# nLab immersion of smooth manifolds

### Context

#### Differential geometry

differential geometry

synthetic differential geometry

# Contents

## Definition

Let $X$ and $X$ be smooth manifolds of finite dimension. Let $f : X \to Y$ be a differentiable function.

###### Definition

The function $f : X \to Y$ is called an immersion precisely if the canonical morphism

$T X \to X \times_Y T Y \eqqcolon f^* T Y$

is a monomorphism.

This morphism is the one induced from the universal property of the pullback by the commuting diagram

$\array{ T X &\stackrel{d f}{\to}& T Y \\ \downarrow && \downarrow \\ X &\stackrel{f}{\to}& Y }$

given by the differential of $f$ going between the tangent bundles.

Equivalently this means the following.

###### Definition

The function $f : X \to Y$ is an immersion precisely if for every point $x \in X$ the differential

$d f|_x : T_x X \to T_{f(y)} Y$

between the tangent space of $X$ at $x$ and the tangent space of $Y$ at $f(y)$ is an injection.

An immersion whose underlying continuous function is an embedding of topological spaces is an embedding of smooth manifolds.

## Properties

### Relation to embeddings

An immersion $f : X \to Y$ is precisely a local embeddings: for every point $x \in X$ there is an open neighbourhood $x \in U \subset X$ such that $f|_U : U \to Y$ is an embedding of smooth manifolds.

### Characterization in infinitesimal cohesion

A smooth function $f : X \to Y$ between smooth manifolds is canonically regarded as a morphism in the cohesive (∞,1)-topos SynthDiff∞Grpd. With respect to the canonical infinitesimal neighbourhood inclusion $i :$ Smooth∞Grpd $\hookrightarrow$ SynthDiff∞Grpd there is a notion of formally unramified morphism in $SynthDiff\infty Grpd$.

$f$ is an immersion precisely if it is formally unramified with respect to this infinitesimal cohesion.

See the discussion at SynthDiff∞Grpd for details.

## Variants

The algebraic geometry analogue of a submersion is a smooth morphism.

The analogue between arbitrary topological spaces (not manifolds) is simply an open map. There is also topological submersion, of which there are two versions.

Revised on March 19, 2012 11:22:01 by Urs Schreiber (89.204.155.155)