FatalMath Mode
Double Subscript
! Double subscript.
What this error means
Two consecutive subscript operators _ were applied to the same base without grouping. LaTeX doesn't allow ambiguous stacked subscripts.
Common causes
- Writing x_i_j instead of x_{ij} or x_{i_j} for nested subscripts
- Accidentally adding a second _ after a command that already has a subscript
How to fix it
Use braces {} to group subscripts. For nested subscripts like x with index i,j write x_{ij} or x_{i_j}. For a subscript that itself has a subscript, wrap the inner expression in braces.
Code examples
Causes the error
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[ x_i_j = 0 \]
\end{document}Fixed
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[ x_{i_j} = 0 \]
\end{document}Related errors
Test the fix live
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