Difference between revisions of "Tangent space"
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<math>\omega\in \mathcal{D}_p(M)\iff\omega:C^\infty_p(M)\rightarrow\mathbb{R}</math> is a derivation | <math>\omega\in \mathcal{D}_p(M)\iff\omega:C^\infty_p(M)\rightarrow\mathbb{R}</math> is a derivation | ||
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| + | |||
| + | See | ||
| + | * [[Motivation for tangent space definitions]] | ||
| + | * [[Motivation for tangent space]] | ||
| + | |||
| + | ==Geometric Tangent Space== | ||
| + | The '''Geometric tangent space to {{M|\mathbb{R}^n}} at {{M|p}}'''<ref>Introduction to smooth manifolds - John M Lee - Second Edition</ref> is defined as follows: | ||
| + | |||
| + | * <math>G_p(\mathbb{R}^n)=\{(p,v)|v\in\mathbb{R}^n\}</math> - the set of all arrows rooted at {{M|p}} | ||
| + | |||
| + | ===Vector space=== | ||
| + | This is trivially a [[Vector space|vector space]] with operations defined as follows: | ||
| + | * <math>v_p+w_p=(v+w)_p</math> | ||
| + | * <math>c(v_p)=(cv)_p</math> | ||
| + | |||
| + | ===Notations=== | ||
| + | * John M Lee uses {{M|\mathbb{R}^n_p}} to mean the same thing ( {{M|G_p(\mathbb{R}^n)}} ) | ||
| + | |||
| + | ==Tangent Space== | ||
| + | The '''Tangent space to {{M|\mathbb{R}^n}} at {{M|p}}'''<ref>Introduction to smooth manifolds - John M Lee - Second Edition</ref> is defined as follows: | ||
| + | |||
| + | * <math>T_p(\mathbb{R}^n)=\{\omega:\omega\text{ is a}</math> [[Derivation|derivation]] <math>\text{at }p\}</math> - that is: | ||
| + | *: <math>\omega\in T_p(\mathbb{R}^n)\iff\omega:C^\infty(\mathbb{R}^n)\rightarrow\mathbb{R}</math> where | ||
| + | *:* {{M|\omega}} is [[Linear map|{{M|\mathbb{R} }}-linear]] | ||
| + | *:* {{M|\omega}} satisfies the [[Leibniz rule]] | ||
| + | |||
| + | ==Isomorphism between geometric tangent space and tangent space== | ||
| + | Infact the geometric tangent space and tangent space to {{M|\mathbb{R}^n}} at {{M|p}} are [[Linear isometry|linearly isomorphic]] to each other. | ||
| + | |||
| + | '''Proposition: ''' | ||
| + | * <math>\alpha:G_p(\mathbb{R}^n)\rightarrow T_p(\mathbb{R}^n)</math> given by: | ||
| + | ** <math>\alpha:v_p\mapsto [D_v|_p:C^\infty(\mathbb{R}^n)\rightarrow\mathbb{R}]</math> | ||
| + | : is a [[Linear isometry|linear isomorphism]] | ||
| + | |||
| + | {{Begin Theorem}} | ||
| + | Theorem: The map <math>\alpha:G_p(\mathbb{R}^n)\rightarrow T_p(\mathbb{R}^n)</math> given by <math>\alpha:v_p\mapsto [D_v|_p:C^\infty(\mathbb{R}^n)\rightarrow\mathbb{R}]</math> is a [[Linear isometry|linear isomorphism]] | ||
| + | {{Begin Proof}} | ||
| + | {{Todo|ITSM p53 if help needed, uses LM has kernel of dim 0 {{M|\implies}} injective}} | ||
| + | {{End Proof}} | ||
| + | {{End Theorem}} | ||
| + | |||
==OLD PAGE== | ==OLD PAGE== | ||
I prefer to denote the tangent space (of a set {{M|A}} at a point {{M|p}}) by {{M|T_p(A)}} - as this involves the letter T for tangent however one author<ref>John M. Lee - Introduction to Smooth Manifolds - second edition</ref> uses {{M|T_p(A)}} as [[Set of all derivations at a point]] - the two are indeed isomorphic but as readers will know - I do not see this as an excuse. | I prefer to denote the tangent space (of a set {{M|A}} at a point {{M|p}}) by {{M|T_p(A)}} - as this involves the letter T for tangent however one author<ref>John M. Lee - Introduction to Smooth Manifolds - second edition</ref> uses {{M|T_p(A)}} as [[Set of all derivations at a point]] - the two are indeed isomorphic but as readers will know - I do not see this as an excuse. | ||
Revision as of 21:32, 13 April 2015
Contents
Types of tangent space
| Name | Symbol | Definition |
|---|---|---|
| Geometric Tangent Space | [ilmath]G_p(\mathbb{R}^n)[/ilmath][1] | The set of tangents to a point in [ilmath]\mathbb{R}^n[/ilmath] [math]G_p(\mathbb{R}^n)=\{(p,v)|v\in\mathbb{R}^n\}[/math] - the set of all arrows at [ilmath]p[/ilmath] |
| Tangent space (to [ilmath]\mathbb{R}^n[/ilmath]) | [ilmath]T_p(\mathbb{R}^n)[/ilmath] | The set of all derivations at [ilmath]p[/ilmath]]] [math]\omega\in T_p(\mathbb{R}^n)\iff \omega:C^\infty(\mathbb{R}^n)\rightarrow\mathbb{R} [/math] is a derivation |
| ([ilmath]T_p(M)[/ilmath] here) | ||
