# I Affine hull and affine combinations equivalence

#### Mr Davis 97

Let $X = \{x_1 , \dots , x_n\}$. Then $\text{aff}(X) = \text{intersection of all affine spaces containing X}$. Let $C(X)$ be the set of all affine combinations of elements of $X$. We want to show that these two sets are equal. First we focus on the $\text{aff}(X) \subseteq C(X)$ inclusion. If we can show that $C(X)$ is affine and that it contains $X$ then this inclusion will hold. Let's just say that it's obvious that it contains $X$. So I want to prove that $C(X)$ is affine. Here is where I hit a roadblock. In my reference it only states that an affine space is a translate of a linear subspace. This is really the only definition I am given. Does this mean that I have to show that $C(X)$ is the translate of some linear subspace?

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#### fresh_42

Mentor
2018 Award
Let $X = \{x_1 , \dots , x_n\}$. Then $\text{aff}(X) = \text{intersection of all affine spaces containing X}$. Let $C(X)$ be the set of all affine combinations of elements of $X$. We want to show that these two sets are equal. First we focus on the $\text{aff}(X) \subseteq C(X)$ inclusion. If we can show that $C(X)$ is affine and that it contains $X$ then this inclusion will hold. Let's just say that it's obvious ...
or just note that $x_i=0+x_i$ is an affine combination
... that it contains $X$. So I want to prove that $C(X)$ is affine. Here is where I hit a roadblock. In my reference it only states that an affine space is a translate of a linear subspace. This is really the only definition I am given. Does this mean that I have to show that $C(X)$ is the translate of some linear subspace?
Yes, and again $0 + \operatorname{span}(X)$ is a linear and as such an affine space, too.

#### Mr Davis 97

or just note that $x_i=0+x_i$ is an affine combination
Yes, and again $0 + \operatorname{span}(X)$ is a linear and as such an affine space, too.
Are you saying that $C(X) = 0 + \text{span}(X)$?

#### fresh_42

Mentor
2018 Award
Are you saying that $C(X) = 0 + \text{span}(X)$?
You're right, that wasn't a valid argument. But $0+\operatorname{span}(X) \subseteq C(X)$ whereas $0+\operatorname{span}(X) \nsubseteq \operatorname{aff}(X)$. What do I miss here? Are you sure the equation holds? I guess I'm currently a bit confused.

Edit: Assume we have two points in the plane. Then $\operatorname{aff}(\{x,y\})$ is the one straight line through $x$ and $y$. But any affine combination $\alpha x + \beta y = c$ yields the entire plane.

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#### Math_QED

Homework Helper
Are you saying that $C(X) = 0 + \text{span}(X)$?
What is span(X)? X are elements in an affine space: writing span around them makes no sense.

Can you give your definition of affine space? I saw it as a set on which a vector space acts satisfying some axioms.

#### fresh_42

Mentor
2018 Award
What is span(X)? X are elements in an affine space: writing span around them makes no sense.
It does. The linear span of all points in $X$ is still an affine space.

#### Math_QED

Homework Helper
It does. The linear span of all points in $X$ is still an affine space.
Span is something we write for vectors. The elements of X are not necessarily vectors if one treats the theory of affine geometry generally. It's why I asked for the OP's definition.

#### fresh_42

Mentor
2018 Award
Span is something we write for vectors. The elements of X are not necessarily vectors if one treats the theory of affine geometry generally. It's why I asked for the OP's definition.
$\vec{0x_i}$ are the corresponding vectors. A space is affine if all $\vec{0x_i} - \vec{0x_j}$ are within. This is the case for $(0+\vec{0x_i}) - (0+\vec{0x_j})$. Nevertheless, I'm not sure what is meant by affine combination, other than all possible $\vec{c} + \sum_i \alpha_i \,\vec{0x_i}$ which is possibly wrong here. It should presumably be $\sum_{i,j} \alpha_{ij} \,(\vec{0x_i}-\vec{0x_j})\,.$

"Affine hull and affine combinations equivalence"

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