Mathematics – Combinatorics
Scientific paper
1994-09-16
Mathematics
Combinatorics
9 pages
Scientific paper
Let H = (H,V) be a hypergraph with edge set H and vertex set V. Then hypergraph H is invertible iff there exists a permutation pi of V such that for all E belongs to H(edges) intersection of(pi(E) and E)=0. H is invertibility critical if H is not invertible but every hypergraph obtained by removing an edge from H is invertible. The degree of H is d if |{E belongs to H(edges)|x belongs to E}| =< d for each x belongs to V Let i(d) be the maximum number of edges of an invertibility critical hypergraph of degree d. Theorem: i(d) =< (d-1) {2d-1 choose d} + 1. The proof of this result leads to the following covering problem on graphs: Let G be a graph. A family H is subset of (2^{V(G)} is an edge cover of G iff for every edge e of G, there is an E belongs to H(edge set) which includes e. H(edge set) is a minimal edge cover of G iff for H' subset of H, H' is not an edge cover of G. Let b(d) (c(d)) be the maximum cardinality of a minimal edge cover H(edge set) of a complete bipartite graph (complete graph) where H(edge set) has degree d. Theorem: c(d)=< i(d)=
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