| Tangent space (in terms of germs) | [ilmath]\mathcal{D}_p(M)[/ilmath] | The set of all derivations of [ilmath]C^\infty_p(M)[/ilmath] - the set of all germs of smooth functions at a point, that is: [math]\omega\in \mathcal{D}_p(M)\iff\omega:C^\infty_p(M)\rightarrow\mathbb{R}[/math] is a derivation |
See
Geometric Tangent Space
The Geometric tangent space to [ilmath]\mathbb{R}^n[/ilmath] at [ilmath]p[/ilmath][2] is defined as follows:
- [math]G_p(\mathbb{R}^n)=\{(p,v)|v\in\mathbb{R}^n\}[/math] - the set of all arrows rooted at [ilmath]p[/ilmath]
Vector space
This is trivially a vector space with operations defined as follows:
- [math]v_p+w_p=(v+w)_p[/math]
- [math]c(v_p)=(cv)_p[/math]
Notations
- John M Lee uses [ilmath]\mathbb{R}^n_p[/ilmath] to mean the same thing ( [ilmath]G_p(\mathbb{R}^n)[/ilmath] )
Tangent Space
The Tangent space to [ilmath]\mathbb{R}^n[/ilmath] at [ilmath]p[/ilmath][3] is defined as follows:
- [math]T_p(\mathbb{R}^n)=\{\omega:\omega\text{ is a}[/math] derivation [math]\text{at }p\}[/math] - that is:
- [math]\omega\in T_p(\mathbb{R}^n)\iff\omega:C^\infty(\mathbb{R}^n)\rightarrow\mathbb{R}[/math] where
- [ilmath]\omega[/ilmath] is [ilmath]\mathbb{R} [/ilmath]-linear
- [ilmath]\omega[/ilmath] satisfies the Leibniz rule
- [math]\omega\in T_p(\mathbb{R}^n)\iff\omega:C^\infty(\mathbb{R}^n)\rightarrow\mathbb{R}[/math] where
Isomorphism between geometric tangent space and tangent space
Infact the geometric tangent space and tangent space to [ilmath]\mathbb{R}^n[/ilmath] at [ilmath]p[/ilmath] are linearly isomorphic to each other.
Proposition:
- [math]\alpha:G_p(\mathbb{R}^n)\rightarrow T_p(\mathbb{R}^n)[/math] given by:
- [math]\alpha:v_p\mapsto [D_v|_p:C^\infty(\mathbb{R}^n)\rightarrow\mathbb{R}][/math]
- is a linear isomorphism
Theorem: The map [math]\alpha:G_p(\mathbb{R}^n)\rightarrow T_p(\mathbb{R}^n)[/math] given by [math]\alpha:v_p\mapsto [D_v|_p:C^\infty(\mathbb{R}^n)\rightarrow\mathbb{R}][/math] is a linear isomorphism
TODO: ITSM p53 if help needed, uses LM has kernel of dim 0 [ilmath]\implies[/ilmath] injective
OLD PAGE
I prefer to denote the tangent space (of a set [ilmath]A[/ilmath] at a point [ilmath]p[/ilmath]) by [ilmath]T_p(A)[/ilmath] - as this involves the letter T for tangent however one author[4] uses [ilmath]T_p(A)[/ilmath] as Set of all derivations at a point - the two are indeed isomorphic but as readers will know - I do not see this as an excuse.
What is defined here may also be called the Geometric tangent space
See also Motivation for tangent space
Definition
It is the set of arrows at a point, the set of all directions essentially. As the reader knows, a vector is usually just a direction, we keep track of tangent vectors and know them to be "tangent vectors at t" or something similar. A tangent vector is actually a point with an associated direction.
Euclidean (motivating) definition
We define [math]T_p(\mathbb{R}^n)=\left\{(p,v)|v\in\mathbb{R}^n\right\}[/math]
Generally then we may say: [math]T_p(A)=\left\{(p,v)|v\in A\right\}[/math]
Notation
A tangent vector (often [ilmath]v[/ilmath] is used) shall be left as just [ilmath]v[/ilmath] if the point to which it is a tangent to is implicit (ie "[ilmath]v[/ilmath] is a tangent at [ilmath]p[/ilmath]")
Rather than writing [ilmath](p,v)[/ilmath] we may write:
- [ilmath]v[/ilmath] (if it is implicitly understood that this is a tangent to the point [ilmath]p[/ilmath])
- [ilmath]v_a[/ilmath]
- [math]v|_a[/math]
Why ordered pairs
Ordered pairs are used because now the tangent space at two distinct points are disjoint sets, that is [math]\alpha\ne\beta\implies T_\alpha(A)\cap T_\beta(A)=\emptyset[/math]
Vector space
[math]T_p(A)[/math] is a vector space when equipped with the following definitions:
- [ilmath]v_a+w_a=(v+w)_a[/ilmath]
- [ilmath]c(v_a)=(cv)_a[/ilmath]
It is easily seen that the basis for this is the standard basis [math]\{e_1|_p,\cdots, e_n|_p\}[/math] and that the tangent space [ilmath]T_p(A)[/ilmath] is basically just a copy of [ilmath]A[/ilmath]
See also
- Set of all derivations at a point
- Set of all derivations of a germ
- The tangent space and derivations at a point are isomorphic
References
- ↑ Alec's notation - John M Lee uses [ilmath]\mathbb{R}^n_p[/ilmath] and it is distinct from [ilmath]T_p(\mathbb{R}^n)[/ilmath]
- ↑ Introduction to smooth manifolds - John M Lee - Second Edition
- ↑ Introduction to smooth manifolds - John M Lee - Second Edition
- ↑ John M. Lee - Introduction to Smooth Manifolds - second edition
