If $E$ is a Banach space, $A$ is a subset of $E$ and is compact with the weak topology $\sigma(E,E')$, that is the most coarse topology which make every $f\in E'$ continuous, is it true that $A$ is weakly sequentially compact?
As we all know, in $C_1$ spaces, compact concludes sequentially compact. So, we should show that $E$ is a $C_1$ spaces with the topology $\sigma(E,E')$.
Some known conclusions:$\forall x_0\in E$ , the basis of neighborhoods of $x_0$ constitutes of $V$ with the below form
$$V=\lbrace x\in E;(f_i,xx_0)\lt\epsilon, i\in I\rbrace$$
where I is a finite set.
Can we prove $E$ is a $C_1$ spaces?
Now, for every such $V$, we can find a $\delta>0$, such that $B(x_0,\delta)\subset V$.
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Yes, it's the Eberlein–Šmulian theorem 

