Show Uniform Convergence of fn to f in Compact Metric Space

In summary, the problem is to show that a sequence of continuous functions fn converges uniformly to an arbitrary function f on a compact metric space X, given that each fn(xn) converges to f(x) for any convergent sequence xn in X. The solution involves assuming that f is not uniformly continuous and creating a sequence of points in X that must have a convergent subsequence due to the compactness of X. The proof then proceeds by contradiction, showing that the sequence of points and functions must converge to f uniformly.
  • #1
Enjoicube
49
1

Homework Statement


Alright, here is the problem. Given a compact metric space [tex]X[/tex], and a sequence of functions fn which are continuous and [tex]f_{n}:X->R[/tex] (reals), also [tex]f_n->f[/tex] (where f is an arbitrary function [tex]f:X->R[/tex]). Also, given any convergent sequence in [tex]X[/tex] [tex]x_{n}->x[/tex], [tex]f_{n}(x_{n})->f(x)[/tex]. The problem is to show that fn converges uniformly to f.

The Attempt at a Solution


Alright, I can prove this relatively easily if I can prove that f (the limit function) is continuous. However, I don't know if this is possible, does anyone see a way to do this? Only little hints if you see a way.
 
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  • #2
Nevermind continuity, I don't think the continuity of f directly matters in this situation. However, maybe a start on the problem would be to assume that f is not uniformly continuous, and then, picking an epsilon e>0, for each n, pick one x in X such that |fn(x)-f(x)|>=e, and so create a sequence of points in X, which then must have a convergent subsequence due to the compactness of X. From there on, I am stumped, even if this is the correct way to solve the problem.
 
  • #3
I would think proof by contradiction is a good way to go. If the fn do not converge to f uniformly, then for all [tex]\epsilon[/tex], there exists an infinite subsequence of functions [tex]f_{n_k}[/tex] such that for each such function, there is a point [tex]x_{n_k}[/tex] with [tex]|f(x_{n_k})-f_{n_k}(x_{n_k})|>\epsilon[/tex]

Relabel these sequences as [tex]f_k[/tex] and [tex]x_k[/tex] for convenience. [tex]x_k[/tex] has a convergent subsequence which we will label [tex]y_k[/tex]. Now let [tex]f_k[/tex] just be the functions corresponding to the points [tex]y_k[/tex]

What can you say about [tex]|f(y_k)-f_k(y_k)|[/tex]
 
  • #4
Haha, took a walk and that's exactly what came to me. I think I had a proof of a similar theorem stuck in my head and wanted to follow that technique.
 

What does it mean for a sequence of functions to converge uniformly?

Uniform convergence of a sequence of functions means that for any given epsilon, there exists a natural number N such that for all n greater than or equal to N, the difference between the nth function and the limiting function is less than epsilon for all points in the domain.

Why is compactness important in proving uniform convergence?

Compactness is important because it guarantees that the sequence of functions will have a limit on the compact metric space. This allows for the use of the definition of uniform convergence, which requires the existence of a limit function.

How does the Cauchy criterion help prove uniform convergence?

The Cauchy criterion states that a sequence of functions converges uniformly if and only if for any given epsilon, there exists a natural number N such that for all m,n greater than or equal to N, the difference between the mth and nth functions is less than epsilon for all points in the domain. This criterion can be used to prove uniform convergence by showing that the sequence of functions satisfies this condition.

Can a sequence of functions converge pointwise but not uniformly?

Yes, a sequence of functions can converge pointwise but not uniformly. This means that for each point in the domain, the sequence of functions converges to the limiting function. However, the rate of convergence may vary at different points, and it is possible that the sequence does not converge uniformly across the entire domain.

What are some common techniques used to prove uniform convergence in compact metric spaces?

Some common techniques used to prove uniform convergence in compact metric spaces include the Cauchy criterion, the Weierstrass M-test, and the Arzela-Ascoli theorem. These methods involve showing that the sequence of functions satisfies certain conditions that guarantee uniform convergence.

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