  We show that, for some Cantor sets in R^d, the capacity g_s associated to the
s-dimensional Riesz kernel x/|x|^{s+1} is comparable to the capacity
C_{2(d-s)/3,3/2} from non linear potential theory. It is an open problem to
show that, when s is positive and non integer, they are comparable for all
compact sets in R^d. We also discuss other open questions in the area.
