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Friday, 5 June 2015

Holder Spaces


Let \Omega \subset \mathbb{R}^n be an open set with k \geq 0 being an integer and 0 < \alpha \leq 1. The space of Holder continuous functions C^{0,\alpha}(\Omega) is defined as consisting of all those continuous functions f \in C(\Omega) so that [f]_{C^{0,\alpha}(K)} := \sup_{x,y \in K, x\neq y} \left\{\frac{|u(x)-u(y)|}{|x-y|^\alpha}\right\} < \infty for every compact K \subset \Omega. Similarly, the space C^{0,\alpha}(\bar{\Omega}) is the set f\in C(\bar{\Omega}) so that [f]_{C^{0,\alpha}(\bar{\Omega})} < \infty. This space is equipped with the norm \|f\|_{C^{0,\alpha}(\bar{\Omega})} := \|f\|_{C^0(\bar{\Omega})} + [f]_{C^{0,\alpha}(\bar{\Omega})}.
(C^{0,\alpha}(\bar{\Omega}), \|\cdot\|_{C^{0,\alpha}(\bar{\Omega})}) is a Banach space
Let f_n be a cauchy sequence in C^{0,\alpha}(\bar{\Omega}). Therefore, for every \epsilon > 0 there exits M such that for all n,m > M, we have \|f_n-f_m\|_{C^{0,\alpha}(\bar{\Omega})} < \epsilon. This gives that [f_n-f_m]_{C^{0,\alpha}(\bar{\Omega})} < \epsilon and \|f_n -f_m\|_{C^0(\bar{\Omega})} < \epsilon. From the latter we get that f_n converges (in the sup norm) to a bounded continuous function f (on \bar{\Omega}). From the former we get that [f_n-f]_{C^{0,\alpha}(\bar{\Omega})} < \epsilon by letting m \to \infty. Therefore \|f_n-f\|_{C^{0,\alpha}(\bar{\Omega})} \to 0 as n \to \infty. It remains to show that f \in {C^{0,\alpha}(\bar{\Omega})} but this follows easily by [f]_{C^{0,\alpha}(\bar{\Omega})} = [f-f_n+f_n]_{C^{0,\alpha}(\bar{\Omega})} \leq [f-f_n]_{C^{0,\alpha}(\bar{\Omega})} + [f_n]_{C^{0,\alpha}(\bar{\Omega})} < \infty

Note that this result does not depend on \alpha.

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