We are going to define for edges, concepts equivalent to the one mentioned above. The edge connectivity  of a graph G, with more than one vertex, is the smallest number of edges by which removal disconnects the graph. In particular, it is 0 if the graph is disconnected. The edge connectivity is considered equal to 0 if the graph has only one vertex.

           We can formalize the definition in this way. If the graph G has at least two vertices, it has a set of edges B, possibly empty, such that G − B is disconnected, and we put in that case:

The set of edges B is what we call an (edge) cut of G. If G has only one vertex, we put:

There is an inequality relation between connectivity and edge connectivity, given in the following proposition.

Proposition 1.1.

 For any simple graph G, we have:

The second inequality is easy, the first can be shown directly but also results easily from Menger’s theorem (see later). These inequalities may be strict (find an example).

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Graph Theory  and Applications ,Jean-Claude Fournier, WILEY, page(63-64)