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.
No comments:
Post a Comment