# Prove that the convex hull of a set is the smallest convex set containing that set

How do you prove that the convex hull of A is the smallest convex set containing A?

edit: definition of a convex hull: Given a set A ⊆ ℝn the set of all convex combinations of points from A is called the convex hull of A.

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That depends: how do you define the convex hull of $A$? –  Gerry Myerson Oct 5 '11 at 2:54
What is your definition of the convex hull? And don’t you mean the smallest convex set containing A? –  Brian M. Scott Oct 5 '11 at 2:54
This is related. –  Did Oct 5 '11 at 5:13

Young, when you wrote

How do you prove that the convex hull of A is the smallest set containing A?

You meant that convex hull of A is the minimal convex set containing A, right?

To show this, which part is your definition? The linear-algebraic characterization?

You can see that any intersection of convex sets containing A is also a convex set containing A.

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Yea, the convex hull of A is the minimal convex set containing A. I guess what I need to prove is that the convex hull of A is itself a convex set containing A. How do I prove that or is it just by definition of a convex hull? –  JYoung Oct 5 '11 at 3:55
@Young: Let $\mathscr{C}$ be the collection of all convex sets containing $A$, and let $K=\bigcap\limits_{C\in\mathscr{C}}C$; certainly $K\supseteq A$, and you need to show that $K$ is convex. Let $x,y\in K$ and $t\in[0,1]$; you need to show that $tx+(1-t)y\in K$. Use the facts that (1) $x,y\in C$ for every $C\in\mathscr{C}$ and (2) every $C\in\mathscr{C}$ is convex. –  Brian M. Scott Oct 5 '11 at 4:05