform hypergraphs and by showing some tight connections with the well known Ramsey numbers for hypergraphs., Comment: 24 pages