# [Tex/LaTex] Customize \chaptermark for \mainmatter

Hey guys it's me again,

I tried to add \leftmark to my header, but I want to change it from Chapter 1. Introduction to 1 Introduction. My MWE:

\documentclass[12pt, oneside, a4paper]{book}
\usepackage{titlesec}
\titleformat{\chapter}{\huge\bfseries}{\thechapter}{20pt}{\huge}
\usepackage{fancyhdr}
\fancypagestyle{plain}
{
\fancyhf{}
\fancyfoot[R]{\thepage}
\renewcommand{\footrulewidth}{0pt}
}

\begin{document}
\frontmatter
\tableofcontents
\mainmatter
\chapter{Introduction}
\chapter{Blablabla}
\end{document}


I googled a lot a found \renewcommand{\chaptermark}[1]{\markboth{#1}{}} and other variations and inserted it at different places of the preamble, but it doesn't work.

Am I missing something important again? Guess so, but I couldn't point it out.

\chaptermark can be redefined the following way:

\makeatletter
\if@twoside
\renewcommand*{\chaptermark}[1]{%
\markboth{%
\ifnum\value{secnumdepth}>-1 %
\thechapter\ %
\fi
#1%
}{}%
}%
\else
\renewcommand*{\chaptermark}[1]{%
\markright{%
\ifnum\value{secnumdepth}>-1 %
\if@mainmatter
\thechapter\ %
\fi
\fi
#1%
}%
\fi
}
\makeatother


CONTENTS
1 Introduction
2 Blablabla


The upper casing of CONTENTS can be removed by the redefinition of \tableofcontents:

\usepackage{etoolbox}
\patchcmd{\tableofcontents}{\MakeUppercase\contentsname}{\contentsname}{}{}
\patchcmd{\tableofcontents}{\MakeUppercase\contentsname}{\contentsname}{}{}


That removed \MakeUppercase from the definition of \tableofcontents (twice).