Arzelà-Ascoli

Totally Bounded

Theorem (Arzelà-Ascoli)

Let (X,d)(X,d) be a Compact Metric Space and F⊂C(X)F \subset C(X). Then F is Totally Bounded for dsupd_{sup} if and only if it is Uniformly Bounded and Equicontinuous, thus uniformly equicontinuous, i.e. ∀ϵ>0\forall\epsilon>0 ∣F∣<∞:∀f∈C(X),∃g∈F:sup⁡x∈X∣f(x)−g(x)∣<ϵ  ⟺  ∃δ>0:d(x,y)<δ  ⟹  ∣f(x)−f(y)∣<ϵ,∀f∈F\begin{gather*} |F|<\infty:\forall f \in C(X),\exists g\in F : \sup_{x \in \mathbb{X}}|f(x)-g(x)|<\epsilon\\ \iff\\ \exists\delta>0:d(x,y)<\delta\implies|f(x)-f(y)|<\epsilon,\forall f\in \mathscr{F} \end{gather*}