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== Definition ==
A [[hypergraph]] is a generalization of a [[Graph (discrete mathematics)|graph]]. Formally, a hypergraph <math>H = (V, E)</math> consists of a set of [[vertex (graph theory)|vertices]] ''V'', and of a set ''E'' of [[hyperedge]]s, each of which is a subset of the vertices ''V''. Given a hypergraph, we can define its ''primal graph'' as the undirected graph defined on the same set of vertices, in which we put an edge between any two vertices which occur together in some hyperedge.
A hypergraph ''H'' is '''α-acyclic''' if it satisfies two conditions: being chordal and being conformal. More precisely, we say that ''H'' is chordal if its primal graph is a [[chordal graph]]. We say that ''H'' is conformal if, for every [[clique (graph theory)|clique]] of the primal graph, there is a hyperedge of ''H'' containing all the vertices of the clique.
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