## 4.3. Graph invariants

Within the scope of the graph -based description this section shows how to use implied constraints, which are systematically linked to the description of a global constraint. This usually occurs in the following context:

In practice the graphs associated with global constraints have a regular structure that comes from the initial graph or from the property of the arc constraints. So, in addition to graph invariants that hold for any graph, we want also tighter graph invariants that hold for specific graph classes. The next section introduces the graph classes we consider, while the two other sections give the graph invariants on one and two graphs.