Q & A for Jackson 3.3 (thanks to Katia for the questions):

> - I calculated the potential along the z axis

Good.

> - I'd like to expand it and identify the coefficient for the all space.

Also good.

> Unfortunatly, the result I have along the z axis is complicated, and not 
> easy to expand. (and I don't know if it's correct) 

It's not too bad - this is the right thing to do, please do it.

> Is there any suggestion concerning an approach to this problem?

Yes, one more hint: the cases r < R and r > R might be quite different, so 
be careful and calculate the integral separately for these two cases and 
then expand separately for the two cases.