I found an answer of \mathit Greek letter: https://tex.stackexchange.com/a/87239/180617

However I found \mathit Greek letter will disappear when loading fontspec package, even if no command like \setmainfont

Here is my mwe:

% compile with xelatex
$a +\Sigma + \pi$

$\mathit{a +\Sigma + \pi}$

will output

enter image description here

If I comment \usepackage{fontspec}, it will output

enter image description here

with both xelatex and pdflatex

Why will this happen?

  • 2
    note that \mathit is the text italic font used in math, so the input \mathit{a +\Sigma + \pi} is rather unexpected. Oct 15 '21 at 15:50

The log will show

Missing character: There is no ^^F (U+0006) in font [lmroman10-italic]:mapping=

as fontspec sets up \mathit to expect Unicode encoding but \Sigma (unless you load unicode-math) will expect to find Sigma in the legacy 7-bit TeX encoding position.

You can use


so that fontspec leaves the math setup untouched, or you could use unicode-math and \symit{\Sigma} to use Unicode fonts throughout.

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