Diamond-Symbol at the end of an environment

Question

I want that theorems, lemmas, examples... are ended with a diamond. Like this:

I found out that I can use \hfill\mbox{$\diamond$}. But I already wrote my document with 100s of examples, lemmas... I don't want to set it manually. So I thought I could set it up in \theoremstyle. Where do I have to set this? Here's my MWE:

\documentclass[ngerman, fontsize=11pt, DIV=15, BCOR = 10mm,parskip=half-, twoside]{scrartcl}
\usepackage{babel}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}

\newtheoremstyle{definition}{}{}{}{}{\bfseries}{:}{5 pt}{}

\theoremstyle{definition}
\newtheorem{defi}[]{Definition}

\begin{document}
      Here's the example.

\begin{defi}
       Does the diamond appear?
\end{defi}
\end{document}

Answer 1

You could use \AtEndEnvironment to automatically add your code (or my slightly adapted version) every time your defi environment ends.

As I use not only \diamonds at the end of some environments but also \triangle, I personally want them to have (approximately) the same size, so I added an environment defiBig which adds a rescaled \diamond. This resizing involves the scalerel package.

\documentclass[ngerman, fontsize=11pt, DIV=15, BCOR = 10mm,parskip=half-, twoside]{scrartcl}
\usepackage{babel}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{scalerel} % only necessary for the version with big \diamond

\newtheoremstyle{definition}{}{}{}{}{\bfseries}{:}{5 pt}{}

\theoremstyle{definition}
\newtheorem{defi}[]{Definition}
\AtEndEnvironment{defi}{\null\hfill\ensuremath{\diamond}}
% For a bigger \diamond you can use the following:
\newtheorem{defiBig}[defi]{DefinitionBig}
\newlength\myheight  \settoheight\myheight{$\triangle$}
\AtEndEnvironment{defiBig}{\null\hfill\ensuremath{\scaleto{\diamond}{\myheight}}}

\begin{document}
      Here's the example.
      \begin{defi}
             Does the diamond appear?
      \end{defi}
      \begin{defiBig}
             Does the big diamond appear?
      \end{defiBig}
\end{document}

Answer 2

Just to be consistent, here is a solution using ntheorem. It has not changed in like 10 years should should not have issues with a 2017 installation. Just remember it takes several compilations for the markers to settle.

Caveat: The ntheorem envs with markers does not like blank lines before the \end{env} part.

Code example including how to define proof:

\documentclass[ngerman, fontsize=11pt, DIV=15, BCOR = 10mm,parskip=half-, twoside]{scrartcl}
\usepackage{babel}
\usepackage{amsmath}
%\usepackage{amsthm}
\usepackage{amssymb}
\usepackage[amsmath,thmmarks]{ntheorem}

\theorembodyfont{\normalfont}
\theoremseparator{.}
\theoremsymbol{\enskip\ensuremath{\diamond}}
\newtheorem{defi}{Definition}

% body font is shared from above
\theoremstyle{nonumberplain}
\theoremheaderfont{\itshape}
\theoremsymbol{\enskip\ensuremath{\Box}}
\newtheorem{proof}{Proof}


\begin{document}
Here's the example.

\begin{defi}
  Does the diamond appear?
\end{defi}

\begin{proof}
  Test
\end{proof}
     
\end{document}

Answer 3

A more elegant way is as follows. By the way, it's better to select the ending symbol as the same as the qed symbol. Just as what I said about theorem formats in my book, the qed symbol indicates "the ending of a proof". So we can generally use the symbol to indicate “the transitory ending of some texts" and extend its use rang to all kinds of propositions, definitions, remarks and so on without proofs.

\documentclass{article}

\usepackage{amsthm}
\usepackage{latexsym}%for \Diamond
\renewcommand{\qedsymbol}{$\Diamond$}

\theoremstyle{theorem}
\newtheorem{theorem}{Theorem}
\newenvironment{thm}{\pushQED{\qed}\theorem}{\popQED\endtheorem}

\theoremstyle{definition}
\newtheorem{definition}{Definition}
\newenvironment{defi}{\pushQED{\qed}\definition}{\popQED\enddefinition}

\begin{document}

Here are the examples.

\begin{theorem}
Does the diamond appear? No!
\end{theorem}

\begin{thm}
Does the diamond appear? Yes!
\end{thm}

\begin{definition}
Does the diamond appear? No!
\end{definition}

\begin{defi}
Does the diamond appear? Yes!
\end{defi}

\end{document}

The following image shows how I use the qed symbol personally.

Answer 4

You could create a wrapper environment which would add a diamond at the end of the environment defined by amsthm. Something around these lines:

\documentclass[ngerman, fontsize=11pt, DIV=15, BCOR = 10mm,parskip=half-, twoside]{scrartcl}
\usepackage{babel}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}

\newtheoremstyle{definition}{}{}{}{}{\bfseries}{:}{5 pt}{}
\theoremstyle{definition}
\newtheorem{ddefi}[]{Definition}
\NewDocumentEnvironment{defi}{s +b}{%
  \begin{ddefi}
    #2\nolinebreak\IfBooleanT{#1}{\hfill}\enspace$\diamond$
  \end{ddefi}}{}


\begin{document}
Here's the example.
\begin{defi}
  Does the diamond appear
\end{defi}

\begin{defi}*
  Does the diamond appear
\end{defi}
\end{document}

---

EDIT. The code based on standard \newenvironment:

\documentclass[ngerman, fontsize=11pt, DIV=15, BCOR = 10mm,parskip=half-, twoside]{scrartcl}
\usepackage{babel}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}

\newtheoremstyle{definition}{}{}{}{}{\bfseries}{:}{5 pt}{}
\theoremstyle{definition}
\newtheorem{ddefi}[]{Definition}
\newenvironment{defi}{\begin{ddefi}}{\nolinebreak\hfill\enspace$\diamond$\end{ddefi}}


\begin{document}
Here's the example.
\begin{defi}
  Does the diamond appear
\end{defi}
\end{document}