证明每一个网孔都是一个独立回路的方法不止一种。这里仅介绍一种证明方法。我们知道,在选回路时,每选一个都含有一条为先选的回路所没有走过的新支路,则这样所选的回路就保证是独立的。对于任何平面网络,我们总可以做到每取一个网孔都将至少包含一条新支路。图1所示为一任意的平面网络,我们总可以由外逐步向里地将它划分为若干层次:第Ⅰ层次为所有边缘网孔;第Ⅱ层次为与第Ⅰ层次的网孔有公共支路的所有网孔;第Ⅲ层次为与第Ⅱ层次的网孔有公共支路的所有网孔;其余依此类推,直 There is more than one way to prove that every cell is a separate loop. Here only introduce a proof method. We know that in choosing a circuit, each of them contains a new branch that has not been traveled by the first-selected circuit, so the selected circuit is guaranteed to be independent. For any planar network, we always have at least one new branch for every cell taken. Figure 1 shows an arbitrary planar network. We can always divide it into several levels from outside to inside: the first level is all the edge cells; the second level is the one with the first level cells All the cells of the road; the third level is all the cells with the public branch of the second level of cells; the rest are