A~series $\displaystyle\sum_{k=1}^\infty a_k$ in $\K$ is convergent if and only if for all $\varepsilon>0$, there exists some $N$ such that for all $n\geq m\geq N$ holds
A~series $\displaystyle\sum_{k=1}^\infty a_k$ in $\mathbb{R}$ is convergent if and only if for all $\varepsilon>0$, there exists some $N$ such that for all $n\geq m\geq N$ holds
\[\left|\sum_{k=m}^n a_k\right|<\varepsilon.\]
\[\left|\sum_{k=m}^n a_k\right|<\varepsilon.\]
\end{Theorem}
\end{Theorem}
{\em Proof:} By the theorems on completeness and convergence of cauchy sequences, a~series converges if and only if the sequence $(s_{n})_{n\in\mathbb{N}}$ of partial sums is a~Cauchy sequence.\\
{\em Proof:} By the theorems on completeness and convergence of cauchy sequences, a~series converges if and only if the sequence $(s_{n})_{n\in\mathbb{N}}$ of partial sums is a~Cauchy sequence.\\
Therefore, the Cauchy criterion is really equivalent to the fact that $(s_{n})_{n\in\mathbb{N}}$ is a~Cauchy sequence in $\K$.\hfill$\Box$
Therefore, the Cauchy criterion is really equivalent to the fact that $(s_{n})_{n\in\mathbb{N}}$ is a~Cauchy sequence in $\mathbb{R}$.\hfill$\Box$
\begin{Remark}{}
% \begin{Remark}{}
%For incomplete spaces, the Cauchy criterion is only necessary (but not sufficient) for convergence of a~series. This is a~consequence of the fact that %the Cauchy criterion is equivalent to the fact that
% %For incomplete spaces, the Cauchy criterion is only necessary (but not sufficient) for convergence of a~series. This is a~consequence of the fact that %the Cauchy criterion is equivalent to the fact that
%the sequence of partial sums is a~Cauchy sequence.\\
% %the sequence of partial sums is a~Cauchy sequence.\\
Reconsidering the example at the very beginning of this chapter, the divergence of this sequence can be directly verified be employing the Cauchy criterion.
% Reconsidering the example at the very beginning of this chapter, the divergence of this sequence can be directly verified be employing the Cauchy criterion.
\end{Remark}
% \end{Remark}
As a corollary, we can formulate the following criterion.
As a corollary, we can formulate the following criterion.
\begin{Theorem}[Necessary criterion for convergence of series]
\begin{Theorem}[Necessary criterion for convergence of series]
\label{eq:conv0}
\label{eq:conv0}
Let \[\sum_{k=1}^\infty a_k\] be a~convergent series in $\K$. Then $(a_n)_{n \in\mathbb{N}}$ is convergent with
Let \[\sum_{k=1}^\infty a_k\] be a~convergent series in $\mathbb{R}$. Then $(a_n)_{n \in\mathbb{N}}$ is convergent with