Missing $ inserted

TeX Output

./example.tex:3: Missing $ inserted.
l.3 Variable x_
               1 is not in math mode.

Explanation

TeX encountered a character or macro that is only valid inside math mode while operating in horizontal (text) mode. Common culprits include:

1. Subscripts (_) or superscripts (^) written outside $ ... $ (e.g. file_name.txt or 10^5).
2. Greek letters or math symbols like \alpha, \sum, \infty in regular prose.
3. Math operators (\to, \times, \le) without surrounding $ delimiters.

How to Fix

• If mathematical: wrap the expression in inline math delimiters $ ... $:

  Variable $x_1$ is in math mode.
  

• If an underscore is intended in text: escape it with \_ (or wrap file paths in \texttt{...} / \url{...}):

  The file name is \texttt{file\_name.txt}.
  

[!NOTE] If this error occurs during \printbibliography or \bibliography, the unescaped underscore or missing dollar sign is inside a .bib database field. See Troubleshooting Bibliography Errors for automated diagnosis with latex_it.