# Remove horizontal space between proof and start of the proof

I am trying to prove some theorem by using proof environment, but somehow the result is different when I use article as my document class and aip-cp document class.

The result when I use article as document class is

The result when I use aip-cp as document class is

Is there a way to use aip-cp document class but when I use proof environment it does not add some horizontal space between the proof and the first text of the proof?

Here is the code

\documentclass{aip-cp}[letter]

\usepackage[numbers]{natbib}
\usepackage{rotating}
\usepackage{graphicx}
\let\iint\relax
\let\iiint\relax
\let\iiiint\relax
\let\idotsint\relax
\let\openbox\relax
\let\proof\relax
\let\endproof\relax
\usepackage{amsmath}
\usepackage{amsthm}

\theoremstyle{lemma}
\newtheorem{lemma}{Lemma}

\setlength{\parindent}{15pt}

\begin{document}

\begin{lemma}
Sample text
\end{lemma}

\begin{proof}
We begin to proof the equation by using the definition of A
\end{proof}

\end{document}


Note: I somehow can reduce the space by using \hspace{-6mm} but it does not look natural.

• Welcome to TeX.Stackexchange! Can you please make a minimal working example (MWE)? – user36296 Sep 19 '18 at 15:49
• Sorry it is my first time here. I have already edited my post. Is that what it means by MWE? – Ben Sep 19 '18 at 15:55

## 2 Answers

The space between "Proof." and the text is controlled by \labelsep You can change this length to your needs:

\documentclass{aip-cp}[letter]

\let\iint\relax
\let\iiint\relax
\let\iiiint\relax
\let\idotsint\relax
\let\openbox\relax

\usepackage{amsmath}
\usepackage{amsthm}

\usepackage{etoolbox}
\AtBeginEnvironment{proof}{
\setlength\labelsep{6pt}
}

\begin{document}

\begin{proof}
We begin to proof the equation by using the definition of A
\end{proof}

\end{document}


The aip-cp class sets a very large value of \labelsep. However, changing the value of this parameter inside the proof environment is not a good solution, because it might make inner lists different from the outer ones.

\documentclass{aip-cp}
\usepackage{showframe}
\usepackage[numbers]{natbib}
\usepackage{rotating}
\usepackage{graphicx}
\let\iint\relax
\let\iiint\relax
\let\iiiint\relax
\let\idotsint\relax
\let\openbox\relax
\let\proof\relax
\let\endproof\relax
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{xpatch}

\makeatletter
\xpatchcmd{\proof}
{\item[\hskip\labelsep\itshape#1\@addpunct{.}]}
{\item[]\textit{#1\@addpunct{.}}\ }
{}{}
\makeatother

\theoremstyle{lemma}
\newtheorem{lemma}{Lemma}

\setlength{\parindent}{15pt}

\begin{document}

\begin{lemma}
Sample text
\end{lemma}

\begin{proof}
We begin to proof the equation by using the definition of A
\begin{itemize}
\item abc
\end{itemize}
and the proof is done.
\end{proof}

\begin{itemize}
\item abc
\end{itemize}

\end{document}