Reference request: $K$ is relatively weakly compact in $C(X)$ iff $K$ is relatively pointwise compact in $C(X)$

functional-analysisgeneral-topologyreal-analysisreference-requestweak-topology

Let $X$ be a compact Hausdorff topological space and $K$ be a norm-bounded subset of $C(X)$(The space of all bounded continuous real valued function on $X$). Then $K$ is relatively compact in $C(X)$ for the weak topology if and only if $K$ is relatively compact in $C(X)$ for the topology of pointwise convergence on $X$.

Proof: $\implies $: since the topology of pointwise convergence is weaker than the weak topology so If $K$ is relatively weakly compact then these two topologies coincide, therefor $K$ is relatively pointwise compact.

the inverse is not clear for me.

Also I would be grateful if you could introduce a reference about this.

Best Answer

Let $K$ be relatively compact for the topology of pointwise convergence. By Eberlian Smulian Theorem it suffices to prove that $K$ is relatively weakly sequentially compact. Let $f_n$ be a sequence in $K$. since $K$ is relatively pointwise compact, $f_n$ has a pointwise convergent subsequence $f_{n_k}$ to a function $f\in C(X)$. Let $x^*\in C(X)^*$ we show that $x^*(f_{n_k})\to x^*(f)$. By Riesz Representation Theorem there exists a regular Borel measure $\mu$ on $X$ such that

$$x^*(g)=\int_X gd\mu$$

then by Lebesgue Dominated Convergence Theorem:

$$\lim_k x^*(f_{n_k})=\lim \int _X f_{n_k}d\mu=\int_Xfd\mu=x^*(f)$$

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