Submersion (mathematics)

Let M and N be differentiable manifolds and ${\displaystyle f\colon M\to N}$ be a differentiable map between them. The map f is a submersion at a point ${\displaystyle p\in M}$ if its differential

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

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 ${\displaystyle q\in N}$ is a regular value of f if all points p in the preimage ${\displaystyle f^{-1}(q)}$ are regular points. A differentiable map f that is a submersion at each point ${\displaystyle p\in M}$ is called a submersion. Equivalently, f is a submersion if its differential ${\displaystyle Df_{p}}$ 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 the more commonly used; e.g., in the formulation of Sard's theorem.

Given a submersion between smooth manifolds ${\displaystyle f\colon M\to N}$ 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 ${\displaystyle f\colon \mathbb {R} ^{3}\to \mathbb {R} }$ given by ${\displaystyle f(x,y,z)=x^{4}+y^{4}+z^{4}.}$ The Jacobian matrix is

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

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

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

are empty for ${\displaystyle t<0}$, and equal to a point when ${\displaystyle t=0}$. Hence we only have a smooth submersion ${\displaystyle f\colon \mathbb {R} ^{3}\to \mathbb {R} _{>0},}$ and the subsets ${\displaystyle M_{t}=\left\{(a,b,c)\in \mathbb {R} ^{3}:a^{4}+b^{4}+c^{4}=t\right\}}$ are two-dimensional smooth manifolds for ${\displaystyle t>0}$.

• Any projection ${\displaystyle \pi \colon \mathbb {R} ^{m+n}\rightarrow \mathbb {R} ^{n}\subset \mathbb {R} ^{m+n}}$
• Local diffeomorphisms
• Riemannian submersions
• The projection in a smooth vector bundle or a more general smooth fibration. The surjectivity of the differential is a necessary condition for the existence of a local trivialization.

If f: M → N is a submersion at p and f(p) = q ∈ N, then there exists an open neighborhood U of p in M, an open neighborhood V of q in N, and local coordinates (x₁, …, x_m) at p and (x₁, …, x_n) 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 preimage f⁻¹(q) in M of a regular value q in N under a differentiable map f: M → N 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 q in N if the map f is a submersion.

Submersions are also well-defined for general topological manifolds.[3] A topological manifold submersion is a continuous surjection f : M → N such that for all p in M, 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 R^m to Rⁿ, where m = dim(M) ≥ n = dim(N).

• Ehresmann's fibration theorem

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