Hanging Indents & troubles with \newenvironment

Question

I'm new to latex, and I've done a lot of searching but can't seem to find a fix for this:

I want a pretty straightforward proof environment, proof followed by a hanging indent.

Like this:

\documentclass{article}
\parindent=0pt
\parskip=\medskipamount
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{amsmath}

\begin{document}
 \paragraph{Proof:}
 \hangindent=4em
 hello there!\\
 testing testing\\
 \begin{flalign*}
 A &= B \\
 C &= D \end{flalign*}
 bla bla bla!\\
\end{document}

Which seems to work nicely.

But when I try to make a \newenvironment out of it, it doesn't seem to work:

\documentclass{article}
\parindent=0pt
\parskip=\medskipamount
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{amsmath}

\newenvironment{pf}
  {\paragraph{Proof:} \hangindent=4em}{}

\begin{document}
\begin{pf}
  hello there!\\
  testing testing\\
  \begin{flalign*}
  A &= B \\
  C &= D \end{flalign*}
  bla bla bla!\\
\end{pf}
\end{document}

The indent is no longer preserved (the last line is off), and I can't figure out why...

In another more convoluted attempt, I've tried \tabu inside of a \tabu:

\documentclass[12pt,parskip=full]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{amsmath, tabu}
\usepackage{graphicx}
\usepackage{tabularx}

\newenvironment{pfa}
  {
    \begin{tabu}{l|p{16cm}}
      \textit{Proof:} & 
      \begin{tabu}{p16cm}
      }
      { \qed
      \end{tabu}
    \end{tabu} 
    }

\begin{document}
\hrulefill

\begin{tabu}{l|p{16cm}}
  Proof: & \begin{tabu}{p{16cm}}
            hello there!\\
            testing testing\\
            {\begin{align}
               A &= B \\
               C &= D \end{align}}
           \qed
           \end{tabu}
\end{tabu}

\hrulefill 

\begin{pfa}
      hello there!\\
      {\begin{align}
               A &= B \\
               C &= D \end{align}}
\end{pfa}

\end{document}

Once more, the manually written version works, but the \newevironment fails and throws errors which I can't seem to fix.

Answer 1

Using a table for the proof is surely wrong, because this will not break across pages. Also \hangindent will not work properly with lists (enumerate or itemize) in the proof.

You can make an easy modification to the proof environment provided by amsthm to use \list instead of \trivlist, thus allowing to set the left margin.

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

% the following two are just for the example
\usepackage[a4paper,margin=3cm]{geometry}
\usepackage{lipsum}

\makeatletter
\renewenvironment{proof}[1][\proofname]
 {\par
  \pushQED{\qed}%
  \normalfont
  \topsep6\p@\@plus6\p@\relax
  \list{}{%
    \settowidth\leftmargin{\textbf{#1:}}%
    \addtolength\leftmargin{\labelsep}%
  }%
  \item[\hskip-\leftmargin\bfseries #1:]\ignorespaces
 }
 {%
  \popQED\endlist\@endpefalse
 }
\makeatother

\begin{document}

\lipsum[2]

\begin{proof}
\lipsum*[3]
\begin{align*}
 A &= B \\
 C &= D
\end{align*}
\lipsum*[3]
\end{proof}

\begin{proof}
\lipsum*[3]
\begin{align*}
 A &= B \\
 C &= D \qedhere
\end{align*}
\end{proof}

\end{document}

Answer 2

You simply can use the ntheorem package, which defines a \theoremindent length. I patched the nonumberplain theorem style to adapt it to this indention. You can use an optional argument to the environment

Another advantage is that you can have an end-of-proof symbol (that you can choose as you want), which will be automatically placed, even if the proof ends in a displayed equation.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.2pt}

\parindent=0pt
\parskip=\medskipamount
%\usepackage{amsthm}
\usepackage{amssymb, amsfonts}
\usepackage{amsmath}

\usepackage[thmmarks,  thref, amsmath]{ntheorem}
\makeatletter
\newtheoremstyle{proof}%
  {\item[\theorem@headerfont\hskip-\dimexpr\labelsep + 0.15em\relax\rlap{##1\theorem@separator}]}%
  {\item[\theorem@headerfont\hskip-\dimexpr\labelsep + 0.15em\relax  ##1\ (##3)\theorem@separator]}
 \makeatother
\theoremindent=4em
\theoremstyle{proof}
\theoremheaderfont{\bfseries}
\theorembodyfont{\normalfont}
\theoremseparator{:}
\theoremsymbol{\ensuremath{\square}}
\newtheorem{proof}{\llap{Proof}}

\begin{document}

 \begin{proof}
 hello there!\\
 testing testing
 \begin{flalign*}
 A &= B \\
 C &= D
 \end{flalign*}
  \end{proof}

 \begin{proof}[of some theorem]
 hello there!\\
 testing testing
 \begin{flalign*}
 A &= B \\
 C &= D
 \end{flalign*}
 Blah blah blah
  \end{proof}

\end{document}