# How to invoke \phi only from newtxmath package?

A minimal working example is as follows:

\documentclass{amsart}
\begin{document}
\Huge

$$\phi$$

\end{document}


Question: How to invoke \phi only from newtxmath package? Note that I don't like to invoke any other letters from newtxmath. Any help is welcome!

There you go.

\documentclass{amsart}
\DeclareSymbolFont{ntxletters}{OML}{ntxmi}{m}{it}
\SetSymbolFont{ntxletters}{bold}{OML}{ntxmi}{b}{it}
\DeclareMathSymbol{\phi}{\mathord}{ntxletters}{30}
\usepackage{bm}
\begin{document}
\Huge

$\phi, \bm{\phi}$

\end{document}


Thanks to @marmot pointing out this previous answer by @AlanMunn. As @AlanMunn said in that answer, one needs some basic knowledge about font declarations before performing the trick. This key line of code in newtxmath.sty:

%\DeclareMathSymbol{\phi}{\mathord}{letters}{30}


tells us that we should be looking for lines that declare letters. Indeed, we find

%%%%%%%%%%%%%%%
% Math Italic %
%%%%%%%%%%%%%%%
... ... ...
\DeclareSymbolFont{letters}{OML}{ntxmi}{m}{it}
\SetSymbolFont{letters}{bold}{OML}{ntxmi}{b}{it}
... ... ...


Thus, the suggested solution.

• It's important to find ntxmi! Although I know the method, but I didn't find ntxmi! – Mogic Oct 2 '18 at 11:37