Trying to write a single vertical bar in mathematical programming latex template https://media.springernature.com/full/springer-cms/rest/v1/content/18782940/data/Download+the+journal+article+template+package
\documentclass[sn-mathphys]{sn-jnl}% Math and Physical Sciences Reference Style
\begin{document}
It thus suffices to prove that $ | $.
\end{document}
