Ak singularity
# Ak singularity

In mathematics, and in particular singularity theory, an $A_k$ singularity, where k ≥ 0 is an integer, describes a level of degeneracy of a function. The notation was introduced by V. I. Arnold.

Let $f:\mathbb {R} ^{n}\to \mathbb {R}$ be a smooth function. We denote by $\Omega (\mathbb {R} ^{n},\mathbb {R} )$ the infinite-dimensional space of all such functions. Let $\operatorname {diff} (\mathbb {R} ^{n})$ denote the infinite-dimensional Lie group of diffeomorphisms $\mathbb {R} ^{n}\to \mathbb {R} ^{n},$ and $\operatorname {diff} (\mathbb {R} )$ the infinite-dimensional Lie group of diffeomorphisms $\mathbb {R} \to \mathbb {R} .$ The product group $\operatorname {diff} (\mathbb {R} ^{n})\times \operatorname {diff} (\mathbb {R} )$ acts on $\Omega (\mathbb {R} ^{n},\mathbb {R} )$ in the following way: let $\varphi :\mathbb {R} ^{n}\to \mathbb {R} ^{n}$ and $\psi :\mathbb {R} \to \mathbb {R}$ be diffeomorphisms and $f:\mathbb {R} ^{n}\to \mathbb {R}$ any smooth function. We define the group action as follows:

    $(\varphi ,\psi )\cdot f:=\psi \circ f\circ \varphi ^{-1}$

The orbit of f , denoted orb(f), of this group action is given by

    ${\mbox{orb}}(f)=\{\psi \circ f\circ \varphi ^{-1}:\varphi \in {\mbox{diff}}(\mathbb {R} ^{n}),\psi \in {\mbox{diff}}(\mathbb {R} )\}\ .$

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

    $f(x_{1},\ldots ,x_{n})=1+\varepsilon _{1}x_{1}^{2}+\cdots +\varepsilon _{n-1}x_{n-1}^{2}\pm x_{n}^{k+1}$

where $\varepsilon _{i}=\pm 1$ and k ≥ 0 is an integer.

By a normal form we mean a particularly simple representative of any given orbit. The above expressions for f give normal forms for the type $A_k-singularities.$ The type $A_k-singularities$ 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 f.

This idea extends over the complex numbers where the normal forms are much simpler; for example: there is no need to distinguish $ε_i$ = +1 from $ε_i$ = −1.

