TITLE: closure, convex hull and closed convex hull
QUESTION [12 upvotes]: Is the closure of the convex hull of some set $A\subseteq\mathbb R^d$ equal to the convex hull of the closure of $A$, i.e.
$$\text{cl}(\text{conv}(A))=\text{conv}(\text{cl}(A))?$$
If not, what are the general relations between them?

REPLY [13 votes]: No.
Let $A = \{(x,e^{-x})\}_{x\geq 0} \cup \{(x,-e^{-x})\}_{x\geq 0}$. Then $A$ is closed, and $\mathrm{co} A = (\{0\}\times [-1,1]) \cup ((0,\infty)\times (-1,1))$, which is not closed (take $(x_n,y_n) = (1, 1-\frac{1}{n})$).
Hence $\mathrm{co} A = \mathrm{co} \overline{A}  $ is strictly contained in $\overline{\mathrm{co}} A = [0,\infty)\times [-1,1]$.
If $A$ is compact, the result is true (using, eg, Carathéodory's theorem).

REPLY [9 votes]: Not necessarily. Let
$$A=\Bigl\{(x,y) : y\geq {1\over 1+x^2}\Bigr\}$$
Then the closure of the convex hull is the closed upper half plane $\{(x,y) : y\geq 0\}$, but the convex hull of the closure is the open upper half plane $\{(x,y) : y > 0\}$.