# Span, linear independence, linear dependence, basis and dimension

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This article includes information on linear dependence and independence an introduction and discussion of these very important concepts can be found on the Basis and coordinates page

## Span

### Definition

Given a set of vectors [ilmath]S[/ilmath] in a vector space [ilmath](V,F)[/ilmath]

$\text{Span}(S)=\left\{\sum^n_{i=1}\lambda v_i|n\in\mathbb{N},\ v_i\in S,\ \lambda_i\in F\right\}$

It is very important that only finite linear combinations are in the span.

## Linear Dependence

A set [ilmath]E[/ilmath] in a vector space [ilmath](V,F)[/ilmath] is linearly dependent if for any finite collection of elements of [ilmath]E[/ilmath] that finite collection is linearly dependent

That is, $\forall n\in\mathbb{N}$ given a subset $\{e_1,...,e_n\}\subset E$

There are solutions to $\sum^n_{i=1}e_i\alpha_i=0$ where the $\alpha_i\in F$ are not all zero.

## Linear Independence

If a set is not linearly dependent it is linearly independent, but formally:

For all finite subsets of a set [ilmath]E[/ilmath], we have only $\alpha_i=0\forall i$ as solutions to $\sum^n_{i=1}e_i\alpha_i=0$

## Basis

Usually a basis will be a finite set, for example, $\{(1,0),(0,1)\}$ is a basis of [ilmath]\mathbb{R}^2[/ilmath].

### Finite case

Given a finite set [ilmath]B\subset V[/ilmath], [ilmath]B[/ilmath] is a basis of [ilmath]V[/ilmath] if $\text{span}(B)=V$ and [ilmath]B[/ilmath] is linearly independent.

### Infinite case

A Hamel basis is any linearly independent subset of [ilmath]V[/ilmath] that spans [ilmath]V[/ilmath] - where linearly independent is given as above.

The definition of independence varies slightly from how it is usually given (I explicitly say for all finite subsets) it is just a stronger form.

#### Example

The set $E=\{1,x,x^2,x^3,...,x^i,...\}$ is a Hamel basis for the space of all polynomials

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