# Get PDF 2-Matchings and 2-covers of hypergraphs Contents:

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The calculation of r and for an arbitrary hypergraph is an NP-hard problem. Thus, any result that gives estimates, at least for a certain class of - hypergraphs, is especially valuable. One of the simplest estimates can be obtained from. The determination of the fractional matching and coveting number is a linear program- ming problem. Massachusetts Institute of Technology, Cambridge, Massachusetts Fractional matchings in graphs. Trotter [NT] proved that even much more , is true. To state their results, define the fractional matching polytope of the hypergraph denoted by FMP , as the set of all fractional matching vectors in IE J I, i.

These are obviously polyhedra.

If we can effectively describe all their vertices and facets, then in a certain sense we can solve any optimization problem concerning fractional matchings and covers. This description was given in [B], [BS], and in [NT] a discussion of this and more graph theoretical background can be found in Lovsz [L79].

Their results imply: all the vertices of the polytopes FMP f and 2. It is obvious that for arbitrary hypergraphs, a statement similar to 2. If v z then necessarily 1. However, his hypergraphs have large ranks.

## Matching Theory, Volume 29

In this section we prove that a similar statement holds even for hypergraphs of rank 3. For the proof we are going to use the following constructions.

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Example 3. We define 0"42 k as follows.

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Define 3 k as follows. For nonnegative integers n, Let q 2ab where b is odd. An upper bound on the denominator. In particular, the set Nr is a discrete sequence. The proof of this result is based on the following ideas. LEMMA 4. Other more general versions of this lemma are well known in the theory of linear programming.

This lemma just means that the number of constraints of a linear program can be reduced to the number of variables without changing the optimal value. Proofof 4.

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There exists an optimal fractional matching w0 i. Applications of these lemmas can be found in [Fii86], [FF]. We now move to the proof of Theorem 4. Proofof Theorem 4. Proof Figure 2 v [denominatorl shows The smallest seven elements of N3 are 1, the incidence matrices of seven hypergraphs of 2. By Lemmas 4.

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A short case-by-case study shows that these are all values of N3 which are not larger than 2. Remark 5. Q: I are now purchase to send pathways. To extract a download 2-matchings and 2-covers or tablet order of a excommunication and Remove it to a paradigm, add left ad macro at for distribution.

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