Editing theorem environments

Question

I have two questions about editing theorem environments on Tex. I did not see concrete answers among answered questions, so I ask here mines, hoping that there are not frequently asked.

First, I would like to know how to create the left line form the underlined title and followed by a little line at the bottom of a theorem like on this picture : Then, which packages are needed to do that - and in which order (I encountered some trouble with adding some packages together, something like incompatibility, especially with "hyperref" and "ntheorem" packages)?

I am not an expert at all with Tex, so an answer like "cut and past it in your code" will be satisfying!

Thanks.

Answer 1

You can do this with thmbox package

\documentclass{article}
\usepackage{thmbox}
\newtheorem[L]{thm}{Theorem}[section]
\newtheorem{lem}[thm]{Lemma}
\newtheorem[S]{cor}[thm]{Corollary}

\begin{document}

\begin{lem}{First resualt}
Here a lemma
\end{lem}

\begin{thm}{Important}
Great theorem
\end{thm}

\begin{cor}{Final}
Here the corollary
\end{cor}

\end{document}

for ntheorem compatibilite may be

\usepackage[amsmath,hyperref,thmmarks]{ntheorem}

or if amsthm theorem style is used

\usepackage[amsthm,amsmath,hyperref,thmmarks]{ntheorem}

Answer 2

I wanted to try an answer with \tcolorbox just as an exercise, but Bernard gave a good reason, thmbox can't break across pages and tcbtheorems can. So, in case someone is interested, here you have a starting point.

A tcbthmbox looks similar to thmbox, tcbSthmbox like [S]-thmbox and tcbLthmbox like [L]-thmbox.

\documentclass{article}
\usepackage{lipsum}
\usepackage{amsmath,amssymb}
\usepackage[most]{tcolorbox}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french]{babel}
\usepackage{lmodern}

\tcbset{
    thmbox/.style={
        enhanced,
        breakable,
        sharp corners=all,
        fonttitle=\bfseries\normalsize,
        fontupper=\normalsize\itshape,
         top=0mm,
         bottom=0mm,
         right=0mm,
        colback=white,
        colframe=white,
        colbacktitle=white,
        coltitle=black,
        attach boxed title to top left,
        boxed title style={empty, size=minimal, bottom=1.5mm},
        overlay unbroken ={
            \draw (title.south west)--(title.south east);
            \draw ([xshift=3.5mm]frame.north west)|-%
                  (frame.south east)--(frame.north east);},
        overlay first={
            \draw (title.south west)--(title.south east); 
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);
            \draw (frame.north east)--(frame.south east);},
        overlay middle={
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);
            \draw (frame.north east)--(frame.south east);},
        overlay last={
            \draw ([xshift=3.5mm]frame.north west)|-%
                  (frame.south east)--(frame.north east);},
        },
    S/.style={thmbox, 
        overlay unbroken ={
            \draw (title.south west)--(title.south east);
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay first={
            \draw (title.south west)--(title.south east); 
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay middle={
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay last={
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        },
    L/.style={thmbox, 
        overlay unbroken ={
            \draw (title.south west)--(title.south east);
            \draw ([xshift=3.5mm]frame.north west)|-([xshift=15mm]frame.south west);},
        overlay first={
            \draw (title.south west)--(title.south east); 
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay middle={
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay last={
            \draw ([xshift=3.5mm]frame.north west)|-([xshift=15mm]frame.south west);},
        },
    LQ/.style={thmbox, 
        overlay unbroken ={
            \draw (title.south west)--(title.south east);
            \draw ([xshift=3.5mm]frame.north west)|-([xshift=15mm]frame.south west);
            \node[anchor=east] at (frame.south east) {$\square$};},
        overlay first={
            \draw (title.south west)--(title.south east); 
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay middle={
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay last={
            \draw ([xshift=3.5mm]frame.north west)|-([xshift=15mm]frame.south west);
            \node[anchor=east] at (frame.south east) {$\square$};},
    },  
}

