On his website andertoons.com, illustrator Mark Anderson has a cartoon in which a boy is handing his teacher an assignment. “To show how well I understand fractions,” he says, “I only did ...

Even a simpler version that Hindman first suggested in the 1970s proved challenging. He conjectured that any coloring of the natural numbers must contain a monochromatic set of the form {x, y, xy, x + ...