The potential W, restricting the admissible values of the
solution to the space of labels, has to take into
account the number and the parameters of the classes. W has
M minima which are the mean values
(i=1..M), and such that
W()=
0, i=1..M.
Moreover, W is quadratic around each minimum and increases at
least linearly at infinity.
In the cases of 2 classes, the form of W may be for instance
to the space of labels, has to take into
account the number and the parameters of the classes. W has
M minima which are the mean values 
(i=1..M), and such that 