Conditionally convergent - limit of series' Let $\sum_{n=1}^\infty a_n$ be conditionally convergent.
Let $k_n:= \max(a_n,0),l_n:=-\min(a_n,0)$ for $n\in \mathbb{N}$ and show that
$\sum_{n=1}^\infty k_n =\infty $ and $\sum_{n=1}^\infty l_n=\infty$.

I found this exercise on some german math forum and the corresponding post was closed for the lack of answers.
Now I tried myself to it and I can visualize that the limits of those series' is $\infty$ because, since it's not absolutely convergent it is $&gt;0$ in case of $k_n$ and also in case of $l_n$ because $\min(a_n,0)&lt;0$ and the $(-)$ would make it positive again.
But how can I prove that mathematically? Or is the argumentation that I gave enough?
First of all we have 
$$\sum k_n+\sum l_n\stackrel{(1)}=\sum(k_n+l_n)\stackrel{(2)}=\sum |a_n|\stackrel{(3)}=\infty $$
where $(1)$ is valid because all $k_n,l_n$ are nonnegative, $(2)$ is valid because $k_n+l_n=|a_n|$ by definition of $k_n,l_n$, and $(3)$ is valid by assumption. Therefore, at least one of the series on the left diverges to $\infty$. If the other were finite (i.e., either $\sum k_n=K&lt;\infty$ or $\sum l_n=L&lt;\infty$), we'd have
$$\sum a_n=\sum k_n-\sum l_n=\begin{cases}K-\infty=-\infty\text{ or}\\\infty-L=+\infty\end{cases} $$
contrary to assumption.
If $\sum a_n$ is conditionally convergent, then $\sum a_n$ converges, but $\sum |a_n|=\infty$ or, in other words

$$\lim_{n\to\infty}\sum_{m=0}^na_m=\infty$$

Let $P=\{n\in\Bbb N: a_n\ge 0\}$.

$$k_n=\begin{cases}a_n, &amp; n\in P\\0, &amp; n\notin P\end{cases}$$

This gives
$$\sum_{n=0}^\infty k_n=\sum_{n\in P} |a_n|=\infty.$$

The last equality holds because $\lim_{n\to\infty}|a_n|=\infty$ implies that any subsequence of partial sums also approaches infinity.

The argument for $\sum l_n$ is similar.