[Tex/LaTex] colon after Appendix name

appendicespunctuation

I am trying to solve a simple problem but have been unable to find a answer on this forum. My question is — how can I put a colon after the appendix name? I feel that it should be pretty simple but the answers I found on this forum mostly relate to listing the appendix with a colon in ToC; I want that colon after the appendix name in the main body (I have no ToC).

Here's my MWE: 

\documentclass[11pt]{article}
\setlength{\pdfpagewidth}{8.5in} \setlength{\pdfpageheight}{11in}
\usepackage[title]{appendix}

\begin{document}
\section{Introduction}

Here is some text ... 

\begin{appendices}
\section{A simple model} \label{app:model}

We present a simple model ...

\end{appendices}

\end{document}

Best Answer

You can easily do that with the \apptocmd command, from etoolbox:

\documentclass[11pt]{article}
\setlength{\pdfpagewidth}{8.5in} \setlength{\pdfpageheight}{11in}
\usepackage[title]{appendix}

\usepackage{etoolbox}
\apptocmd{\appendices}{\apptocmd{\thesection}{: }{}{}}{}{}

\begin{document}
\section{Introduction}

Here is some text ...

\begin{appendices}
\section{A simple model} \label{app:model}

We present a simple model ...

\end{appendices}

\end{document}

enter image description here

Added: To avoid side effects, you either redefine the appendices environment to add a colon in the right place – a bit long in my opinion. Or give up the title option of appendix and add it yourself, with etoolbox and titlesec:

\documentclass[11pt]{article}
\setlength{\pdfpagewidth}{8.5in} \setlength{\pdfpageheight}{11in}
\usepackage{appendix}
\usepackage{titlesec}
 \usepackage{etoolbox, chngcntr}
\AtBeginEnvironment{appendices}{%
 \titleformat{\section}{\bfseries\Large}{\appendixname~\thesection:}{0.5em}{}%
 \titleformat{\subsection}{\bfseries\large}{\thesubsection}{0.5em}{}%
\counterwithin{equation}{section}
}

\begin{document}

\section{Introduction}

Here is some text ...

\begin{appendices}%
\section{A simple model} \label{app:model}
We present a simple model ...
\begin{equation}
    a = b
\end{equation}
\subsection{A still simpler model}
Blah blah
\end{appendices}

\end{document}

enter image description here

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