I am trying to show that the Hölder space $C^{k,\gamma}(\bar{U})$ is a Banach space. To do this, I successfully proved that the mapping $\| \quad \| : C^{k,\gamma}(\bar{U}) \to [0,\infty)$ is a norm, by proving its properties.
But how do I show that the sequence $\{u_k\}_{k=1}^\infty \subset C^{k,\gamma}(\bar{U})$ converges to $u \in C^{k,\gamma}(\bar{U})$, that is, how do I show that $$\lim_{k \to \infty}\|u_k-u\|=0,$$ which would mean the normed linear space is complete, and hence a Banach space?
Here are the following taken from PDE Evans, 2nd edition, page 255:
Definition. The Hölder space $$C^{k,\gamma}(\bar{U})$$ consists of all functions $u \in C^k(\bar{U})$ for which the norm $$\|u\|_{C^{k,y}(\bar{U})}:= \sum_{|\alpha|\le k} \|D^\alpha u \|_{C(\bar{U})}+\sum_{|\alpha|=k} [D^\alpha u]_{C^{0,\gamma}(\bar{U})}$$ is finite.
Also from page 254,
Definitions. (i) If $u : U \to \mathbb{R}$ is bounded and continuous, we write $$\|u\|_{C(\bar{U})}:=\sup_{x\in U}|u(x)|.$$
(ii) The $\gamma$th-Hölder seminorm of $u : U \to \mathbb{R}$ is $$[u]_{C^{0,\gamma}(\bar{U})}:=\sup_{\substack{x,y\in U \\ x \neq y}} \left\{\frac{|u(x)-u(y)|}{|x-y|^\gamma} \right\},$$ and the $\gamma$th-Hölder norm is $$\|u\|_{C^{0,\gamma}(\bar{U})}:=\|u\|_{C(\bar{U})}+[u]_{C^{0,\gamma}(\bar{U})}.$$
This is all I got so far:
\begin{align}
\|u_k-u\|_{C^{k,\gamma}(\bar{U})}&=\sum_{|\alpha|\le k} \|D^\alpha u \|_{C(\bar{U})}+\sum_{|\alpha|=k} [D^\alpha u]_{C^{0,\gamma}(\bar{U})} \\
&= \sum_{|\alpha|\le k} \sup_{x\in U} |u_k(x)-u(x)|+ \sum_{|\alpha|=k} \sup_{\substack{x,y\in U \\ x \neq y}} \left\{\frac{|[u_k(x)-u(x)]-[u_k(y)-u(y)]|}{|x-y|^\gamma} \right\}.
\end{align}
Now, where can I go from here, to show that $\lim_{k \to \infty}\|u_k-u\|_{C^{k,\gamma}(\bar{U})}=0$? The sequence is Cauchy, and I have to use that fact somehow.
Best Answer
We have to show the following: given a Cauchy sequence $(u_n)_{n\in \mathbb N}$ in $C^{k,\gamma}(\overline U),$ there is a $u \in C^{k,\gamma}(\overline U)$ such that $\lim_{n\rightarrow\infty}u_n = u$ in $C^{k,\gamma}(\overline U).$
Note that the Hölder norm is the "sum" of the $C^k$ norm (i.e. sup-norm up to the $k$-th derivatives) and the Hölder condition with parameter $\gamma \in (0,1)$ for the $k$-th derivatives. From the $C^k$ part of the Hölder norm and the completeness of $C^k(\overline U),$ we get a $u \in C^k(\overline U)$ with $\lim_{n\rightarrow\infty}u_n = u$ in $C^k(\overline U).$ Now let's look at the Hölder condition. Let's fix a multi-index $\alpha$ with $|\alpha| = k$ and write $$ v := D^\alpha u\qquad and \qquad v_n := D^\alpha u_n $$ for short. Then we have for fixed $x\neq y$ $$ \begin{align} \frac{|v(x) - v(y)|}{|x-y|^\gamma} & = \frac{|v(x) - v_m(x) + v_m(x) - v_m(y) + v_m(y) - v(y)|}{|x-y|^\gamma} \\ & \leq \frac{|v(x) - v_m(x)|}{|x-y|^\gamma} + \frac{|v_m(x) - v_m(y)|}{|x-y|^\gamma} + \frac{|v_m(y) - v(y)|}{|x-y|^\gamma} \end{align} $$ for arbitrary $m \in \mathbb N.$ Here, the first and third summands on the r.h.s. can be made arbitrarily small by choosing $m$ large enough, since $v_m \rightarrow v$ uniformly, and the second summand on the r.h.s. is bounded independent of $m$ and $x\neq y$ since the sequence $u_n$ is Cauchy, hence bounded, in the Hölder norm. From all this we get a constant $M_\alpha$ such that $$ \frac{|v(x) - v(y)|}{|x-y|^\gamma} \leq M_\alpha \qquad independent\ of\ x\neq y. $$ This shows that we have $$ u\in C^{k,\gamma}(\overline U). $$ So far so good. Finally, we have to show that $u_n \rightarrow u$ in $C^{k,\gamma}(\overline U).$ From the construction of $u,$ we already have $u_n \rightarrow u$ in $C^k(\overline U),$ so we only have to show convergence in the Hölder seminorms of the $k$-th derivatives. We use the notation $\alpha,v,v_n$ from above. We have $$ \begin{align} \frac{|(v-v_m)(x) - (v-v_m)(y)|}{|x-y|^\gamma} & = \frac{|v(x) - v_m(x) - v(y) + v_m(y)|}{|x-y|^\gamma} \\ & = \frac{|(\lim_{k\rightarrow\infty}v_k(x)) - v_m(x) - (\lim_{k\rightarrow\infty}v_k(y)) + v_m(y)|}{|x-y|^\gamma} \\ & = \lim_{k\rightarrow\infty}\frac{|v_k(x) - v_m(x) - v_k(y) + v_m(y)|}{|x-y|^\gamma} \\ & = \lim_{k\rightarrow\infty}\frac{|(v_k - v_m)(x) - (v_k - v_m)(y)|}{|x-y|^\gamma}. \end{align} $$ Here, the r.h.s. can be made arbitrarily small independent of $x\neq y$ by choosing $m$ large enough, since the sequence $u_n$ is Cauchy in the Hölder norm. So we find $v_n \rightarrow v$ in the Hölder seminorm. Since there are only finitely many multi-indices $\alpha$ (or equivalently, $k$-th derivatives) to consider, we can conclude $$ u_n \rightarrow u\qquad in\ C^{k,\gamma}(\overline U), $$ as desired.