Thank you, KevinH, for your answer. BoD ist "book on Demand", a publisher for books and e-books.
Yes, <p> may contain <math>, that' what SIGIL allows.
There is only one thing I didn't try: previously, my ebook was epub-2, than I inserted mathml. Should create a epub-3 with Sigil and copy my html to this, then I'll have another try.
Thank's a lot,
Hans