
theorem
  for G being locally-finite with_max_out_degree _Graph, n being Nat
  holds G.maxOutDegree() = n iff
    ex v being Vertex of G st v.outDegree() = n &
      for w being Vertex of G holds w.outDegree() <= v.outDegree()
proof
  let G be locally-finite with_max_out_degree _Graph, n be Nat;
  hereby
    assume G.maxOutDegree() = n;
    then consider v being Vertex of G such that
      A1: v.outDegree() = n and
      A2: for w being Vertex of G holds w.outDegree() c= v.outDegree()
      by Th81;
    take v;
    thus v.outDegree() = n by A1;
    let w be Vertex of G;
    Segm w.outDegree() c= Segm v.outDegree() by A2;
    hence w.outDegree() <= v.outDegree() by NAT_1:39;
  end;
  given v being Vertex of G such that
    A3: v.outDegree() = n and
    A4: for w being Vertex of G holds w.outDegree() <= v.outDegree();
  now
    let w be Vertex of G;
    Segm w.outDegree() c= Segm v.outDegree() by A4, NAT_1:39;
    hence w.outDegree() c= v.outDegree();
  end;
  hence thesis by A3, Th50;
end;
