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单词 ThomassensTheoremOn3connectedGraphs
释义

Thomassen’s theorem on 3-connected graphs


Every 3-connectedPlanetmathPlanetmath (http://planetmath.org/KConnectedGraph) graph G with morethan 4 vertices has an edge e such that G/e (http://planetmath.org/EdgeContraction) is also 3-connected.

Note: G/e denotes the graph obtained from G by contracting the edge e.If e=xy we also use the notation G/xy.

Suppose such an edge doesn’t exist. Then, for every edge e=xy, thegraph G/e isn’t 3-connected and can be made disconnected byremoving 2 vertices. Since κ(G)3, our contracted vertexvxy has to be one of these two. So for every edge e, G has avertex zx,y such that {vxy,z} separates G/e. Any 2vertices separated by {vxy,z} in G/e are separated in G byS:={x,y,z}. Since the minimalPlanetmathPlanetmath size of a separating set is 3, everyvertex in S has an adjacent vertex in every component of G-S.

Now we choose the edge e, the vertex z and the component C suchthat |C| is minimal. We also choose a vertex v adjacent to z inC.

By construction G/zv is not 3-connected since removing xydisconnects C-v from G/zv. So there is a vertex w such that{z,v,w} separates G and as above every vertex in {z,v,w} hasan adjacent vertex in every component of G-{z,v,w}. We nowconsider a component D of G-{z,v,w} that doesn’t contain x ory. Such a component exists since x and y belong to the samecomponent and G-{z,v,w} isn’t connected. Any vertex adjacent tov in D is also an element of C since v is an element ofC. This means D is a proper subsetMathworldPlanetmathPlanetmath of C which contradicts ourassumptionPlanetmathPlanetmath that |C| was minimal.

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