Theorem environment with hanging indentation, again

Question

I would like to have formatting of theorem environment (and only this environment) with indentation, like this

It is exactly the same question as Theorem environment with hanging indentation

The solutions from there do not want to work well in amsart document class which I am working with.

I have a simple solution

\documentclass{amsart}

\makeatletter
\def\th@fancyindent{
    \itshape
    \addtolength{\leftskip}{2.5em} %indentation here
}
\makeatother

\newtheorem{defi}{Definition}
\theoremstyle{fancyindent}
\newtheorem{thm}{Theorem}

\begin{document}
Text body. Text body. Text body.
\begin{thm}[Pythagoras]
    Let $a,b,c$ the sides of a rectangular triangle.
    Without loss of generality, we assume that  $a<b<c$ .
    Then, the following equality holds:
          \[a^2 + b^2 = c^2\]
\end{thm}
More text. And even more text.
\begin{defi}  
Everything is normal here, no indentation whatsoever. Everything is normal here, no indentation whatsoever. 
\end{defi}
\end{document}

But it is not exactly what I am going for, since it indents the word Theorem too. How can I make my desired indented formatting starting with the Theorem on top with no indent?

Answer 1

A simple modification of https://tex.stackexchange.com/a/106582/4427

\documentclass[a4paper]{scrartcl}
\usepackage{amsthm,thmtools}
\usepackage{etoolbox}
\usepackage{lipsum}
\usepackage{showframe}

\declaretheoremstyle[
  headfont=\kern-2.5em\bfseries,
  bodyfont=\itshape,
  headindent=0pt,
  headpunct=\newline,
]{INDENTthm}
\declaretheoremstyle[
  headfont=\bfseries,
  bodyfont=\itshape,
  headindent=0pt,
  headpunct={},
]{NOINDENTthm}

\declaretheorem[
  within=section,
  style=INDENTthm,
  name=Theorem
]{theorem}

\makeatletter
\AtBeginEnvironment{theorem}{%
  \patchcmd\@thm
    {\trivlist}
    {\list{}{\leftmargin2.5em\itemindent-15em}}
    {}{}%
}
\makeatother

\declaretheorem[
  style=NOINDENTthm,
  name=Definition
]{definition}

\begin{document}

Text body. Text body. Text body.
\begin{theorem}[Pythagoras]
    Let $a,b,c$ the sides of a rectangular triangle.
    Without loss of generality, we assume that  $a<b<c$ .
    Then, the following equality holds:
          \[a^2 + b^2 = c^2\]
\end{theorem}
More text. And even more text.
\begin{definition}
Everything is normal here, no indentation whatsoever. Everything is 
normal here, no indentation whatsoever. 
\end{definition}

\end{document}

Answer 2

I am posting this answer for future reference (because I am not an advanced LaTeX user and did not know of existence of amsthm package before).

This solution uses amsthm package and works nicely in amsart as well as when changing to other standard document class. It adjusts to global rules of indentation and does not break existing theorem styles.

\documentclass{amsart}
\usepackage{amsthm}

\newtheoremstyle{fancyindent}               % name of theoremstyle
{.5\baselineskip±.2\baselineskip}           % Space above, these are standard values for AMS class document
{.5\baselineskip±.2\baselineskip}           % Space below
{\itshape\addtolength{\leftskip}{15mm}\setlength{\parindent}{0em}}      % Body font +indent added +no indent between paragraphs
{-15mm}                                     % Indent of header amount
{\bfseries}                                 % Theorem head font
{\newline}                                  % Punctuation after theorem head and start at new line
{.5em}                                      % Space after theorem head
{\thmname{#1}\thmnumber{ #2}. \thmnote{[#3]}}% Theorem head spec (can be left empty, meaning ‘normal’)

\newtheoremstyle{fancyindentwithindents}    % name of theoremstyle
{.5\baselineskip±.2\baselineskip}           % Space above, these are standard values for AMS class document
{.5\baselineskip±.2\baselineskip}           % Space below
{\itshape\addtolength{\leftskip}{15mm}}     % Body font +indent added
{-15mm}                                     % Indent of header amount
{\bfseries}                                 % Theorem head font
{}                                          % Punctuation after theorem head
{.5em}                                      % Space after theorem head
{\thmname{#1}\thmnumber{ #2}. \thmnote{[#3]}}% Theorem head spec (can be left empty, meaning ‘normal’)

\newtheorem{defi}{Definition}

\theoremstyle{fancyindent}
\newtheorem{thm}{Theorem}

\theoremstyle{fancyindentwithindents}
\newtheorem{thmind}{Theorem}

%\setlength{\parindent}{0em}

Now in the body of document you will produce a following outcome:

Details can be found here: http://www.ams.org/arc/tex/amscls/amsthdoc.pdf

Drawback is that it indeed does not work well with enumerate, to fix this one needs to follow advice here: A theoremstyle with complete indentation using amsthm . For instance the solution of Philippe Goutet works well with indenting enumerate

\makeatletter
\newtheoremstyle{indentedenumerate} % name of theoremstyle
{.5\baselineskip±.2\baselineskip}           % Space above, these are standard values for AMS class document
{.5\baselineskip±.2\baselineskip}           % Space below
{\itshape\addtolength{\@totalleftmargin}{15mm}
   \addtolength{\linewidth}{-15mm}
   \parshape 1 15mm \linewidth}             % Body font +indent added +will work well with enumerate! (you need to add makeatletter)
{-15mm}                                     % Indent of header amount
{\bfseries}                                 % Theorem head font
{}                                          % Punctuation after theorem head
{.5em}                                      % Space after theorem head
{\thmname{#1}\thmnumber{ #2}. \thmnote{[#3]}}% Theorem head spec (can be left empty, meaning ‘normal’)
\makeatother

but in amsart fails to indent equations (sic!).