Definition:Affine Space

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Definition

Associativity Axioms

Let $K$ be a field.

Let $\struct {V, +_V, \circ}$ be a vector space over $K$.

Let $\EE$ be a set on which two mappings are defined:

$+ : \EE \times V \to \EE$
$- : \EE \times \EE \to V$

satisfying the following associativity conditions:

\((\text A 1)\)   $:$     \(\displaystyle \forall p, q \in \EE:\) \(\displaystyle p + \paren {q - p} = q \)             
\((\text A 2)\)   $:$     \(\displaystyle \forall p \in \EE: \forall u, v \in V:\) \(\displaystyle \paren {p + u} + v = p + \paren {u +_V v} \)             
\((\text A 3)\)   $:$     \(\displaystyle \forall p, q \in \EE: \forall u \in V:\) \(\displaystyle \paren {p - q} +_V u = \paren {p + u} - q \)             


Then the ordered triple $\struct {\EE, +, -}$ is an affine space.


Group Action

Let $K$ be a field.

Let $\left({V, +_V, \circ}\right)$ be a vector space over $K$.

Let $\mathcal E$ be a set.

Let $\phi: \mathcal E \times V \to \mathcal E$ be a free and transitive group action of $\struct{V, +_V}$ on $\mathcal E$.


Then the ordered pair $\tuple{\mathcal E, \phi}$ is an affine space.


Weyl's Axioms

Let $K$ be a field.

Let $\struct{V, +_V, \circ}$ be a vector space over $K$.

Let $\EE$ be a set on which a mapping is defined:

$- : \EE \times \EE \to V$

satisfying the following associativity conditions:

\((\text W 1)\)   $:$     \(\displaystyle \forall p \in \EE: \forall v \in V: \exists ! q \in \EE:\) \(\displaystyle v = q - p \)             
\((\text W 2)\)   $:$     \(\displaystyle \forall p, q, r \in \EE:\) \(\displaystyle \paren{r - q} +_V \paren{q - p} = r - p \)             


Then the ordered pair $\tuple {\EE, -}$ is an affine space.


Addition

Let $\tuple {\EE, +, -}$ be an affine space.


Then the mapping $+$ is called affine addition.


Subtraction

Let $\tuple {\EE, +, -}$ be an affine space.


Then the mapping $-$ is called affine subtraction.


Tangent Space

Let $\tuple {\EE, +, -}$ be an affine space.

Let $V$ be the vector space that is the codomain of $-$.


Then $V$ is the tangent space of $\EE$.


Vector

Let $\EE$ be an affine space.

Let $V$ be the tangent space of $\EE$.


Any element $v$ of $V$ is called a vector.


Point

Let $\mathcal E$ be an affine space.


Any element $p$ of $\mathcal E$ is called a point.


Also see


Sources