# Banach space

by dirk_mec1
Tags: banach, space
 P: 683 1. The problem statement, all variables and given/known data 2. Relevant equations A banach space is a complete normed space which means that every Cauchy sequence converges. 3. The attempt at a solution I'm stuck at exercise (c). Suppose $$(f_n)_n$$ is a Cauchy sequence in E. Then $$|f_n-f_m| < \epsilon\ \forall\ n,m \leq N$$ so $$|f'_n - f'_m| \leq |f'_n - f'_m|_{\infty} < \epsilon$$ Am I going in the right direction?
 Sci Advisor HW Helper Thanks P: 25,228 You have that backwards. ||fn-fm||_E
P: 683
 Quote by Dick You have that backwards. ||fn-fm||_E
Really? I don't see why this is so.

 Can you use the fact the difference in derivatives of fn and fm is small to prove the difference between fn and fm is small? Hence that fn(x) is a cauchy sequence for each x?
But what good will that do?

So here is the interpretation of the assignment in my eyes:

Given a Cauchy sequence $$(f_n)_n \in\ E$$ prove that $$||f_n-f||_E \rightarrow 0$$ and that f is in E.

So we have:

$$||f'_n -f'_m||_{\infty} < \epsilon\ \forall m,n \geq N$$

and we want: $$||f'_n -f'||_{\infty} \rightarrow 0\ \forall n \geq N$$

Is this correct?

 Sci Advisor HW Helper Thanks P: 25,228 Banach space Yes, that's it. Show f exists and has bounded derivative.
 P: 683 I'm sorry Dick, I have have been thinking about this but I can't seem to get f in E and converging. Your posts imply that I should prove that f_n is a Cauchy sequence but what do I get from that? You also mention to use the deratives to prove that f_n is Cauchy: do you mean that I should use the definition of the derative?
 Sci Advisor HW Helper Thanks P: 25,228 You know there is a limiting function f', since f'_n(x) is a cauchy sequence in R for every x. So that sequence has a limit, define f'(x) to be that limit. f' is also continuous since it's a uniform limit of continuous functions. Once you have f' just define f to be the integral from 0 to x of f'(t)dt.

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