Do not get a square ntheorem, proof

Question

Here is my code:

\documentclass[12pt,a4paper,twoside]{article}

\usepackage{amsmath}
\usepackage{amssymb}

\usepackage[thmmarks]{ntheorem}
\theoremsymbol{\ensuremath\square}
\newtheorem{Beweis}{Beweis}

\begin{document}

\begin{Beweis}
\begin{align*}
test
\end{align*}
\end{Beweis}
\end{document}

But no matter what I do I do not get a square at the end of the proof. What do I do wrong?

Answer 1

I suggest to use amsthm. The name of the proof will automatically be translated to "Beweis" thanks to the babel package and it solves your additional problem with the number you mentioned [in your comment][1.

Thanks to barbara beeton for the hint with the qed symbol!

\documentclass[12pt,a4paper,twoside]{article}

\usepackage[ngerman]{babel}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}

\begin{document}

\begin{proof}
Es ist (geometrische Reihe):
\begin{align*}
    \sum_{k=0}^{\infty}\Big(\frac{1}{4}\Big)^k=&\frac{1}{1-\frac{1}{4}}\\
    =&\frac{4}{3}\\
    \iff \sum_{k=1}^{\infty}\Big(\frac{1}{4}\Big)^k=&\frac{1}{3}
\end{align*}
Und damit ist bei nur n Viertelungen:
\begin{align*}
    \sum_{k=1}^{\infty}\Big(\frac{1}{4}\Big)^k=\sum_{k=1}^{n}\Big(\frac{1}{4}\Big)^k+\sum_{k=n+1}^{\infty}\Big(\frac{1}{4}\Big)^k=&\frac{1}{3}\\
    \iff \sum_{k=1}^{n}\Big(\frac{1}{4}\Big)^k=&\frac{1}{3}-\sum_{k=n+1}^{\infty}\Big(\frac{1}{4}\Big)^k\qedhere
\end{align*}
\end{proof}
\end{document}