Fubini's theorem on differentiation

Assume ${\displaystyle I\subseteq \mathbb {R} }$ is an interval and that for every natural number k, ${\displaystyle f_{k}:I\to \mathbb {R} }$ is an increasing function. If,

${\displaystyle s(x):=\sum _{k=1}^{\infty }f_{k}(x)}$

exists for all ${\displaystyle x\in I,}$ then,

${\displaystyle s'(x)=\sum _{k=1}^{\infty }f_{k}'(x)}$

almost everywhere in I.[1]

In general, if we don't suppose f_k is increasing for every k, in order to get the same conclusion, we need a stricter condition like uniform convergence of ${\displaystyle \sum _{k=1}^{n}f_{k}'(x)}$ on I for every n.[2]