I noticed that while \mathit{AHfi} sets the spaces and ligatures correctly (like \operatorname), the letters look narrower than the ones produced by \mathnormal{AHfi}. Is there a way to typeset a multi-letter math operator in the mathnormal font?
[Tex/LaTex] \mathit spacing with \mathnormal font
math-mode
Related Solutions
It's not clear exactly what layout you want, (perhaps a full minimal example and image in your question would have helped).
The main function of \operatorname is to provide multi-letter operators like log etc, if you want a math italic f it seems that you just need one of these, depending on where you want the limits to go.
The other function is to give it operator spacing, ie a small gap before the argument, so perhaps the first or second form is what you are looking for?
$ f_1^2 x $
$ \mathop{f_1^2} x $
$ \mathop{{}f}_1^2 x $
$ \mathop{{}f}\limits_1^2 x $
This is the consequence of a bad decision in unicode-math: they named \mathit the normal math italic letters, instead of respecting the LaTeX convention of referring to them as \mathnormal; \mathit should choose the text italic font, in order to make unicode-math a drop-in replacement.
I wouldn't expect different output when unicode-math is loaded or not; but this simple example shows the bug:
\documentclass{article}
%\usepackage{unicode-math}
\begin{document}
$\mathit{different}$
$different$
\end{document}
If the line with unicode-math is commented out, we get
If I uncomment the line, I get
which is definitely wrong.
Workaround:
Define a new math alphabet:
\documentclass{article}
\usepackage{unicode-math}
\DeclareMathAlphabet{\Lmathit}{\encodingdefault}{\familydefault}{m}{it}
\begin{document}
$\Lmathit{different}$
$different$
\end{document}
If you use lhs2TeX, you can add
\renewcommand{\Conid}[1]{\Lmathit{#1}}
\renewcommand{\Varid}[1]{\Lmathit{#1}}
after loading it.
This shouldn't raise the Too many math alphabets error; if it does, then add the code you find between \makeatletter and \makeatother in https://tex.stackexchange.com/a/100428/4427
Best Answer
No, you should use
\mathitor\mathrm(or higher level constructs like\DeclareMathOperator) for multi-letter identifiers. The math italic (\mathnormal) font is specifically designed so that characters have larger sidebearings to separate them so thatabclooks like an implied product ofatimesbtimescrather than the identifierabc. This is a property of the font itself and not something under the control of TeX.