All the material should be accessible to upper-level undergraduates with some background in number theory, and anything beyond this level is fully explained in the text.

This is not a monograph in the usual sense concentrating on a specific topic. Instead, it narrates in five parts ? Numbers, Trees, Groups, Words, Finale ? the story of a discovery in one field and its many manifestations in others, as a tribute to a great mathematical achievement and as an intellectual pleasure, contemplating the marvellous unity of all mathematics.]]>