--- WARNING ---

Preface to the english edition (2001)
\*

The first seven chapters are a substantially revised and enlarged
translation of the italian edition of the book with the same title by
one of us (GG). It is still under revision. The chapters 8,9 are
entirely new.

The necessary work will be done slowly and the files will be
continuously updated (we hope). 

The various  versions will be recognized
by the date or by the number.  Fully revised versions will have the
version number of the form *.*. The circulation of versions *.* has
been started because the italian version is out of print and I use the
book (or better parts of it) in courses at the Universit\`a di Roma.

Giovanni Gallavotti
Federico Bonetto
Guido Gentile

Roma 2 giugno 2001