The generalized Ulam spiral [La spirale d'Ulam généralisée].
Starting from the center of the picture, the points are
numbered (1, 2, 3,...) when following a
square spiral. The radius of the N-th sphere is proportional
to the number of divisors of N.
(CMAP28 WWW site: this page was created on 02/01/1999 and last updated on 04/26/2015
12:24:17 -CEST-)
