# The difference between affine set and affine hull

According to the definition of affine hull and affine set.
$$aff [C] = [\theta_1x_1+...+\theta_nx_n|x_1,...x_n \in C, \theta_1+...+\theta_n=1]$$
The data in affine hull is also in affine set. And vice versus. So what's difference between them?

• I find your question confusing. If two things are defined the same way, then they are the same thing. If you want to ask the difference between two definitions, please report both of them properly. – Giovanni Mascellani Oct 19 '14 at 14:37
• An affine hull must be the affine hull of some set. An affine set is just a set with a particular property (the property of being affine). So the definitions aren’t really the same. An affine hull is something you define in terms of a given set. To define an affine set, you can define the property “affine,” and an affine set is a set with that property. – Steve Kass Jun 14 '18 at 14:50

An affine hull is always an affine set.

However, affine hull is connected to a set $C$ containing $n$ distinct points. The smallest affine set that contains those points will be affine hull of $C$.

For example, in vector space $\mathbb{R}^2$, if all points in $C$ are colinear, the line passing through those points would the affine hull of $C$.

But, if the points in $C$ are not colinear, the affine hull of $C$ will be the entire plane $\mathbb{R}^2$.

Similar, in vector space $\mathbb{R}^3$, if all points in $C$ are coplanar, the plane passing through those points would the affine hull of $C$.

It can also occur that the points in $C$ are also colinear. Hence, the affine hull of $C$ will be the line passing through these points.

If these four points are neither colinear nor coplanar, then the affine hull of $C$ will be the entire space vector space $\mathbb{R}^3$.

• @Henry sorry it was on my tongue. I meant affine hull (of course). – Shah Rukh Qasim Jun 14 '18 at 14:35

Affine set, Affine hull, Convex set and Convex hull

Affine set is a set which contains every affine combinations of points in it. For example, for two points $$x,y\in R^2$$, an affine set is the whole line passing through these two points. (Note: $$\theta_i$$ could be negative as long as $$\theta_1 + \theta_2 =1$$. If all $$\theta_i \geq 0$$, it is called a convex set and it is the line segment between two points instead of the whole line.)

For affine hull, convex hull might be easier to visualize first. Let's say $$A= \{ x,y,z | x,y,z \in R^2 \}$$. Convex hull of A is a plastic wrap around x,y,z containing every points within the wrap and on the border. The reason it is within the wrap is because coefficients $$\theta_i \geq 0$$. affine hull allow $$\theta_i$$ to be negative, so affine hull will be entire $$R^2$$ unless all three points lie on a line, in that case , affine hull will be the whole line passing through these three points.

Defintion of Affine hull for an arbitrary set A is the smallest affine set that can contains all points in A. Affine hull for an affine set is the affine set itself.

Hope it helps !!

$A$ is an affine set iff for any $x,y \in A$ the line passing through $A$ is also contained in $A$.

The affine hull of a set $S$, denoted by $\operatorname{aff} S$, is defined to be the smallest affine set that contains $S$. (By smallest, I mean the intersection of all affine sets containing $S$.)

At some level the terms mean the same thing in the sense that an affine hull is an affine set, and any affine set can be written as the affine hull of itself, but generally when the term affine hull is used, the underlying constituent set is being highlighted in some way.

aff C is the smallest affine set that contains set C. So by definition a affine hull is always a affine set.

e.g.

• The affine hull of 3 points in a 3-dimensional space is the plane passing through them.
• The affine hull of 4 points in a 3-dimensional space that are not on the same plane is the entire space.