Understanding Uniform Convergence: The Role of N and A

In summary, the conversation discusses a theorem about the convergence of a sequence of measurable functions to a real-valued function on a measurable set of finite measure. The conclusion of the theorem states that given ε>0 and \delta>0, there is a set A\subsetE with mA<\delta, and an N such that for all x\notinA and all n≥N, the sequence converges uniformly to the function on the set E~A. However, the individual discussing the theorem questions why the conclusion needs to include the conditions of ε and \delta, as the difference between uniform and pointwise convergence is small. The main difference being that in uniform convergence, N does not depend on x.
  • #1
EV33
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Homework Statement



I would just like to be pointed in the right direction. I have this theorem:

Let E be a measurable set of finite measure, and <fn> a sequence of measurable functions that converge to a real-valued function f a.e. on E. Then given ε>0 and [itex]\delta[/itex]>0, there is a set A[itex]\subset[/itex]E with mA<[itex]\delta[/itex], and an N such that for all x[itex]\notin[/itex]A and all n≥N,
lfn(x)-f(x)l<ε

To me it appears to be concluding:
Given ε>0 and [itex]\delta[/itex]>0, there is a set A[itex]\subset[/itex]E with mA<[itex]\delta[/itex], and an N such that for all x[itex]\notin[/itex]A and all n≥N,
<fn> converges uniformly to a real-valued function f on E~A.



I know that this isn't case but I don't see why. So my question is what would the conclusion of this theorem need to say, in terms of ε and [itex]\delta[/itex], so that <fn> converges uniformly to a real-valued function f on E~A?


Thank you for your time.
 
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  • #2
The difference between definitions of uniform and pointwise convergence is really small. What you have there is really uniform convergence. Although I don't see why you need delta and A..
Anyway, the difference is that in uniform convergence N does not depend on x. Swapping the conditions "there exists N" and "for all x" would produce the definition of pointwise convergence.
 

FAQ: Understanding Uniform Convergence: The Role of N and A

What is the difference between pointwise and uniform convergence?

Pointwise convergence is when a sequence of functions converges to a particular point for each value of x in the domain. Uniform convergence is when a sequence of functions converges to a particular point for all values of x in the domain simultaneously. In other words, pointwise convergence only requires that the functions get close to the limiting function at each point, while uniform convergence requires that the functions get uniformly close to the limiting function at all points.

How is uniform convergence related to continuity?

Uniform convergence of a sequence of functions implies that the limiting function is continuous. This means that the sequence of functions converging uniformly will always result in a continuous function.

What is the Cauchy criterion for uniform convergence?

The Cauchy criterion states that a sequence of functions converges uniformly if and only if for every positive number ε, there exists a natural number N such that for all n,m≥N, |fn(x) - fm(x)| < ε for all x in the domain.

Can a uniformly convergent sequence of functions converge to a function that is not in the sequence?

Yes, it is possible for a uniformly convergent sequence of functions to converge to a function that is not in the sequence. This is because uniform convergence only requires that the functions get close to the limiting function at all points, not necessarily that they are equal to the limiting function.

How is uniform convergence related to series convergence?

Uniform convergence of a sequence of functions is closely related to series convergence. In fact, a sequence of functions converges uniformly if and only if the corresponding series of functions converges uniformly. This means that the concepts of uniform convergence and series convergence are essentially interchangeable.

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