The Ulam spiral (or Ulam cloth) consists of integers, starting from a given at the center, written in a spiral with the prime numbers highlighted or emphasized in some way. For example, writing a spiral with 42 at the center,
and then simply blanking the composites, we obtain
Carrying on this process in more layers will show that most of the primes tend to fall on certain diagonals and not others.
This formation was first pondered by Stanisław Ulam with 1 at the center. He tried carrying out the process much further, and also tried different center values, but in each case the primes would cluster on certain diagonals and sparsely populate others. Because almost all primes are odd, it is easy to explain why they would tend to form diagonals, but much more difficult to explain why they fall on certain diagonals.
- 1 Gardner, M. “Mathematical Recreations: The Remarkable Lore of the Prime Number” Scientific American 210 3: 120 - 128