The Ulam spiral (or prime spiral) is a simple method of graphing the prime numbers that reveals a pattern. It was discovered by the mathematician Stanislaw Ulam in 1963, while he was doodling on scratch paper at a scientific meeting. Ulam wrote down a regular rectangular grid of numbers, starting with 1 at the center, and spiraling out. He then circled all of the prime numbers, and to his surprise, the circled numbers tended to line up along diagonal lines. This demo is a 100×100 Ulam spiral, where primes are blue. Diagonal lines are clearly visible, confirming the pattern. You need to run it for a few minutes. Eduard Muntaner http://www.eduard.cat
