The problem is what color should be used at least when painting maps. Eh! You think that you can’t do it with 4 coolers because the rainbow has 7 colors , you are art-oriented. Or you should paint with 2 colors, red, blue, red, blue … alternately, don’t you forget the color of the sky? Or you can do it in one color by painting it with black and shade, you are genius who don’t follow the rules.

In fact, this four-color problem was finally proved in 1975 using a computer and became a four-color theorem. When l looked at the map book at elementary school, I thought that the prefecture l lived in was ○ color, why it was the same or different from other prefectures, in any case I thought it would be better to paint it in 47 colors, but 4 colors had a reason.

We often think of the “three major ○○”. It may be an eco-system that we should remember only three typical ones. For example, there are many genres, such as the Three Great Lakes, the Three Great Nutrients, the Three Great Obligations, and the Three Great Festivals, and I have no time to list them. However, don’t you forget “other” like the sky blue? If you put it in, this is also a four color problem.

By the way, a map of Japan that I see everyday in the news for the past two months. The number of prefectures painted in red or yellow has decreased. The long-awaited emergency has begun to be lifted. Some prefectures still have colors, but they will be released soon.

After that, I hope that the 47 prefectures will remain white color until the medical precautions are complete. In other words.

This issue is a one-color problem.

(2020年5月22日@nortan)