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?