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1

A hierarchy of maximal intersecting triple systems

Polcyn J., Ruciński A.

Opuscula Mathematica

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2017

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Vol. 37, no. 4

597--608

EN

We reach beyond the celebrated theorems of Erdös-Ko-Rado and Hilton-Milner, and a recent theorem of Han-Kohayakawa, and determine all maximal intersecting triples systems. It turns out that for each n ≥ 7 there are exactly 15 pairwise non-isomorphic such systems (and 13 for n = 6). We present our result in terms of a hierarchy of Turan numbers [formula], s ≥ 1, where [formula] is a pair of disjoint triples. Moreover, owing to our unified approach, we provide short proofs of the above mentioned results (for triple systems only). The triangle C3 is defined as C3 = {{x1,y3,x2}, {x1,y2,x3}, {x2, y1,x3}}. Along the way we show that the largest intersecting triple system H on n ≥ 6 vertices, which is not a star and is triangle-free, consists of max{10, n} triples. This facilitates our main proof's philosophy which is to assume that H contains a copy of the triangle and analyze how the remaining edges of H intersect that copy.

2

Towards the boundary between easy and hard control problems in multicast Clos networks

Obszarski P., Jastrzębski A., Kubale M.

Bulletin of the Polish Academy of Sciences. Technical Sciences

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2015

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Vol. 63, nr 3

739--744

EN

In this article we study 3-stage Clos networks with multicast calls in general and 2-cast calls, in particular. We investigate various sizes of input and output switches and discuss some routing problems involved in blocking states. To express our results in a formal way we introduce a model of hypergraph edge-coloring. A new class of bipartite hypergraphs corresponding to Clos networks is studied. We identify some polynomially solvable instances as well as a number of NP-complete cases. Our results warn of possible troubles arising in the control of Clos networks even if they are composed of small-size switches in outer stages. This is in sharp contrast to classical unicast Clos networks for which all the control problems are polynomially solvable.

3

Heredity properties of connectedness in edge-coloured complete graphs

Idzik A., Tuza Z.

Prace Instytutu Podstaw Informatyki Polskiej Akademii Nauk

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1999

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nr 876

3-12

EN

If the monochromatic graphs G1 and G2 in a 2-edge-coloured complete graph Km(m>6) are connected, then there exist at least two vertices x such that the graphs G1\x and G2\x are also connected. Similar theorems are proved for k-edge-coloured complete graphs. They generalize earlier results of Idzik, Komar and Malawski (Discrete Math. 66(1987), 119-125). Examples are shown that analogous theorems are no longer true for 3-uniform complete hypergraphs.

PL

Jeśli monochormatyczne grafy G1 i G2 w 2-krawędziowo pokolorowanym grafie zupełnym Km(m>6) są spójne, to istnieją co najmniej dwa wierzchołki x takie, że grafy G1\x i G2\x są również spójne. Podobne twierdzenia są udowodnione dla k-krawędziowo pokolorowanych grafów zupełnych. Twierdzenia te uogólniają wcześniejsze rezultaty Idzika, Komara i Malawskiego (Discrete Math. 66 (1987), 119-125). Sa pokazane przykłady, że analogiczne twierdzenia nie są prawdziwe dla 3-jednostajnych hipergrafów zupełnych.