Problem: embedding in a element

Hello,
I'm writing a math-book with SIGIL and I'm using mathml.
So far all is fine.
My problem:
if I embed a <math>-Tag in <p> and </p>, then I get an error when uploading my ebook to BoD. It says that it is not allowed to do this. Leaving out the <p> </p> tag returns the error that <p> </p> must be present. Any suggestions?

Sample code:
<p><math ><mstyle displaystyle="true"><mrow class="MJX-TeXAtom-ORD"><msub><mi>δ<!-- δ --></mi>

<mrow class="MJX-TeXAtom-ORD"><mi>i</mi>

<mi>j</mi></mrow></msub></mrow></mstyle></math>
</p>

I hope someone has a good idea.
Thanks,
Hans