Fancyhdr and \thesection command

Question

I have a problem with \thechapter implementation in fancyhdr package. As you can see in the photo I attached at the bottom of the post, the section number (RO) and its title (LO) don't match at the header: it should say "Propietats bàsiques de la divisibilitat" and then "1.3", or "Equacions diofantines lineals" and then "1.4". If I had to choose, I would prefer the latter (I think it's more complex to achieve, though). My pagestyle{fancy} config is the following:

---

% Header
\pagestyle{fancy}
\renewcommand{\headrulewidth}{0.1pt}
\fancyhead[RE]{\nouppercase{\leftmark}}
\fancyhead[LO]{\nouppercase{\rightmark}}
\fancyhead[RO]{\thesection}
\fancyhead[LE]{\thechapter}
\fancyfoot[RO, LE]{\thepage}
\fancyfoot[CO, CE]{}
\fancyheadoffset[LE,RO]{+0.025\textwidth}
\renewcommand{\chaptermark}[1]{\markboth{ #1}{}}
\renewcommand{\sectionmark}[1]{\markright{ #1}}

---

If it can be of any help, I am working on a book, twoside environment.

\documentclass[a4paper,12pt,twoside]{book}

Thanks in advance!

Answer 1

As I stated in my comment, the section number \thesection must be communicated through a mark (for \thechapter this isn't necessary as chapters always start at a new page). But normal LaTeX doesn't have an extra mark for this section number, so I use the extramarks package with the \extramarks command and \firstrightxmark and \firstleftxmark.

\documentclass{book}

\usepackage{fancyhdr}
\usepackage{extramarks}
\usepackage{lipsum}

% Header
\pagestyle{fancy}
\renewcommand{\headrulewidth}{0.1pt}
\fancyhead[RE]{\nouppercase{\leftmark}}
\fancyhead[LO]{\nouppercase{\firstrightxmark}}
\fancyhead[RO]{\firstleftxmark}
\fancyhead[LE]{\thechapter}
\fancyfoot[RO, LE]{\thepage}
\fancyfoot[CO, CE]{}
\fancyheadoffset[LE,RO]{+0.025\textwidth}
\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\renewcommand{\sectionmark}[1]{\extramarks{\thesection}{#1}}

\begin{document}
\chapter{First Chapter}

\lipsum

\newpage

\section{First section}

\lipsum[1]

\section{Second section}

\lipsum[2]

\end{document}

Answer 2

A workaround which redefines \sectionmark to use only the left header of odd pages:

\renewcommand{\sectionmark}[1]{\markright{#1\hfill \thesection}}

MWE

\documentclass[a4paper,12pt,twoside]{book}
\usepackage{lipsum}
\usepackage{fancyhdr}
  \pagestyle{fancy}
  \renewcommand{\headrulewidth}{0.1pt}
  \renewcommand{\chaptermark}[1]{\markboth{\thechapter\hfill #1}{}}
  \renewcommand{\sectionmark}[1]{\markright{#1\hfill \thesection}}
  \fancyhf{}
  \fancyhead[LE]{\leftmark}
  \fancyhead[LO]{\rightmark}
  \fancyfoot[RO, LE]{\thepage}
  \fancyfoot[CO, CE]{}
  \fancyheadoffset[LE,RO]{+0.025\textwidth}

\begin{document}
 \chapter{Chapter title}
  \lipsum[1-8]
 \section{First section}
  \lipsum[1]
 \section{Second section}
  \lipsum 
\end{document}