2 research outputs found
Covering complete partite hypergraphs by monochromatic components
A well-known special case of a conjecture attributed to Ryser states that
k-partite intersecting hypergraphs have transversals of at most k-1 vertices.
An equivalent form was formulated by Gy\'arf\'as: if the edges of a complete
graph K are colored with k colors then the vertex set of K can be covered by at
most k-1 sets, each connected in some color. It turned out that the analogue of
the conjecture for hypergraphs can be answered: Z. Kir\'aly proved that in
every k-coloring of the edges of the r-uniform complete hypergraph K^r (r >=
3), the vertex set of K^r can be covered by at most sets,
each connected in some color.
Here we investigate the analogue problem for complete r-uniform r-partite
hypergraphs. An edge coloring of a hypergraph is called spanning if every
vertex is incident to edges of any color used in the coloring. We propose the
following analogue of Ryser conjecture.
In every spanning (r+t)-coloring of the edges of a complete r-uniform
r-partite hypergraph, the vertex set can be covered by at most t+1 sets, each
connected in some color.
Our main result is that the conjecture is true for 1 <= t <= r-1. We also
prove a slightly weaker result for t >= r, namely that t+2 sets, each connected
in some color, are enough to cover the vertex set.
To build a bridge between complete r-uniform and complete r-uniform r-partite
hypergraphs, we introduce a new notion. A hypergraph is complete r-uniform
(r,l)-partite if it has all r-sets that intersect each partite class in at most
l vertices.
Extending our results achieved for l=1, we prove that for any r >= 3, 2 <= l
= 1+r-l, in every spanning k-coloring of the edges of a complete
r-uniform (r,l)-partite hypergraph, the vertex set can be covered by at most
1+\lfloor \frac{k-r+\ell-1}{\ell}\rfloor sets, each connected in some color.Comment: 14 page