Sequences of Lipschitz Functions

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SUMMARY

The discussion focuses on the properties of sequences of Lipschitz functions defined on the interval [a, b] with a Lipschitz constant C. It establishes that if a sequence of functions (f_n) converges uniformly to a function f, then f belongs to the space Lip_C([a, b]). The participants also clarify that Lip_C([a, b]) is not a subvector space of B([a, b]), as demonstrated by the example of the function f(x) = Cx, where 2f does not satisfy the Lipschitz condition. The conversation highlights the importance of uniform convergence in maintaining the Lipschitz property.

PREREQUISITES
  • Understanding of Lipschitz continuity and the definition of Lip_C([a, b])
  • Knowledge of uniform and pointwise convergence of sequences of functions
  • Familiarity with bounded functions and the space B([a, b])
  • Basic concepts of real analysis and function spaces
NEXT STEPS
  • Study the properties of Lipschitz functions in real analysis
  • Learn about the implications of uniform convergence on function spaces
  • Explore counterexamples in functional analysis, particularly regarding subspaces
  • Investigate the relationship between pointwise and uniform convergence in detail
USEFUL FOR

Mathematicians, students of real analysis, and anyone studying functional analysis, particularly those interested in the properties of Lipschitz functions and convergence of function sequences.

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Let )<C<\infty and a,b \in \mathbb{R}. Also let
Lip_{C}\left(\left[a,b\right]\right) := \left\{f:\left[a,b\right]\rightarrow \mathbb{R} | \left|f(x) - f(y)\right| \leq C \left|x-y\right| \forall x,y \in \left[a,b\right]\right\}<br />.

Let \left(f_{n}\right) _{n \in \mathbb(N)} be a sequence of functions with f_{n} \in Lip_{C}\left(\left[a,b\right]\right) for all n.

i) Show that if \left(f_{n}\right) _{n \in \mathbb(N)} converges uniformly to a function f : \left[a,b\right]\rightarrow \mathbb{R}, then f \in Lip_{C}\left(\left[a,b\right]\right).

ii) Is Lip_{C}\left(\left[a,b\right]\right) a sub vector space of B\left(\left[a,b\right]\right) := \left\{f:\left[a,b\right]\rightarrow \mathbb{R} | f bounded }?____________________________________
Useful formula:

The sequence converges uniformly, so:
For all \epsilon &gt; 0, there exists N \in \mathbb{N} so that for all n,m &gt; N, |f_{n}(x)-f_{m}(x)|&lt;\epsilon for all x \in \left[a,b\right]\.

For (i), I honestly have no idea.

It seems that I can just say:

|f(x)-f(y)|
= |\lim_{n \to \infty} f_{n}(x) - \lim_{n \to \infty} f_{n}(y)|
= |\lim_{n \to \infty} (f_{n}(x) - f_{n}(y))|
= \lim_{n \to \infty} |(f_{n}(x) - f_{n}(y))| \leq C |x-y|

But I know that has to be wrong. It's not even using the uniformity.For (ii) I know the answer is of course not.

Let f(x):= Cx. Then 2 f \notin Lip_{C}\left(\left[a,b\right]\right), since

|2f(x)-2f(y)| = 2C|x-y| &gt; C|x-y|.

Help on part (i) please?
 
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Hmm.. after a bit more thinking...
I think that the sequence only needs to converge pointwise (not uniformly) to a function in order for the limit function to also be in LipC([a,b]).

So, my proof for (i) pretty much works. (I think) Please tell me if I am wrong?!

Also, interestingly, I think that a pointwise convergent series of functions in LipC([a,b]) is also uniformly convergent.
 

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