In mathematics, and in particular singularity theory, an singularity, where is an integer, describes a level of degeneracy of a function. The notation was introduced by V. I. Arnold.

be a smooth function the infinite-dimensional space denote the infinite-dimensional Lie group of diffeomorphisms The product group acts any smooth function. We define the group action as follows:

The orbit of , denoted , of this group action is given by

The members of a given orbit of this action have the following fact in common: we can find a diffeomorphic change of coordinate in and a diffeomorphic change of coordinate in such that one member of the orbit is carried to any other. A function is said to have a type -singularity if it lies in the orbit of

and is an integer.

By a normal form we mean a particularly simple representative of any given orbit. The above expressions for give normal forms for the type -singularities. The type -singularities are special because they are amongst the simple singularities, this means that there are only a finite number of other orbits in a sufficiently small neighbourhood of the orbit of.

This idea extends over the complex numbers where the normal forms are much simpler; for example: there is no need to distinguish from .

See also

• , an ADE classification of singularities

References