Some version of the theorem of Arzelà-Ascoli

arzela-ascolifunctional-analysisreal-analysisreference-requestuniform-convergence

I have a question concerning the theorem of Arzelà-Ascoli. Let $(f_n)_n:[0,T]\to \mathbb{R}$ be a family of functions so that $(f_n)_n$ is uniformly bounded on $[0,T]$ and $(\frac{d}{dt}f_n)_n$ is uniformly bounded on $[0,T]$. In the paper I read $f_n$ is some integral operator for which they have shown $\frac{d}{dt}f_n\to \frac{d}{dt}f$ in $L^1((0,T))$. Then they conclude that by this $f_n\to f$ uniformly on $[0,T]$. Do we really need this last convergence? Due to https://en.wikipedia.org/wiki/Arzel%C3%A0%E2%80%93Ascoli_theorem it should be enough to have a uniform bound on the derivative which then gives
$$|f_n(t)-f_n(\tilde{t})|\leq |\frac{d}{dt}f(\xi_{t,\tilde{t}})||t-\tilde{t}|\leq \|\frac{d}{dt}f\|_{L^{\infty}}|t-\tilde{t}|.$$
Does anybody have a reference for this situation?

Best Answer

Yes, we need. Arzela-Ascoli theorem gives the convergence of some subsequence. The reasoning is the following.

  • Take any subsequence of our sequence.
  • Use A-A theorem to prove that it has convergent sub-subsequence.
  • Use your convergence in $L^1$ to prove that this sub-subsequence converges to $f$.
  • You got that each subsequence has a sub-subsequence convergent to $f$. This implies that the whole sequence converges to $f$.
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