Rules / Build

LaTeX Error: Environment theorem undefined — Unknown environment

A \begin{…} names an environment that is not defined (theorem environments must be declared).

Fix

Load the package that defines it, declare it with \newtheorem, or fix the spelling.

Example

Reproduced on TeX Live 2026 by CI · pdflatex · corpus/environment-undefined

theorem environment used without defining it

Fails: main.tex
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{theorem}
Every bounded monotone sequence converges.
\end{theorem}
\end{document}
Fixed
\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}
Every bounded monotone sequence converges.
\end{theorem}
\end{document}
What TeX prints in main.log
! LaTeX Error: Environment theorem undefined.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.4 \begin{theorem}
                   
Your command was ignored.
Type  I <command> <return>  to replace it with another command,
or  <return>  to continue without it.

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