LuaTeX: Map \mathrm\mathring to main font glyph

Question

I'm using LuaTeX via LyX with OpenType fonts and have some trouble setting the Ångström unit in math mode.

Using this order works, but composes the character instead of using the font's glyph:

$\mathring{\mathrm{A}}$

Using this order, which is effectively what is generated by using the units package, fails:

$\mathrm{\mathring{A}}$

The error message is:

Missing character: There is no (U+0017) in font DejaVuSerif:mode=node;...

This isn't technically wrong, but we've already left the TeX math font and should produce glyphs from the main font. "Latin Capital Letter A with Ring Above" is U+00C5, which the font contains and renders just fine in text mode.

What can I do to achieve this? LyX always decomposes the character Å into \mathring{A} in math blocks, so I can't just input that (which would work, I've tested it). I don't need a general solution, this is the only character where it needs to work. It's also the only use of \mathring, so I'd have no qualms redefing the accent to something else...

Thanks!

Answer 1

Use \textnormal{Å}.

\documentclass{article}
\usepackage{amsmath}
\usepackage{units}
\usepackage{unicode-math}

\setmainfont{DejaVu Serif}
\setmathfont{TeX Gyre DejaVu Math}

\newcommand{\angstrom}{\textnormal{Å}}

\begin{document}

$a=\unit[1.4]{\angstrom}$

\end{document}

Of course, nowadays siunitx should be preferred to a package that hasn't been maintained for several years.

\documentclass{article}
\usepackage{amsmath}
\usepackage{siunitx}
\usepackage{unicode-math}

\setmainfont{DejaVu Serif}
\setmathfont{TeX Gyre DejaVu Math}

\DeclareSIUnit\angstrom{\text{Å}} % angstrom is deprecated

\begin{document}

$a=\qty{1.4}{\angstrom}$

\end{document}

Answer 2

Answering my own question. I've been thinking down this line since before asking, but missed that fontspec does its magic AtBeginDocument...

Simply hard-wire \mathring{A} to the character.

\AtBeginDocument{
 \let\@oldmathring\mathring
 \renewcommand{\mathring}[1]{%
  \if\detokenize{A}\detokenize{#1} Å \else \@oldmathring{#1} \fi%
 }
}

This breaks when it isn't used in operator font (cmmi doesn't have nor need the glyph), but that is not a situation I will ever have. U+C5 is always the unit and should never be printed in variable font anyway.