English: Four color theorem. While the four color problem is less interesting to cartographers, it can help other areas such as cell tower placement. If each cell tower broadcast channel is like a color, and channel–colors are limited to four, then the problem of where to position broadcast towers for maximum coverage and minimal cost is the same as the four-color problem. The challenge? Eliminate non-coverage spots (red) along the highway. In places where multiple channels overlap (dark blue), the channels must be different––colors must be different, so the cell phone signal can be handed off to a different channel.