# Coordinate space

(Redirected from Coordinate vector space)

In mathematics, specifically in linear algebra, the coordinate space, Fn, is the prototypical example of an n-dimensional vector space over a field F. It can be defined as the product space of F over a finite index set.

## Definition

Let F denote an arbitrary field (such as the real numbers R or the complex numbers C). For any positive integer n, the space of all n-tuples of elements of F forms an n-dimensional vector space over F called coordinate space and denoted Fn.

An element of Fn is written

$\mathbf x = (x_1, x_2, \cdots, x_n)$

where each xi is an element of F. The operations on Fn are defined by

$\mathbf x + \mathbf y = (x_1 + y_1, x_2 + y_2, \cdots, x_n + y_n)$
$\alpha \mathbf x = (\alpha x_1, \alpha x_2, \cdots, \alpha x_n).$

The zero vector is given by

$\mathbf 0 = (0, 0, \cdots, 0)$

and the additive inverse of the vector x is given by

$-\mathbf x = (-x_1, -x_2, \cdots, -x_n).$

### Matrix notation

In standard matrix notation, each element of Fn is typically written as a column vector

$\mathbf x = \begin{bmatrix} x_1 \\ x_2 \\ \vdots \\ x_n \end{bmatrix}$

and sometimes as a row vector:

$\mathbf x = \begin{bmatrix} x_1 & x_2 & \dots & x_n \end{bmatrix}.$

The coordinate space Fn may then be interpreted as the space of all n×1 column vectors, or all 1×n row vectors with the ordinary matrix operations of addition and scalar multiplication.

Linear transformations from Fn to Fm may then be written as m×n matrices which act on the elements of Fn via left multiplication (when the elements of Fn are column vectors) or right multiplication (when they are row vectors).

## Standard basis

The coordinate space Fn comes with a standard basis:

$\mathbf e_1 = (1, 0, \ldots, 0)$
$\mathbf e_2 = (0, 1, \ldots, 0)$
$\vdots$
$\mathbf e_n = (0, 0, \ldots, 1)$

where 1 denotes the multiplicative identity in F. To see that this is a basis, note that an arbitrary vector in Fn can be written uniquely in the form

$\mathbf x = \sum_{i=1}^n x_i \mathbf{e}_i.$

## Discussion

It is a standard fact of linear algebra that every n-dimensional vector space V over F is isomorphic to Fn. It is a crucial point, however, that this isomorphism is not canonical. If it were, mathematicians would work only with Fn rather than with abstract vector spaces.

A choice of isomorphism is equivalent to a choice of ordered basis for V. To see this, let

A : FnV

be a linear isomorphism. Define an ordered basis {ai} for V by

ai = A(ei) for 1 ≤ in.

Conversely, given any ordered basis {ai} for V define a linear map A : FnV by

$A(\mathbf x) = \sum_{i=1}^n x_i \mathbf a_i.$

It is not hard to check that A is an isomorphism. Thus ordered bases for V are in 1-1 correspondence with linear isomorphisms FnV.

The reason for working with abstract vector spaces instead of Fn is that it is often preferable to work in a coordinate-free manner, i.e. without choosing a preferred basis. Indeed, many vector spaces that naturally show up in mathematics do not come with a preferred choice of basis.

It is possible and sometimes desirable to view a coordinate space dually as the set of F-valued functions on a finite set; that is, each "point" of Fn is viewed as a function whose domain is the finite set {1,2....n} and codomain F. The function sends an element i of {1,2....n} to the value of the i'th coordinate of the "point", so Fn is, dually, a set of functions.