Starting from the center of the picture, the 81 little squares are
numbered (N=1, 2, 3,..., 81) when following a
square spiral.
The number 1 is the green square when the prime numbers are the white ones.
The other squares are red with a luminance proportional to the number of divisors of N.
![The generalized Ulam spiral displaying 81 numbers [La spirale d'Ulam généralisée montrant 81 nombres entiers ]](image.jpg "The generalized Ulam spiral displaying 81 numbers [La spirale d'Ulam généralisée montrant 81 nombres entiers ]")
