Abstract
In this paper, we settle a problem of Frankl and Filredi, which is a special case of a problem of Erdt~s, determining the maximum number of hyperedges in a 3-uniform hypergraph in which no two pairs of distinct hyperedges have the same union. The extremal case corresponds to the existence of weakly union-free twofold triple systems, which is settled here with six definite and four possible exceptions. An application to group testing is also given.
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