Let denote the space of smooth map germs Let be the group of diffeomorphism germs and be the group of diffeomorphism germs The group acts on in the natural way: Under this action we see that the map germs are -equivalent if, and only if, lies in the orbit of , i.e. (or vice versa).
A map germ is called stable if its orbit under the action of is open relative to the Whitney topology. Since is an infinite dimensional space metric topology is no longer trivial. Whitney topology compares the differences in successive derivatives and gives a notion of proximity within the infinite dimensional space. A base for the open sets of the topology in question is given by taking -jets for every and taking open neighbourhoods in the ordinary Euclidean sense. Open sets in the topology are then unions of these base sets.
Consider the orbit of some map germ The map germ is called simple if there are only finitely many other orbits in a neighbourhood of each of its points. Vladimir Arnold has shown that the only simple singular map germs for are the infinite sequence (), the infinite sequence (), and
- K-equivalence (contact equivalence)
- M. Golubitsky and V. Guillemin, Stable Mappings and Their Singularities. Graduate Texts in Mathematics, Springer.