It was almost four hundred years ago that Galileo wrote in Il Saggiatore that the "Book of Nature is written in mathema ti ca 1 characters." Thi s sentence, i nspi red at the dawn of physics has proved with the passage of time to contain a deep truth and also a warning: in order to understand Nature, first we must learn to read mathema tical characters. Indeed, writing physical law in such characters has proved not as hard as unraveling the content of the resulting equations. In particular, the lack of knowledge in the field of nonlinear mathematics has been a severe limita tion in the past....