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Suppose $f_{n}$ in $C[0,1]$ all have Lipschitz constant $L$. Show that if $f_{n}$ converges pointwise to $f$, then the convergence is uniform and $f$ is Lipschitz with constant $L$.

Attempt

Want to show: $\forall \epsilon$ there exists a $N > 0$ s.t. $\|f_n - f\| < \epsilon$ for all $x \in [0,1]$ and for all $n \geq N$. Also that $\|f(x) - f(y)\| \leq L\|x-y\|$ for all $x,y \in [0,1]$

What we know

$f_n$ Lipschitz means $\|f(x) - f(y)\| \leq L\|x-y\|$ for all $x,y \in [0,1]$.

By Pointwise convergence: For all $\epsilon > 0$ and for all $x \in [0,1]$, there exists $N > 0$ s.t. $\lim_{n \rightarrow \infty} f(x_{0})= f(x_{0})$ for fixed $x_{0}$. Which also means $\|f_{n}(x_{0}) - f(x_{0})\| < \epsilon$ for all $n \geq N$ (maybe I should use different notation for absolute value bars here).

Proof Attempt: Let $\epsilon > 0$ and take $n \geq N$ to satisfy the sequence condition.

$$\|f_{n}(x) - f(x)\| = \|f_{n}(x) - f_{n}(x_{0}) + f_{n}(x_{0}) - f(x_{0}) + f(x_{0}) - f(x)\| \leq \|f_{n}(x) - f_{n}(x_{0})\| + \|f_{n}(x_{0}) - f(x_{0})\| + \|f(x_{0}) - f(x)\|$$.

I feel that the first and second expression I could control by the Lipschitz constant and pointwise continuity respectively, so the next line would look something like:

$$\leq L\|x-x_{0}\| +\frac{\epsilon}{3} + something $$

I also feel that if I could show all three expressions in the form of a Lipschitz constant I could take each of their limits and I would get zero which could show uniform continuity. Am I on the right track and what hints could you give me to continue solving?

Mars Plastic
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D.C. the III
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  • Also: https://math.stackexchange.com/q/583692/42969. – Martin R Jul 19 '19 at 20:44
  • Taking limits of $\lvert f_n(x)-f_n(x_0)\rvert\leq L\lvert x-x_0\rvert$ reveals that $f$ is also Lipschitz with Lipschitz constant $L$. – user375366 Jul 19 '19 at 20:46
  • @user375366 how does that work out? – D.C. the III Jul 19 '19 at 20:48
  • @dc3rd It follows from the assumption that $f_n$ converges pointwise. If all of the $f_n$ satisfy the Lipschitz inequality, then so too does the limit function. – user375366 Jul 19 '19 at 20:52
  • @user375366 I actually am also supposed to prove that as well, so it is not given right away. – D.C. the III Jul 19 '19 at 20:53
  • @dc3rd If you are happy that $\lvert f_n(x)-f_n(x_0)\rvert\rightarrow\lvert f(x)-f(x_0)\rvert$ and you are able to use the fact that $a_n\leq K$ implies $a\leq K$ for convergence sequences $(a_n)$ then it will follow from these two facts. Otherwise you might have to establish these two in your proof or follow the linked questions for their method. – user375366 Jul 19 '19 at 20:57
  • @user375366 well previous to this question I did prove that $f$ was bounded if the $f_{n}$ were bounded. I haven't shown (to myself) that $\lvert f_n(x)-f_n(x_0)\rvert\rightarrow\lvert f(x)-f(x_0)\rvert$ is true, but I could probably do that. – D.C. the III Jul 19 '19 at 21:03

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