\newtcbtheorem[]{tcbthmbox}{Theorem}{thmbox}{theo}
\newtcbtheorem[]{tcbSthmbox}{Theorem}{thmbox,S}{theo}
\newtcbtheorem[]{tcbLthmbox}{Theorem}{thmbox,L}{theo}
\newtcbtheorem[]{proof}{Démonstration}{thmbox,LQ}{theo}

\begin{document}

\begin{tcbthmbox}{First result}{}
\lipsum[3]
\end{tcbthmbox}

\begin{tcbSthmbox}{Important}{}
\lipsum[2]
\end{tcbSthmbox}

\begin{tcbLthmbox}{Final}{}
\lipsum[1]
\end{tcbLthmbox}

\begin{proof}{Proof}{}
\lipsum[1]
\end{proof}
\end{document}

Update:

\newtcbtheorem[<init options>]{<name>}{<display name>}{<options>}{<prefix>}

has five parameters <init options> are related with automatic numbering; name is the name used as environment \begin{name}...\end{name}; display name, the word or words used before title: Theorem, Lemma, Démonstration, ...; options are all tcolorbox options to define its appearance and prefix a prefix used before the label assigned to each environment. Once defined, every tcbtheorem environment is declared with

\begin{name}{title}{label}
...
\end{name} 

which uses two mandatory arguments, title and label. They are mandatory but can be empty. If a proof doesn't have any title except Démonstration, just leave the title empty. In this case : (or any other character used between display name and title) will vanish.

If you also want to suppress theorem numbers, you have two options, the first one is \begin{name*}...\end{name*}. The starred variant doesn't have number but it's also unlabeled and not listed in list of theorems. A second possibility consists in using option theorem name is theorem options. This options can be added inside \tcbset command, in \newtcbtheorem <options> parameter or through an optional argument in theorem environment: \begin{proof}[theorem name]{}{}.

Following code which has been a little bit simplified from previous version, shows some examples with proof:

\documentclass{article}
\usepackage{lipsum}
\usepackage{amsmath,amssymb}
\usepackage[most]{tcolorbox}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french]{babel}
\usepackage{lmodern}

\tcbset{
    thmbox/.style={
        enhanced,
        breakable,
        sharp corners=all,
        fonttitle=\bfseries\normalsize,
        fontupper=\normalsize\itshape,
         top=0mm,
         bottom=0mm,
         right=0mm,
        colback=white,
        colframe=white,
        colbacktitle=white,
        coltitle=black,
        attach boxed title to top left,
        boxed title style={empty, size=minimal, bottom=1.5mm},
        overlay unbroken ={
            \draw (title.south west)--(title.south east);
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay first={
            \draw (title.south west)--(title.south east); 
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay middle={
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        overlay last={
            \draw ([xshift=3.5mm]frame.north west)--([xshift=3.5mm]frame.south west);},
        },
    Boxed/.style={ 
        overlay unbroken and last app ={
            \draw ([xshift=3.5mm]frame.south west)-|(frame.north east);},
        },
    L/.style={ 
        overlay unbroken and last app ={
            \draw ([xshift=3.5mm]frame.south west)--++(0:11.5mm);},
        },
    Q/.style={ 
        overlay unbroken and last app={
            \node[anchor=east] at (frame.south east) {$\square$};},
    },  
}

\newtcbtheorem[]{tcbthmbox}{Theorem}{thmbox,Boxed}{theo}
\newtcbtheorem[]{tcbSthmbox}{Theorem}{thmbox}{theo}
\newtcbtheorem[]{tcbLthmbox}{Theorem}{thmbox,L}{theo}
\newtcbtheorem[]{proof}{Démonstration}{thmbox,Q}{theo}

\begin{document}

\begin{proof}{Title}{}
\textcolor{red}{A demonstration with title and number}

\lipsum[2]
\end{proof}

\begin{proof}{}{}
\textcolor{red}{A demonstration without title but with number}

\lipsum[3]
\end{proof}

\begin{proof}[theorem name]{}{}
\textcolor{red}{A demonstration without title and number}

\lipsum[3]
\end{proof}

\begin{proof*}{}

\textcolor{red}{Starred version}

\lipsum[3]
\end{proof*}

\end{document}