The dual tree of a fold map germ from R3 to R4

Let f : (R-3,0) -> (R-4,0) be an analytic map germ with isolated instability. Its link is a stable map which is obtained by taking the intersection of the image of f with a small enough sphere S-epsilon(3) centred at the origin in R-4. If f is of fold type, we define a tree, that we call dual tree, that contains all the topological information of the link and we prove that in this case it is a complete topological invariant. As an application we give a procedure to obtain normal forms for any topological class of fold type.