The P+41 spiral contains an unusually large number of primes because 163 is a Heegner number (https://en.wikipedia.org/wiki/Heegner_number https://en.wikipedia.

The P+41 spiral contains an unusually large number of primes because 163 is a Heegner number (https://en.wikipedia.org/wiki/Heegner_number https://en.wikipedia.org/wiki/Heegner_number).