Starting from the center of the picture -red square-, all the little squares (even the black ones) are
numbered (N=1, 2, 3,...) when following a square spiral.
The yellow squares display the first prime numbers ({2,3,5,7,11,13,17,19,23,29,31,37,41,...}).
![The Ulam spiral [La spirale d'Ulam ]](image.jpg "The Ulam spiral [La spirale d'Ulam ]")
