A maximal connected subgraph without a tangent point in G is called a block of G. If H is a block of G, then H itself does not contain a tangent point, and the following conditions are met: If H adds an edge but no nodes, then H is not a subgraph of G; If you add nodes or edges to H and expand H into a larger connected graph, then H will contain a tangent point.

Extended data:

If the proper subset t of the vertex set of graph G satisfies G-T unconnected or trivial graph, if the proper subset s of the edge set of graph G satisfies G-S unconnected or trivial graph.

The necessary condition for a graph G to have a strongly connected directed graph is that G is 2- edge connected. Otherwise, there is pruning in G, which contradicts the strongly connected digraph of G.

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