Thomassen’s theorem on -connected graphs
Every -connected (http://planetmath.org/KConnectedGraph) graph with morethan 4 vertices has an edge such that (http://planetmath.org/EdgeContraction) is also -connected.
Note: denotes the graph obtained from by contracting the edge .If we also use the notation .
Suppose such an edge doesn’t exist. Then, for every edge , thegraph isn’t -connected and can be made disconnected byremoving 2 vertices. Since , our contracted vertex has to be one of these two. So for every edge , has avertex such that separates . Any 2vertices separated by in are separated in by. Since the minimal size of a separating set is 3, everyvertex in has an adjacent vertex in every component of .
Now we choose the edge , the vertex and the component suchthat is minimal. We also choose a vertex adjacent to in.
By construction is not -connected since removing disconnects from . So there is a vertex such that separates and as above every vertex in hasan adjacent vertex in every component of . We nowconsider a component of that doesn’t contain or. Such a component exists since and belong to the samecomponent and isn’t connected. Any vertex adjacent to in is also an element of since is an element of. This means is a proper subset of which contradicts ourassumption
that was minimal.