QED-Symbol in last line of a cases-environment

Question

I know there are several topics about this, but for any reason, no one answered my exact problem. So, I end a proof with a cases-environment and I want to set the QED-Square at the end of the last line. But it is either

set right after the last symbol (but in the correct line) or
in a new line (but at the correct position).

Here is my MWE which results in case 1. If I type

 \qedhere

before the \end{cases} command, it results in case 2.

\documentclass{article}
\usepackage[german, linesnumbered, boxruled]{algorithm2e}
\usepackage[utf8]{inputenc}
\usepackage[german]{babel}
\usepackage[T1]{fontenc}
\usepackage{empheq}
\usepackage{amsthm}
\begin{document}
There is
\begin{proof}
    this proof for
    [
    \begin{cases}
        a \
        b. 
    \end{cases}
    ]
    \end{proof}
\end{document}

related: qed when cases is the last element of the proof-ending display with svmono class without amsthm and ntheorem — dexteritas, Oct 16 '23 at 15:59
Welcome to tex,sx, You're using amsthm, which cooperates with amsmath in placing the QED symbol. But you haven't loaded amsmath. If you do that, it should place the QED at the right margin of the baseline of the equation containing the cases, but there are known ways to adjust that. — barbara beeton, Oct 16 '23 at 15:59

score 1 · Accepted Answer · answered Oct 16 '23 at 16:00

I'm not sure I would do this, but I guess you want

\documentclass{article}
\usepackage[german, linesnumbered, boxruled]{algorithm2e}
\usepackage[utf8]{inputenc}
\usepackage[german]{babel}
\usepackage[T1]{fontenc}
\usepackage{empheq}
\usepackage{amsthm}
\begin{document}
There is
\begin{proof}
    this proof for
    [
    \begin{cases}
        a &xxx\
        b.&yyy
    \end{cases}
    ]
\vspace{-1.2\baselineskip}
\mbox{}\qedhere
    \end{proof}
\noindent X\dotfill X % just for testing
\end{document}

That is exactly what I wanted to! Thank you, dear Sir! – Lereu Oct 16 '23 at 16:16 — Lereu, Oct 16 '23 at 16:16

score 1 · Answer 2 · answered Oct 16 '23 at 17:11

You can slightly change the code at https://tex.stackexchange.com/a/282772/4427

\documentclass[twocolumn]{article}
\usepackage{amsmath}
\usepackage{amsthm}
\begin{document}
\begin{proof}
this proof for
[
\begin{gathered}[b]
\begin{cases}
  a \
  b.
\end{cases}\[-1.2\normalbaselineskip]\mathstrut
\end{gathered}\qedhere
]
\end{proof}
\end{document}

Note. twocolumn is used just to make a smaller picture that better shows the alignment.

Anyway, the simplest way is to avoid such situations and finish the proof with some explicatory words.

QED-Symbol in last line of a cases-environment

2 Answers2