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{{redirect|Regular point|"regular point of an algebraic variety"|Singular point of an algebraic variety}} | |||
In [[mathematics]], a '''submersion''' is a [[differentiable map]] between [[differentiable manifold]]s whose [[pushforward (differential)|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 (mathematics)|immersion]]. | |||
== Definition == | |||
Let ''M'' and ''N'' be [[differentiable manifold]]s and ''f'' : ''M'' → ''N'' be a [[differentiable map]] between them. The map ''f'' is a '''submersion at a point''' ''p'' ∈ ''M'' if its [[pushforward (differential)|differential]] | |||
:<math>Df_p : T_p M \to T_{f(p)}N\,</math> | |||
is a [[surjective]] [[linear map]]. In this case ''p'' is called a '''regular point''' of the map ''f'', otherwise, ''p'' is a [[Critical point (mathematics)|critical point]]. A point ''q'' ∈ ''N'' is a '''regular value''' of ''f'' if all points ''p'' in the pre-image ''f''<sup>−1</sup>(''q'') are regular points. A differentiable map ''f'' that is a submersion at each point is called a '''submersion'''. Equivalently, ''f'' is a submersion if its differential ''Df''<sub>''p''</sub> has [[rank (differential topology)|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 (linear algebra)|rank]] of the [[Jacobian matrix]] of ''f'' at ''p'' is not maximal.<ref>{{cite book |last1= Arnold |first1= V.I. |last2 = Gusein-Zade |first2 = S.M. |last3 = Varchenko |first3 = A.N. |title=Singularities of Differentiable maps |publisher=Birkhäuser}}</ref> 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]]. | |||
== Examples == | |||
* Any projection <math>\pi:\mathbb{R}^{m+n}\rightarrow\mathbb{R}^n\subset\mathbb{R}^{m+n}</math> | |||
* [[Local diffeomorphism]]s | |||
*[[Riemannian submersion]]s | |||
* 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]]. | |||
== Local normal form == | |||
If ''f'': ''M'' → ''N'' is a submersion at ''p'' and ''f''(''p'') = ''q'' ∈ ''N'' then there exist an [[open neighborhood]] ''U'' of ''p'' in ''M'', an open neighborhood ''V'' of ''q'' in ''N'', and local coordinates (''x''<sub>1</sub>,…,''x''<sub>''m''</sub>) at ''p'' and (''x''<sub>1</sub>,…,''x''<sub>''n''</sub>) at ''q'' such that ''f''(''U'') = ''V'' and the map ''f'' in these local coordinates is the standard projection | |||
: <math>f(x_1, \ldots, x_n, x_{n+1}, \ldots, x_m) = (x_1, \ldots, x_n).</math> | |||
It follows that the full pre-image ''f''<sup>−1</sup>(''q'') in ''M'' of a regular value ''q'' ∈ ''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'' ∈ ''N'' if the map ''f'' is a submersion. | |||
== See also == | |||
*[[Ehresmann's fibration theorem]] | |||
==References== | |||
{{reflist}} | |||
*{{Citation|first=V. I.|last=Arnold|first2=A. N.|last2=Varchenko|first3=S. M.|last3=Gusein-Zade|title=Singularities of Differentiable Maps: Volume 1|publisher=Birkhäuser|year=1985|ISBN=0-8176-3187-9}} | |||
*{{Citation|first=J. W.|last=Bruce|first2=P. J.|last2=Giblin|title=Curves and Singularities|publisher=Cambridge University Press|year=1984|ISBN=0-521-42999-4}} | |||
[[Category:Maps of manifolds]] | |||
[[Category:Smooth functions]] |
Revision as of 20:50, 18 May 2013
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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 : M → N be a differentiable map between them. The map f is a submersion at a point p ∈ M if its differential
is a surjective linear map. In this case p is called a regular point of the map f, otherwise, p is a critical point. A point q ∈ N 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 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.[1] 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.
Examples
- 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.
Local normal form
If f: M → N is a submersion at p and f(p) = q ∈ N 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
It follows that the full pre-image f−1(q) in M of a regular value q ∈ 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 ∈ N if the map f is a submersion.
See also
References
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