I have:
\documentclass{article}
\usepackage{fontspec}
\usepackage{unicode-math}
\setmainfont{STIX Two Text}[Ligatures=TeX]
\setmathfont{STIX Two Math}
\usepackage{amsmath}
% need a suitable redefinition of ‘\gg’
\begin{document}
$\gg$
\end{document}
with Lualatex but I want to redefine \gg to print a boldface g (such as \symbf{g}). I even asked ChatGPT but it could not find any solution which would work. Is there any? (of course I know that I could use a different command....)
I'm convinced that it's unadvisable to modify substantially the meaning of an existing command. By default, \gg produces ≫ (“much greater than” symbol).
In any case, if you really want to make your document unportable, the redefinition has to be done at begin document, after unicode-math has done its work.
Note that loading fontspec and amsmath is redundant, and Ligatures=TeX is on by default.
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\documentclass{article}
\usepackage{unicode-math}
\setmainfont{STIX Two Text}
\setmathfont{STIX Two Math}
\AtBeginDocument{%
\NewCommandCopy\mg\gg
\renewcommand{\gg}{\symbf{g}}%
}
\begin{document}
$\gg\mg x$
\end{document}
\aa, \ll, \ss and \tt are defined. Redefining \aa and \ss might be troublesome as they're part of the LaTeX Internal Character Representation (LICR).
OK, I adapted a similar solution found here.
\AtBeginDocument{%
\let\mg\gg
\renewcommand\gg{\symbf{g}}%
}
does the trick nicely. Surprised ChatGPT couldn't find it.
Disappointing.
Maurizio
\let with LaTeX (because \let cannot handle, i.e., protected commands correctly and it does not prevent you from unrecognized overwriting of an existing command). Instead use \NewCommandCopy (or if intended \RenewCommandCopy or \DeclareCommandCopy) as shown by @egreg.
In a comment below @egreg's answer, the OP wrote
The reason for which I like
\ggto give me a boldfacegis that I use\vv,\cc,\xxetc. as a shorthand for boldface letters of which I have many.
Rather than having to keep track of a plethora of two-letter macro names -- some of which will, sooner or later, interfere with existing macro names (such as \gg) -- that all accomplish the same thing, I suggest you create a single, specialized macro called, say, \bb, which bold-faces its argument. In a way, \bb is a shortcut to \symbf. Of course, it's up to you if you prefer writing \bb g or \bb{g}.
An advantage of using a single macro (\bb) instead of \vv, \cc, \xx, etc is that if you -- or the journal you're submitting a paper to -- decide that you'd like to generate bold-italic rather than bold-upright symbols, all you'll have to do is to change \newcommand\bb[1]{\symbf{#1}} to \newcommand\bb[1]{\symbfit{#1}}.
% !TEX TS-program = lualatex
\documentclass{article} % or some other suitable document class
\usepackage{unicode-math} % 'unicode-math' loads 'amsmath' and 'fontspec' automatically
\setmainfont{STIX Two Text} % 'Ligatures=TeX' is enabled by default
\setmathfont{STIX Two Math}
\newcommand\bb[1]{\symbf{#1}} % handy shortcut macro
\begin{document}
$\bb g \gg g$
\end{document}