deletions | additions       diff --git a/friends.tex b/friends.tex  index 4dc9f12..0a0e6bb 100644  --- a/friends.tex  +++ b/friends.tex 
...
\begin{quote}  Multiplying a bit by $2$ we always obtain $0$.  \end{quote}  There exist particular phenomena such the one related with time that have a kind of {\em ciclicity}.  This has a precise meaning from a mathematical point of view, the concept of \emph{characteristic}.\\  Given a field (in our case $\Fb$), it can happen that   some positive\footnote{notice: positive means \emph{different} from $0$!!} integer numbers, multiplied to all the elements of the field, give $0 $ as