The Surprises Never Eend: The Ulam Spiral of Primes : Good Math, Bad Math: "That's the Ulam spiral filling a 200x200 grid. Look at how many diagonal line segments you get! And look how many diagonal line segments occur along the same lines! Why do the prime numbers tend to occur in clusters along the diagonals of this spiral? I don't have a clue. Nor, to my knowledge, does anyone else!"