
theorem
  for G1 being _Graph, G2 being DSimpleGraph of G1
  for v1 being Vertex of G1, v2 being Vertex of G2
  st v1 = v2 holds v1 is cut-vertex iff v2 is cut-vertex
proof
  let G1 be _Graph, G2 be DSimpleGraph of G1;
  consider H being removeDParallelEdges of G1 such that
    A1: G2 is removeLoops of H by Th120;
  let v1 be Vertex of G1, v2 be Vertex of G2;
  assume A2: v1 = v2;
  reconsider v3 = v2 as Vertex of H by A1, GLIB_000:def 33;
  v3 is cut-vertex iff v2 is cut-vertex by A1, Th67;
  hence thesis by A2, Th110;
end;
