Unnumbered part with "Part" header

Question

I am writing a thesis, whose outline is a bit treelike, so I would like to have a trunk part (that contains several chapters) and several branches (also containing some chapters).

I changed the name of the parts using

\renewcommand\partname{Trunk}
\part{Polish groups}

and then

\renewcommand\partname{Branch}
\part{The first branch},

this works fine.

However, since there is only one trunk, I would like the trunk part not to be numbered Trunk 1. Also, I would like the branches to start being numbered from Branch 1 (and not Branch 2).

But when I write

\part*{Polish groups},

only the title "Polish groups" appears on the page, with no indication "Trunk" like it would do with "Part 1".

Do you know how to do this? Thanks.

Edit. Here is a (hopefully) compilable code:

 \documentclass[12pt]{amsbook}

    \usepackage[margin=2cm]{geometry}
    \usepackage[english]{babel}
    \usepackage{amsmath}
    \usepackage{amsthm}
    \usepackage{amssymb}
    \usepackage{enumerate}
    \usepackage{ucs}
    \usepackage[utf8]{inputenc}
    \usepackage[T1]{fontenc}
    \usepackage{color}
    \usepackage{ stmaryrd }
    \usepackage{hyperref}
    \usepackage[retainorgcmds]{IEEEtrantools}
    \usepackage{tikz}
    \usepackage[initials, shortalphabetic]{amsrefs}
    \usepackage{footnote}

      \title{Thèse}
    \author{}

    \begin{document}

    \tableofcontents


    \renewcommand\partname{Trunk}
    \part{Polish groups}

    \renewcommand\partname{Branch}
        \part{First branch}

\part{Second branch}

    \end{document}

Answer 1

From line 867 to 897 of amsbook.cls class is where the \partname, \thepart and \part are defined. Correspond to the problem that the number is not needed for Trunk, we can redefine the \thepart to be empty. And to the problem that the next part must number from one, we just reset the counter part to 0.

\documentclass{amsbook}
\begin{document}

    \renewcommand\thepart{}
    \renewcommand\partname{Trunk}
    \part{Polish groups}

    \setcounter{part}{0}
    \renewcommand\thepart{\arabic{part}}
    \renewcommand\partname{Branch}
    \part{First branch}

    \part{Second branch}

\end{document}

Answer 2

Special care must be taken, of course, in order to produce the correct output also in the table of contents. The \@part command, as defined in the amsbook document class, issues an \addcontentsline command as follows:

\addcontentsline{toc}{part}{\partname\ \thepart. \protect\enspace\protect\noindent#1}%

As you can see, a period is added after \thepart; thus, redefining the latter command to produce no output is not enough: a patch to the above code in \@part would also be necessary, whose details may depend on the document class.

But you can also make up for this problem with a quick trick, which is illustrated by the following MWE:

\documentclass[a4paper]{amsbook}
\usepackage[T1]{fontenc}
\usepackage{lipsum}

\begin{document}

\tableofcontents

\begingroup
    \renewcommand*\partname{}
    \renewcommand*\thepart{\unskip Trunk}
    \part{Polish groups}
\endgroup
\setcounter{part}{0}
\chapter{Aaaaaa}
\lipsum[1]
\chapter{Bbbbbb}
\lipsum[2]

\renewcommand*\partname{Branch}
\part{First branch}
\chapter{Cccccc}
\lipsum[3-4]
\chapter{Dddddd}
\lipsum[5-6]

\part{Second branch}
\chapter{Eeeeee}
\lipsum[7]
\chapter{Ffffff}
\lipsum[8]

\end{document}

This alternative is less tricky than it might seem at first sight: after all, you could actually think of the word “trunk” as the “number” of the first part, but a “number” that, being a noun in itself, and not a numeral adjective, does not require another noun on which it depends. Note that \unskip is necessary to remove the preceding space. The .toc file will contain the line

\contentsline {part}{\ \unskip Trunk. \enspace \noindent Polish groups}{3}

which shows that the spacing will look right in the table of contents too.