Indent text after line 1

Question

How can I accomplish something like this:

Basically, I want my text to be "indented" after the first line. It would be great if somebody knew an answer. Ideally I would like to do this, because I want my theorems/proofs to look like this:

In my opinion, it makes the whole format much clearer.

Answer 1

Using the enumitem package, you can fairly easily muck around with the dimensions of a list:

\documentclass{article}
\usepackage{enumitem}
\usepackage{lipsum}
\pagestyle{empty}
\begin{document}
\lipsum[1]
\begin{itemize}[label={},itemindent=-2em,leftmargin=2em]
\item \lipsum[1]
\end{itemize}

\end{document}

If you want to do this for every paragraph, that's a bit more involved.

It doesn't appear that amsthm, or even thm-tools, provide the means for easily creating a theorem with a hanging indent. But, theorem-like environments are built up from \trivlist. So, if you know what you want, you can define your own theorem style from a list environment. You'll probably also want to define your own counter to go with it.

To begin to get a theorem-like structure, you can do something like the following:

\newcounter{mytheoremcounter}[chapter]%% to number within chapters
\newenvironment{mytheorem}
    {\refstepcounter{mytheoremcounter}%% so ref/labels work as you expect
     \begin{list}
           {\bfseries\upshape Theorem \arabic{mytheoremcounter}.}
           {\setlength{\labelwidth=2em}
            \em%
          }
          \item 
    }
    {\end{list}}

Per @GonzaloMedina You can get the amsthm package to work for this purpose. You cleverly sneak the formatting through the argument of a \newtheoremstyle for the body font. As in

\documentclass{book}
\usepackage{enumitem}
\usepackage{lipsum}
\pagestyle{empty}
\usepackage{amsthm}
\newtheoremstyle{mythrm}%
    {0pt}{0pt}
    {\hangindent 2.5em}% body font
    {}
    {\bfseries}
    {.}% punctuation
    {0.5em}
    {}
\theoremstyle{mythrm}
\newtheorem{mytheorem}{Theorem}

\setlength{\parindent}{0pt}
\pagestyle{empty}
\begin{document}

\chapter{Hi}

\lipsum[2]

\begin{mytheorem}{}
\lipsum[2]
\end{mytheorem}

A second theorem
\begin{mytheorem}{}
\lipsum[3]
\end{mytheorem}

\chapter{There}


\begin{mytheorem}{}
\lipsum[2]
\end{mytheorem}

A second theorem
\begin{mytheorem}{}
\lipsum[3]
\end{mytheorem}

\end{document}