Uniform Convergence & Boundedness

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Discussion Overview

The discussion centers around the implications of uniform convergence of a sequence of functions on Rn and the boundedness of the limit function. Participants explore the conditions under which a uniformly convergent sequence of bounded functions leads to a bounded limit function, addressing both theoretical and conceptual aspects.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant states that if fk converges uniformly to f and each fk is bounded by Ak, then f should be bounded, but questions the reasoning behind this implication.
  • Another participant provides a mathematical inequality involving ε and Ak, suggesting that uniform convergence allows ε to be used uniformly across all x.
  • Several participants express confusion about the role of ε, questioning whether it is fixed and why uniform convergence is necessary compared to pointwise convergence.
  • A participant explains that the norm being defined as the supremum allows for the establishment of a bound for f, emphasizing that uniform convergence provides a single ε applicable to all x.
  • Some participants challenge the notion that ε should always be given and argue that a constant bound M independent of k is desired, while others suggest that M may depend on k.
  • One participant summarizes that uniform convergence guarantees a bound for f, but notes that the proof does not specify the minimum value of this bound.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of uniform convergence and the nature of the bounds involved. There is no consensus on the implications of ε or the dependence of the bound M on k, indicating ongoing debate and uncertainty in the discussion.

Contextual Notes

Participants highlight limitations in their understanding of the relationship between uniform convergence and boundedness, particularly regarding the fixed nature of ε and the independence of the bound M from k.

kingwinner
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"Let fk be functions defined on Rn converging uniformly to a function f. IF each fk is bounded, say by Ak, THEN f is bounded."

fk converges to f uniformly =>||fk - f|| ->0 as k->∞
Also, we know|fk(x)|≤ Ak for all k, for all x

But why does this imply that f is bounded? I don't see why it is necessarily true.

Any help is appreciated!
 
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|f|=|f-fk+fk| ≤ |f-fk| + |fk|≤ ε + Ak

You need uniform for ε to be usable for all x.
 
Why does it imply f is bounded? Is ε fixed here?

Also, why do we need uniform convergence? (why is pointwise convergence not enough...I still don't see why)

Can someone explain this, please? Thanks!
 
Since the norm is defined to be the sup over all values of x
We can write:
||f||=||f-fk+fk|| ≤ ||f-fk|| + ||fk||≤ ε + Ak
Therefore ||f||≤ ε + Ak, which is a bound. ε will depend on k, but not on x.

The uniform convergence means that we can use one ε to be a bound for ||f-fk||. If it was simply pointwise convergence, ε would be a function of x and might not be bounded.
 
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Sorry, I don't think it makes sense. ε is supposed to be given, always. And we're supposed to find N such that for k>N, ... will be < ε

Also, Ak depends on fk. What we want is a bound M which is a constant and does not depend on k, right?
 
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You need to show that f is bounded, i.e. find some positive real number M such that for all x we have |f(x)|<M. Here M cannot depend on x, but it may ver well depend on k (why not?).

mathman found such M in his first post.
 
kingwinner said:
Sorry, I don't think it makes sense. ε is supposed to be given, always. And we're supposed to find N such that for k>N, ... will be < ε

Also, Ak depends on fk. What we want is a bound M which is a constant and does not depend on k, right?

put ε =1 and k = N+1
 
(Uniform convergence) For every ε > 0 there exists an N so that for all k > N, ||f-fk|| < ε.
(bounded) ||fk|| < Ak. (Use any k > N)

Therefore ||f|| < ε + Ak. Thus, in plain English, f is bounded! This proof does not determine the value of the minimum bound.
 

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