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Friday, 5 June 2015

The space C(\bar{\Omega})


This post tries to establish that the above mentioned space is Banach. The proof is straight-forward but it is here for reference sake! Let \Omega \subset \mathbb{R}^d be an open set. Denote by C(\bar{\Omega}) the set of all bounded continuous functions u: \Omega \to \mathbb{R} which can be continuously extended to \bar{\Omega} in a bounded manner. (These are exactly the same functions which are bounded and continuous on \bar{\Omega}) Now, we put the norm (called the sup norm) \|f\| = \sup_{x \in \bar{\Omega}} f(x) on this space. Clearly, C(\bar{\Omega}) is a vector space and \|f\| = \sup_{x \in \bar{\Omega}} |f(x)| defines a norm on C(\bar{\Omega}). The only non-trivial thing needed to prove this is that \sup_x (|f(x)| + |g(x)|) \leq \sup_x |f(x)| + \sup_x |g(x)| (note that these two values need not be equal).
C(\bar{\Omega}) with the norm defined above is a Banach space
Let f_n be a cauchy sequence in C(\bar{\Omega}), i.e. for every \epsilon > 0, there exists N such that for all n,m > N, we have \|f_n-f_m\| < \epsilon, i.e. \sup_x |f_n(x) - f_m(x)| < \epsilon. This gives that f_n(x) is a cauchy sequence for each x\in \bar{\Omega}. As \mathbb{R} is complete, f_n(x) converges for each x. Let the limit be f(x) (i.e. f(x) = \lim_{n \to \infty} f_n(x))

Note that we have convergence f_n to f in sup norm. (Just take the limit m \to \infty in the cauchy criterion). It remains to show that f is a bounded continuous function on \bar{\Omega}. \sup_x|f(x)| \leq \sup_x|f(x) - f_n(x)| + \sup_x|f_n(x)| For large enough n the first term on the r.h.s can be made small and second term is finite as f_n is bounded. Therefore f is bounded.

As for continuity, we have |f(x)-f(y)| \leq |f(x)-f_n(x)| + |f_n(x)-f_m(x)| + |f_m(x)-f_m(y)| + |f_m(y)-f(y)| The first, second and the fourth terms can be made small by choosing large enough n,m independent of x,y. The third term can be made arbitrarily small by choosing y sufficiently close to x due to the continuity of f_m.

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