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I just got a mail from a person who told me that - while reading with a Kobo H2O an ebook I wrote - he could not see the character aleph in "ℵ0", and moreover the reader would not get past that page. (The ebook reads fine with the Firefox plugin, and with other ebook readers). I may understand that the Kobo does not contain the character ℵ, but I am puzzled that it blocks the flow.
Besides converting the character to a picture, does anybody have an idea about what to do in such cases?
AFAIK, Kobos have two reading engines: ADE/RMSDK for ePub2 books and NetFront ACCESS for ePub3 (kepub) books.
In addition to checking your book with the IDPF Online Validator, you might want to test your book with an older ADE version, e.g. ADE 2.0.1. If it displays OK with ADE, the reader probably uses an older or incompatible reading app.
If it doesn't look OK with ADE, you'll need to embed a font with math symbols and/or convert your book to an ePub3 book and upload the (kepub) ePub3 version to the Kobo bookstore.
The ebook was checked with Sigil. Note that the Sigil FlightCrew validator occasionally misses problems that epubcheck will flag. I think I'll end up embedding a suitable font. I'd recommend the STIX fonts. (You can subset them with Calibre to keep the file size down.)