This front cover shows 2000 circles in a pattern generated by a simple algorithm: Place a circle at the origin and choose an arbitrary direction. Place another circle as close to the origin as possible along that direction without overlapping another circle. Rotate the direction a fixed angle and place another circle as close as possible to the origin without overlapping. Repeat. The fixed angle used here is 360°/φ, where φ is the golden ratio. See page 26 for examples of patterns obtained for other choices of angle.