# Weak* extreme points

Suppose $x$ is an extreme point of the unit ball of a Banach space $E$. Embed $E$ in $E^{**}$ in the standard way. Is $x$ an extreme point of the unit ball of $E^{**}$?

• With regard to the title, where does the weak-* topology enter this question? Feb 21, 2016 at 14:40
• The general definition I have in mind is: an extreme point of $K \subset E$ is weak* extreme if it is still an extreme point of the weak* closure of $K$ in $E^{**}$. I guess it's not hard to come up with examples of extreme points that are and that are not weak* extreme. Feb 21, 2016 at 14:48
• Come to think of it, that may be a good way to find a counterexample to my question ... Feb 21, 2016 at 14:48

The answer is no and $E=\mathscr{K}(\ell_p)$ for $p\in (1,\infty)\setminus \{2\}$ is already a counterexample. See the proof of Proposition 2.3 in

J. Hennefeld, Compact extremal operators, Illinois J. Math. 21 (1997) 61-65.

Here we identifty $\mathscr{K}(\ell_p)^{**}$ with $\mathscr{B}(\ell_p)$.

This question is discussed in:

S. Dutta, T. S. S. R. K. Rao, On Weak-${}^\ast$ Extreme Points in Banach Spaces, Journal of Convex Analysis, 10 (2003), No. 2, 531-539.

where further examples are given.

• Oh great. That answers it nicely! Feb 21, 2016 at 15:37