Show only chapter number (and not name) as chapter heading?

Question

The chapters in the book I'm working on are all named. However, while I want the names to show up in the TOC, I want the numbers and the numbers only to show up as the chapter headings. (I know that sounds weird, and I'll be happy to explain why if it matters.)

So, for example, the name of chapter I is "De Puero Superstite." I want "De Puero Superstite" to show up in the TOC, but only the number I to show up as the chapter heading. (I've already got it set to Roman numerals.)

Is there a way to do this? If I need to I can use titlesec to redefine the title format, leave the numbers out entirely, and do eg \chapter[De Puero Superstite]{I}, but there's got to be a more elegant way to do it.

Any thoughts?

EDIT: By "Chapter Heading" I mean not the page header—the thing that appears at, for example, the very top of every left page (or every right page, or every page) but the thing that appears once before the text of the chapter begins.

Answer 1

It depends on whether you want the ToC entry to appear in the page headers or not. For the ToC entry and page header entries to be the same.

% chapternameprob.tex  SE 544473 no name in division header
%\documentclass{memoir}
\documentclass{book}
\usepackage{lipsum}
\renewcommand{\chaptername}{} % eliminate Chapter in divisional heading
\begin{document}
\tableofcontents
\chapter[toc and header entry]{} % empty divisional title
\lipsum
\end{document}

For them to be different use the memoir class:

% chapternameprob.tex  SE 544473 no name in division header
\documentclass{memoir}
%\documentclass{book}
\usepackage{lipsum}
\renewcommand{\chaptername}{}
\begin{document}
\tableofcontents
\chapter[toc entry][header entry]{}
\lipsum
\end{document}

The memoir class enables you to have different entries in the ToC, the page header, and the main document for all the divisional commands (\chapter ... \subparagraph)

Answer 2

Try this.

\documentclass{book}
\usepackage{titlesec}
\usepackage{xpatch} % for \xpatchcmd
\usepackage{lipsum} % for \lipsum to produce dummy texts 

\makeatletter
% hide chapter title (e.g., "first title")
\xpatchcmd\ttl@mkchap
  {\ttl@mkchap@i{#1}{#2}{#3}{#4}{#5}{#6}{#7}}
  {\ttl@mkchap@i{#1}{#2}{#3}{#4}{#5}{#6}{}}
  {}{\fail}
\makeatother

% hide \chaptername (the word "Chapter" followed by a space)
% based on `texdoc titlesec`, sec. 9.2
\titleformat{\chapter}[display]
  {\normalfont\huge\bfseries}
  % before: {\chaptername\ \thechapter}
  {\thechapter}
  {20pt}
  {\Huge}

\renewcommand\thechapter{\Roman{chapter}}

\begin{document}
\chapter{first title}
\lipsum

\chapter{second title}
\lipsum
\end{document}