Along this overview I have used the tensorial calculus from [87, 21, 36], with the appropriate
gauge to have simpler expressions. Instead of reproducing here the concepts involved in this calculus
I'm going to express *in words* the geometric meaning of the symbols that appear along the overview, which,
I think, is a more practical solution in this case. We have the following link
between 'symbols' and 'meaning':

being and unit vectors. Therefore

In the 2-dimensional case we have

In *d* dimensions, expressions generalize according to the tensorial calculus in [87, 21, 36].

Wed Oct 8 17:04:50 MET DST 1997