A positive integer (1, 2, 3, ... ) greater than 1 is called

The Ulam spiral of prime numbers is constructed as follows: Consider a rectangular grid. We start with the central point and arrange the positive integers in a spiral fashion (anticlockwise) as at below left. The prime numbers are then marked (here with blue boxes). Since the primes occur in an irregular manner in the sequence of numbers one might expect that in this grid they would occur more or less at random, and so form something like a random pattern. But, on the contrary, there is a clear tendency for the prime numbers to form diagonal lines. This can be seen more clearly in the image below right, in which each of the 70,255 pixels corresponds to a position in the number sequence and the primes are marked by white pixels. This pattern is puzzling, since no complete explanation has been given for why the prime numbers should line up in this way.

This

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