In the 1700′s the Swiss-born mathematician, Leonhard Euler worked out a way to tell whether or not a particular design was traceable in this way simply by counting the number of lines at each intersection in the design. He found that if all the intersections had an even number of lines coming into them, then you could start at any point in the design and trace the whole thing without lifting your pen or going over any line twice. Today, a design that meets these requirements is called an Euler circuit after the eighteenth-century mathematician. So, if you’re planning a paper route, you might want to figure out whether the streets you’ve been given make up an Euler circuit. Once again, count the number of roads coming in to each intersection. If all intersections have an even number of roads, then you’re in luck.