节外生枝一般来说,一张长方形纸条很容易做成一个纸圈,这个纸圈有上下两条边和正反两个面(见图1)。可当我们把纸条拧转180度,A点和C点、B点和D点分别重合时,做成的纸圈却只有一条边、一个面(见图2)。这就是莫比乌斯圈。“莫比乌斯圈”是德国数学家莫比乌斯在1858年研究“四色定 Extroverted branches In general, a rectangular strip of paper is easy to make a paper ring, the paper has two upper and lower edges and the positive and negative sides (see Figure 1). When we twist the strip 180 degrees, point A and point C, point B and point D coincide, the paper ring made of only one side, one side (see Figure 2). This is Mobius Circle. “Mobius Circle” is a German mathematician Mobius in 1858 to study “four-color set.”