Mr Wu, a mathematics tutor in Singapore and author of the blog about that, offered this week’s topic. It’s about one of the iconic mathematics shapes.

Torus

When one designs a board game, one has to decide what the edge of the board means. Some games make getting to the edge the goal, such as Candy Land or backgammon. Some games set their play so the edge is unreachable, such as Clue or Monopoly. Some make the edge an impassible limit, such as Go or Scrabble or Checkers. And sometimes the edge becomes something different.

Consider a strategy game like Risk or Civilization or their video game descendants like Europa Universalis. One has to be able to go east, or west, without limit. But there’s no making a cylindrical board. Or making a board infinite in extent, side to side. Instead, the game demands we connect borders. Moving east one…