Difference between revisions of "Tangent space"
m |
m |
||
| Line 2: | Line 2: | ||
What is defined here may also be called the '''Geometric tangent space''' | What is defined here may also be called the '''Geometric tangent space''' | ||
| + | |||
| + | See also [[Motivation for tangent space]] | ||
==Definition== | ==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. | 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. | ||
Revision as of 18:11, 10 April 2015
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[1] 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
Contents
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
- ↑ John M. Lee - Introduction to Smooth Manifolds - second edition
