Double subscript
Description
Two consecutive subscript characters _ were applied to the same math symbol without grouping braces.
Remediation
Group the subscripts with curly braces: $x_{a_b}$ or $x_{a,b}$.
[!NOTE] If this error occurs during
\printbibliographyor\bibliography, consecutive unescaped underscores (e.g.journal = {NORDIC_J_COMP}) exist inside a.bibdatabase field. See Troubleshooting Bibliography Errors for automated diagnosis withlatex_it.