Primal graph
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In graph theory, the primal graph of a hypergraph is a graph with the same vertices of the hypergraph, and an edge between any pair of vertices contained in the same hyperedge. Alternatively, a primal graph may be the planar graph from which a dual graph is formed.
In constraint satisfaction, the primal graph or primal constraint graph of a constraint satisfaction problem is the graph whose nodes are the variables of the problem and an edge joins a pair of variables if the two variables occur together in a constraint. The primal graph of a constraint satisfaction problem can also be defined as the primal graph of the hypergraph made of the variables and the constraint scopes. In the case of binary constraint network any two nodes having missing arcs between each other means these two nodes have the universal binary relation between each pair of values.
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