3. The method as claimed in claim 2 , wherein said criterion is satisfied for a given vertex v if a characteristic vector A M i (v) of the neighborhood of the vertex v in the current minor M i of the graph and the characteristic vector A G (v) of the neighborhood of the vertex v in the constructed graph G, satisfy the following relation: 
 | A M i ( v )|≦β| A g ( v )− A M i ( v )| where β is a convergence parameter.
2. The method as claimed in claim 1 , wherein step b) includes a step of processing the graph G by an iterative process, each iteration comprising generating a minor M i+1 of a current minor M i of the graph by deletion from the current minor M i of the graph of at least one vertex v satisfying a predefined criterion relating to a comparison of its neighborhood in the current minor M i of the graph and of its neighborhood in the constructed graph G, the iterative process ending when the number of vertices of the current minor M i of the graph becomes less than or equal to N.