Special characters in epub not appearing on Kobo Libra 2

[Buckaroo]

Hi, All - I've got an epub of a physics book that contains lots of special characters, most of which are appearing correctly on my Kobo. There are a few, though, that aren't, and I can't figure out what's happening. The characters in question are the "double bar" style, e.g. "ℤ", "ℝ", etc., and show up just fine in both the Calibre edit source window and the reader app, but not on the Kobo, where they show up as blank spaces. I've tried using the Kobo Extended plugin to send kepub, as well as changing the font on the device, but nothing has worked. The characters are apparently UTF-8 format, if that matters.

Any ideas how to fix this?

[DNSB]

That would indicate the font you are using to display the ePub does not include those fonts. If you use kepub format, you can add a supplementary font to try to cover those special cases. Otherwise you will need to find a font that supplies them. A math oriented font would probably be what you are looking for. Noto Sans Math for instance includes quite a few of the double struck glyphs.

[Buckaroo]

Hmmm, that makes sense, except that the epub is already embedding fonts that I would assume should be comprehensively UTF-8 compliant for special characters... in this case, the embedded fonts are "Augusta Cancellaresca Std," "Free Serif," and "Shift Light." I assume those are the fonts being used when it displays correctly on my computer screen. Looks like I'm not understanding something - I'm new to e-book formatting with Calibre.

[DNSB]

calibre is almost useless for checking ebooks for font usage since it will pull from the fonts installed on your computer to display glyphs that are not in the embedded fonts. calibre is also very forgiving of errors in an ebook whereas most devices are not.

Looking at Free Serif, it should have most of the double struck characters but you would need to specify that font for those characters.

[Buckaroo]

Ok, gotcha. I'd bet that's the problem. Thanks!