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**Contents**

The internal representation of a mesh must allows us to find a particular value (quantity stored in the representation) rapidly, or to calculate a value easily from information presented (quantity deduced from stored information). A mesh is therefore, in general, an assembly of values and arrays of values containing the set of geometric elements which constitute it. The following information is necessary for each element:

- the
*nature*of the element: segment, triangle, quadrangle, tetrahedra, pentahedra, hexahedra, or a particular element; - the list of the
*vertices*of the element; - the element
*connectivity*and*topology*(the connectivity describes the connectivity between the vertices, the topology defines the edges and faces of the elements in relation to the vertices); - the
*number*and list of the nodes; - the
*coordinates*of the vertices; - the
*physical attributes*of the element:- a
*sub-domain number*or material number. This number, assigned to each element, characterizes it with regard to a treatment of type:sub-domain number "volumetric" treatment -
*reference numbers*or*references*. This number is assigned to the items which constitute an element (faces, edges, points) and enables these items to be characterized with regard to a treatment of type:reference number "sub-volumetric" treatment

- a

The MODULEF code stores this information in a ** NOPO** data structure, constituting the natural interface
between the different mesh modules.

** Remark:**

- The sub-domains : The notion of materials is refined to distinguish two identical media subject to different conditions: as many sub-domain numbers are assigned as required to make this distinction;
- The references : This attribute has two functions:
- to characterize the items (edges and points) with respect to a boundary condition type treatment (loads, prescribed values, etc.)
- to describe a boundary from a geometric point of view: i.e. to associate a curve equation to a portion of the boundary and force, in this way, the mesh to approach this curve.

As many reference numbers are assigned as required in order to incorporate the entire set of descriptions.