Showing a limit exists using differentiability

B3NR4Y
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Homework Statement


Assume f:(a,b)→ℝ is differentiable on (a,b) and that |f'(x)| < 1 for all x in (a,b). Let an
be a sequence in (a,b) so that an→a. Show that the limit as n goes to infinity of f(an) exists.

Homework Equations


We've learned about the mean value theorem, and all of that fun stuff.

The Attempt at a Solution


I don't really know where to start so I brainstormed a couple of things I noticed[/B]
I know that since |f'(x)| is always less than one, any sequence of points will be bounded. Since they are bounded they are cauchy.
I also know that
## lim_{h \rightarrow 0} \frac{f(x+h) - f(x)}{h} ## exists for all x. I assume this should also mean that ## lim_{n \rightarrow \infty} \frac{f(a_n)-f(a)}{a_n - a} ## exists. I think with these two facts I can construct a proof but I don't know if either of the two are correct
 
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B3NR4Y said:

Homework Statement


Assume f:(a,b)→ℝ is differentiable on (a,b) and that |f'(x)| < 1 for all x in (a,b). Let an
be a sequence in (a,b) so that an→a. Show that the limit as n goes to infinity of f(an) exists.

Homework Equations


We've learned about the mean value theorem, and all of that fun stuff.

The Attempt at a Solution


I don't really know where to start so I brainstormed a couple of things I noticed[/B]
I know that since |f'(x)| is always less than one, any sequence of points will be bounded. Since they are bounded they are cauchy.
I also know that
## lim_{h \rightarrow 0} \frac{f(x+h) - f(x)}{h} ## exists for all x. I assume this should also mean that ## lim_{n \rightarrow \infty} \frac{f(a_n)-f(a)}{a_n - a} ## exists. I think with these two facts I can construct a proof but I don't know if either of the two are correct
Hint:

Take two element of the sequence, ##a_n## and ##a_m##, and apply the mean value theorem for these two values.
That should give you an interesting upper bound for ## |f(a_n)-f(a_m)|##.

It is not true that a bounded sequence is Cauchy.
But, a convergent sequence is a Cauchy sequence, and in ℝ a Cauchy sequence has a limit. This, together with the upper bound for ## |f(a_n)-f(a_m)|## should lead to a solution.
 
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Ok so applying the mean value theorem, there is an x in (am,an) so that ## \frac{f(a_m) - f(a_n)}{a_m - a_n} = f'(x) ## taking the absolute value of both sides, ## |f(a_m) - f(a_n)| < |a_m - a_n| ##. Since an is convergent, |am - an| is always less than some epsilon greater than zero (because it is a convergent sequence in R and therefore Cauchy), and so |f(am) - f(an)| is also always less than that epsilon. This is the definition of a cauchy sequence, so the sequence f(an) must have a limit.

Is this the right path?
 
B3NR4Y said:
Ok so applying the mean value theorem, there is an x in (am,an) so that ## \frac{f(a_m) - f(a_n)}{a_m - a_n} = f'(x) ## taking the absolute value of both sides, ## |f(a_m) - f(a_n)| < |a_m - a_n| ##. Since an is convergent, |am - an| is always less than some epsilon greater than zero (because it is a convergent sequence in R and therefore Cauchy), and so |f(am) - f(an)| is also always less than that epsilon. This is the definition of a cauchy sequence, so the sequence f(an) must have a limit.

Is this the right path?
Yes.
Essentially you have that ##(f(a_n))_n## is a Cauchy sequence, therefore convergent.

Maybe a remark: where you say "is always less than some epsilon", you should add "for n and m sufficiently large". But that is probably what you implicitely meant.
 
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