Record the nature of the plan, I am afraid that some places will be tested for tumors in the future .
Some things are specified below:
\ (v \) points, \ (e \) edges.
\ (f \) The number of faces, that is, the plane is divided into several faces (it can be imagined that an infinite cake is cut into several pieces).
\ (c \) Number of Unicom blocks.
The number of edges in a plane graph is very small, and it is of the same order as the number of points \ (e \ le 3v-6 \) .
Euler's theorem of the plane graph \ (v-e + f = c + 1 \) , if the graph is connected, it is \ (v-e + f = 2 \) .