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daniel_i_l
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Homework Statement
1) f is some function who has a bounded derivative in (a,b). In other words, there's some M>0 so that |f'(x)|<M for all x in (a,b).
Prove that f is bounded in (a,b).
2) f has a bounded second derivative in (a,b), prove that f in uniformly continues in (a,b).
Homework Equations
The Attempt at a Solution
1) For all x in (a,b), (f(x)-f(a))/(x-a) = f'(c) =>
|f(x)-f(a)| = |f'(c)||x-a| =< M|b-a| (c is in (a,x))
and so
f(a) - M|b-a| =< f(x) >= f(a) + M|b-a| and so f(x) is bounded in (a,b).
2)Using (1) we see that f'(x) is bounded in (a,b). So there's some M>0 so that |f'(x)|<=M. And so for all x,y in (a,b)
(f(x)-f(y))/(x-y) = f'(c) =< |M| and so
|f(x)-f(y)| =< |x-y|M and it's easy to see that f is UC in (a,b).
Did I do that right? It some what bothers me that in (1) I also could have proved that f is UC which is a stronger result than the fact that it's bounded the same way that I proved it in (2). Is that right? If so then why did they only ask to prove that it was bounded?
Thanks.