[Math] Uniformly convergent subsequence: Arzela-Ascoli

functional-analysisreal-analysis

Let $f_n$ be a sequence of differentiable functions on $[0,1]$ such that $|f_n'(x)| \le M$ (the absolute value of the derivative of $f_n$ at $x$) for all $n$ and
for all $x$ in $[0,1]$. Show that $f_n$ has a uniformly convergent subsequence.

Partial solution: The $f_n$s are equicontinuous (by the mean value theorem). How to prove that the
$f_n$s are pointwise bounded, so that we can use Arzela-Ascoli?

Best Answer

As stated, the sequence need not be uniformly (in your words, pointwise, but I have not heard this term used this way before) bounded, and in fact need not have a uniformly convergent subsequence. Take for example the sequence of functions $f_n(x)=n$. I suspect that you are missing an additional assumption, of the form "each $f_n$ satisfies $f_n(0)=0$". If this is the case, then we can apply the Fundamental Theorem of Calculus: $$|f_n(x)|=\left|f(0)+\int_0^xf_n'(t)dt\right|\leq 0+\int_0^x|f_n(t)|dt\leq \int_0^xMdt=Mx\leq M$$