Theorem environment with numbers on the left

Question

I am trying to construct a theorem environment which roughly looks like this:

1.1   Theorem   Statement of the theorem. Here is some extra text so you can see how the
      theorem body text should be indented (aligned with theorem head). 

The theorem head should be bold, with no punctuation after. The spacing between thmnumber and thmname should be adjustable. Body text normal.

I want to be able to use the environment for definitions as well (which won't be numbered), so it should be possible to remove the number without affecting the alignment of the theorem head. Example:

      Definition   Statement of the definition. It should be aligned the same way as a 
      theorem. 

Here is some body text in the document. Note that the theorem numbers are not in the
margin. 

1.1   Theorem   Statement of the theorem. Here is some extra text so you can see how the
      theorem body text should be indented (aligned with theorem head). 

Answer 1

Here's a way to do it; this will also comply with lists such as enumerate in the statement, which solutions with \hangindent won't.

\documentclass{article}
\usepackage{showframe} % just for the example
\usepackage{amsthm}
\usepackage{enumitem}
\usepackage{xparse}

\usepackage{lipsum}

\newtheoremstyle{fctaylor}% name
  {\topsep}%      Space above
  {\topsep}%      Space below
  {\normalfont}%         Body font
  {}%         Indent amount (empty = no indent, \parindent = para indent)
  {\bfseries}% Thm head font
  {}%        Punctuation after thm head
  {0pt}%     Space after thm head: " " = normal interword space;
  {\makethmhead{#1}{#2}{#3}}

\newlength\fctaylortheoremindent
\AtBeginDocument{\setlength\fctaylortheoremindent{3em}} % <- customize here
\newlength\fctaylorlabelsep
\AtBeginDocument{\setlength\fctaylorlabelsep{1em}} % <- customize here

\makeatletter
\newcommand{\makethmhead}[3]{%
  \gdef\thisthmhead{%
    \makebox[\fctaylortheoremindent][l]{\bfseries#2}%
    {\bfseries#1}%
    \@ifnotempty{#3}{ (#3)}%
    \hspace{\fctaylorlabelsep}%
  }%
}
\makeatother

\newenvironment{fctayloritemize}
 {\list{}{%
    \leftmargin=\fctaylortheoremindent
    \labelwidth=\dimexpr\fctaylortheoremindent-\labelsep\relax
    \itemindent=0pt
  }}
 {\endlist}

\NewDocumentCommand{\newfctaylortheorem}{smomo}{%
  \IfBooleanTF{#1}
   {\newtheorem*{fctaylor@#2}{#4}}
   {\IfNoValueTF{#3}
     {\IfNoValueTF{#5}
       {\newtheorem{fctaylor@#2}{#4}}
       {\newtheorem{fctaylor@#2}{#4}[#5]}}
     {\newtheorem{fctaylor@#2}[fctaylor@#3]{#4}}}%
  \NewDocumentEnvironment{#2}{o}
   {\IfNoValueTF{##1}{\begin{fctaylor@#2}}{\begin{fctaylor@#2}[##1]}%
    \begin{fctayloritemize}\item[\thisthmhead\hfill]}
   {\end{fctayloritemize}\end{fctaylor@#2}}%
}

\theoremstyle{fctaylor}
\newfctaylortheorem{thm}{Theorem}[section]
\newfctaylortheorem*{defn}{Definition}

\begin{document}
\section{One}

\begin{defn}
\lipsum*[2]
\end{defn}

\begin{thm}\label{A}
\lipsum*[2]
\end{thm}

\begin{thm}[Somebody]\label{B}
Something that should show how the text is split across line boundaries
and is correctly indented. And some equivalent conditions:
\begin{enumerate}[label=\upshape(\alph*),ref=(\alph*)]
\item a condition
\item another
\item and another
\end{enumerate}
which show the point made.
\end{thm}

\ref{A} and \ref{B}

\end{document}

Answer 2

Here's a possibility using thmtools as a front-end for amsthm:

The code (adjust the settings according to your needs; in particular, change \thmindent to get the desired hanging indentation for the theorem-like structures):

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{thmtools}
\usepackage{lipsum}

\newlength\thmindent
\setlength\thmindent{2.5em}

\declaretheoremstyle[
  spaceabove=6pt, 
  spacebelow=6pt,
  headfont=\normalfont\bfseries,
  notefont=\normalfont\bfseries, 
  notebraces={(}{)},
  bodyfont=\normalfont,
  postheadspace=0.5em,
  numberwithin=section,
  headformat={\makebox[\thmindent]{\NUMBER\hfill}\NAME\NOTE},
  postheadhook=\hangindent=\thmindent
]{nuswapindented}
\declaretheoremstyle[
  spaceabove=6pt, 
  spacebelow=6pt,
  headfont=\normalfont\bfseries,
  notefont=\normalfont\bfseries, 
  notebraces={(}{)},
  bodyfont=\normalfont,
  postheadspace=0.5em,
  numbered=no,
  headformat={\makebox[\thmindent]{\mbox{}\hfill}\NAME\NOTE},
  postheadhook=\hangindent=\thmindent
]{unnuswapindented}
\declaretheorem[style=nuswapindented,name=Theorem]{theo}
\declaretheorem[style=unnuswapindented,name=Definition]{defi}

\begin{document}

\section{Test section}

\lipsum[4]
\begin{defi}
\lipsum[4]
\end{defi}
\lipsum[4]
\begin{theo}
\lipsum[4]
\end{theo}

\end{document}

Answer 3

It can be done with the ntheorem package and its change theoremstyle. The spacing between theorem name and theoremnumber can be adjusted patching the change theorem style: I introduce a new dimension, thlabelsep which defaults to 0.5em and can changed in the preamble:

\documentclass{article}
\usepackage[utf8]{inputenc}

 \usepackage{amsmath}
\usepackage[thmmarks, amsmath, thref]{ntheorem}
\usepackage{cleveref}

\newdimen\thlabelsep
\global\thlabelsep0.5em
\makeatletter
\renewtheoremstyle{change}%
{\item[\hskip\labelsep \theorem@headerfont ##2\hskip\thlabelsep##1\theorem@separator]}%
{\item[\hskip\labelsep \theorem@headerfont ##2\hskip\thlabelsep##1\ (##3)\theorem@separator]}
\makeatother
\theoremstyle{change}
\thlabelsep1.5em
\theoremheaderfont{\bfseries\upshape}
\theoremseparator{}
\theorembodyfont{\upshape}
\newtheorem{thm}{Theorem}[section]

\theoremstyle{nonumberplain}
\newtheorem{defn}{Definition}
\begin{document}

\section{Two Conjectures}

\begin{defn}
  A Sophie Germain prime is a prime number $ p $ such that $ 2p + 1$ is also prime.
\end{defn}

\begin{thm}
There is an infinity of twin primes. Related: There is an infinity of Sophie Germain primes. 
\end{thm}

\end{document}