How to change the font of math symbol?

How can I change the font of setR symbol to other? I downloaded STIX font and I’d like to change all math symbols to STIX’s font. But Format - Font changes only non-math symbols (plain text in “quotes”) and keeps setR and -> the same.

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Is there a “reasonable reason”? Shouldn’t mathematical symbols follow simple concepts and be simple in themselves?
As I see it this should imply to prefer printable symbols being obviously very similar to representations easily usable in handwriting.
Is there still somebody worrying about
How to get better scalable bracket? and
tdf#32362 ?

Math formulae are edited in Math. When you return from Math to Writer, formulae are “imported” as pictures as can be seen by the light gray border around it. Consequently, changing styles in Writer has no effect on them. You must make your changes in Math.

The R-set symbol is inserted with reserved word setR.

A naive approach would be to write the formula as font serif setR (more on that later). Unfortunately, this setR symbol is hard-linked to OpenSymbol font. For a font change to be effective, you must drop usage of setR and directly paste the appropriate Unicode character, e.g. U+211D DOUBLE-STRUCK CAPITAL R.

Modified formula is now: font serif { ℝ } but ℝ is considered a variable and italicised. The final formula is then:

nitalic font serif { ℝ }

How do you set Serif to point to your chosen math font?

Format>Fonts shows three Custom fonts with names Serif, Sans-serif and Fixed-width corresponding to keywords serif, sans and fixed in the font command. Set whatever font you please there and use the keyword to change the formatting. Note that the name has no semantic implication and you can use a proportional font under Fixed-width.

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@ajlittoz The font instruction only let you choose from the font faces defined in Format → Font (within a Math object): It accept only three options, serif, sans and fixed. Try

font serif {text}

font sans {text}

font fixed {text}

OK, they can be customised under the Custom Fonts category. It works.

As @ajlittoz pointed out, LibreOffice Math uses OpenSymbol for all its symbols. Bug 101174 ask for a way to select a different font, but in the meantime there is a way to trick Math to use another font: Go to Tools → Options → LibreOffice → Fonts and create a font substitution that always replace OpenSymbol for STIX Two Math.

Be aware, though, that you may get some glitches: Math has its limitations so spacing issues may be a problem with this substitution.

A more complete “solution” is to completely switch from Math to XeLaTeX: with TexMaths extension it’s possible to use all the power of XeTeX in LibreOffice and thus select any mathematical font you want. I wrote an article about that, if you’re interested.

You can also add your specific symbol in “symbols” and use it from there.