Chapter with specific number (1, 1b, 2, 2b, 3, 4)

Question

Currently I use a manually declared version of the chapter title. But the idea is to extend it to use specific and manually chosen number of the chapters.

I try to obtain numbers like this 1, 1b, 2, 2b, 3, 4. But for obtaining b, I want to specify it manually with for example an option in the \chapter command (something like this \chapter{...} for obtaining 1 then \chapter[b]{...} for obtaining 1b.

Do you have any ideas for obtaining this result?

Answer 1

You could add a + specifier for the “repeat chapters”; the proposed code doesn't interfere with usages of \chapter*.

\documentclass{book}

\makeatletter
\newcounter{repchapter}
\let\latex@chapter\chapter
\renewcommand{\chapter}{\@ifstar{\latex@chapter*}{\guuk@chapter}}
\newcommand{\guuk@chapter}{\@ifnextchar+{\repeat@chapter}{\restore@chapter\latex@chapter}}
\newcommand{\repeat@chapter}[1]{% #1 is +
  \addtocounter{chapter}{-1}%
  \addtocounter{repchapter}{1}%
  \gdef\thechapter{\arabic{chapter}\alph{repchapter}}%
  \latex@chapter
}
\newcommand{\restore@chapter}{%
  \setcounter{repchapter}{1}%
  \gdef\thechapter{\arabic{chapter}}%
}
\makeatother

\begin{document}

\tableofcontents

\chapter{Title}
x
\chapter+{Repeat}
x
\chapter{Title}
x
\chapter+{Repeat}
x
\chapter+{Again}
x
\end{document}

If you load hyperref, then add

\renewcommand{\theHchapter}{\thechapter}

just after \makeatletter. Make sure \usepackage{hyperref} is before the shown code.