# Submersion (mathematics)

(Redirected from Regular point)

In mathematics, a submersion is a differentiable map between differentiable manifolds whose differential is everywhere surjective. This is a basic concept in differential topology. The notion of a submersion is dual to the notion of an immersion.

## Definition

Let M and N be differentiable manifolds and f : MN be a differentiable map between them. The map f is a submersion at a point pM if its differential

${\displaystyle Df_{p}:T_{p}M\to T_{f(p)}N\,}$

is a surjective linear map.[1] In this case p is called a regular point of the map f, otherwise, p is a critical point. A point qN is a regular value of f if all points p in the pre-image f−1(q) are regular points. A differentiable map f that is a submersion at each point pM is called a submersion. Equivalently, f is a submersion if its differential Dfp has constant rank equal to the dimension of N.

A word of warning: some authors use the term "critical point" to describe a point where the rank of the Jacobian matrix of f at p is not maximal.[2] Indeed, this is the more useful notion in singularity theory. If the dimension of M is greater than or equal to the dimension of N then these two notions of critical point coincide. But if the dimension of M is less than the dimension of N, all points are critical according to the definition above (the differential cannot be surjective) but the rank of the Jacobian may still be maximal (if it is equal to dim M). The definition given above is more commonly used, e.g. in the formulation of Sard's theorem.

## Submersion Theorem

Given a submersion between smooth manifolds ${\displaystyle f:M\to N}$ the fibers of ${\displaystyle f}$, denoted ${\displaystyle M_{x}=f^{-1}(\{p\})}$ can be equipped with the structure of a smooth manifold. This theorem coupled with the Whitney embedding theorem implies that every smooth manifold can be described as the fiber of a smooth map ${\displaystyle f:\mathbb {R} ^{n}\to \mathbb {R} ^{m}}$.

For example, consider

${\displaystyle f:\mathbb {R} ^{3}\to \mathbb {R} }$ where ${\displaystyle f(x,y,z)=x^{4}+y^{4}+z^{4}}$

The jacobian matrix is given by

${\displaystyle {\begin{bmatrix}{\frac {\partial f}{\partial x}}&{\frac {\partial f}{\partial y}}&{\frac {\partial f}{\partial z}}\end{bmatrix}}={\begin{bmatrix}4x^{3}&4y^{3}&4z^{3}\end{bmatrix}}}$

This has maximal rank at every point except for ${\displaystyle (0,0,0)}$. Also, the fibers

${\displaystyle f^{-1}(\{t\})=\{(a,b,c)\in \mathbb {R} ^{3}:a^{4}+b^{4}+c^{4}=t\}}$

are empty for ${\displaystyle t\leq 0}$. Hence we only have a smooth submersion

${\displaystyle f:\mathbb {R} ^{3}\to \mathbb {R} _{>0}}$

and the subsets

${\displaystyle M_{t}=\{(a,b,c)\in \mathbb {R} ^{3}:a^{4}+b^{4}+c^{4}=t\}}$

are smooth manifolds for ${\displaystyle t>0}$.

## Local normal form

If f: MN is a submersion at p and f(p) = qN then there exist an open neighborhood U of p in M, an open neighborhood V of q in N, and local coordinates (x1,…,xm) at p and (x1,…,xn) at q such that f(U) = V and the map f in these local coordinates is the standard projection

${\displaystyle f(x_{1},\ldots ,x_{n},x_{n+1},\ldots ,x_{m})=(x_{1},\ldots ,x_{n}).}$

It follows that the full pre-image f−1(q) in M of a regular value qN under a differentiable map f: MN is either empty or is a differentiable manifold of dimension dim M − dim N, possibly disconnected. This is the content of the regular value theorem (also known as the submersion theorem). In particular, the conclusion holds for all qN if the map f is a submersion.

## Topological manifold submersions

Submersions are also well defined for general topological manifolds.[3] A topological manifold submersion is a continuous surjection f : MN such that for all pM, for some continuous charts ψ at p and φ at f(p), the map ψ−1 ∘ f ∘ φ is equal to the projection map from Rm to Rn, where m=dim(M) ≥ n=dim(N).