How to Access Neighbor Nodes and Elements

In a finite element context it is often useful to have a fast access ( O(N) time with N number of nodes ) to the objects which are "around" a certain node. This can be achieved by storing on each node the list of all of the nodes and elements which are close to it.

Before proceeding it is useful to clarify the meaning of this "around". If we consifer the mesh we will define as "NEIGHBOUR NODES" of node the n