B.8 Display math should end with $$
You may have a dollar sign ($) in a displayed maths
environment, such as the equation environment.
Remember that $ is short hand for \begin{math} or
\end{math}, so you can't end one of the other environments
with a $. (This error message is in fact a bit confusing,
as it seems to be suggesting that you end a displayed maths
environment with $$ which you also shouldn't do[Why use
\[...\] in place of $$...$$].)
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