# How to change a single character font after writing long document?

Using classicthesis. How to change a single math caracter, say J? I know how to do it by just by adding an extra symbol:

\DeclareSymbolFont{CMletters}{OML}{cmm}{m}{it}
\DeclareMathSymbol{\J}{\mathord}{CMletters}{"4B}


I'd like to obtain J permanently leaving the rest of the alphabet untouched, though. Of course

  $\J$


produces the desired result and it wouldn't matter to begin a document writing an extra slash for each math-J. But I already wrote it and I'd like to change the font to \mathlord-J. In my document I would like to add some type of preamble that allows me to change all the

 $J$


in math mode, so that they appear with the mentioned font. Is that possible?

• To be honest, i am not sure what the current situation is and what you are trying to achieve. Can you provide a minimal test case that shows the current state? – Johannes_B Mar 24 '15 at 10:04
• It should be \DeclareMathSymbol{J}{\mathalpha}{CMletters}{J} – egreg Mar 24 '15 at 10:09

The correct way would be

\DeclareMathSymbol{J}{\mathalpha}{CMletters}{J}


but I'm not really sure you want it.

The main ingredient is that classicthesis loads mathpazo, so we can show a minimal example as follows:

\documentclass{article}
\usepackage{mathpazo}

\mathchardef\oldJ=\mathcodeJ % just to show the difference

\DeclareSymbolFont{CMletters}{OML}{cmm}{m}{it}
\DeclareMathSymbol{J}{\mathalpha}{CMletters}{J}

\begin{document}
$A+J=I$ is with the J from CM

$A+\oldJ=C$ is with the J from mathpazo
\end{document}


Needless to say, the “old” version is much better.