LaTeX warning · Warnings: references, floats and boxes

LaTeX Warning: Citation `vaswani2017' on page 1 undefined

The warning in the log

This is how the message appears in the .log file (and in the compile output). The line starting with l. is the line number where TeX noticed the problem; the real mistake is there or just before it.

LaTeX Warning: Citation `vaswani2017' on page 1 undefined on input line 3.

LaTeX Warning: There were undefined references.

Why it happens

• The key in \cite does not match the key in the .bib file or \bibitem exactly (keys are case-sensitive): vaswani2017 versus vaswani2017attention.
• BibTeX or Biber has not been run, or failed: look in the .blg file for "I couldn't open database file" or syntax errors in the entry.
• The bibliography is never printed: \bibliography{refs} or \printbibliography is missing, or the .bib file name is wrong.
• Mixing systems: \addbibresource (biblatex) with BibTeX, or \bibliographystyle together with biblatex.

How to fix it

Copy the key from the warning and search for it in your bibliography. If it is not there, add the entry or correct the key in \cite. If it is there, compile the full sequence (LaTeX, BibTeX or Biber, LaTeX twice) and read the .blg log for problems with the .bib file.

The example below uses thebibliography so it is self-contained; with a .bib file the fix is the same: the key in \cite must match the key on the first line of the entry.

Broken: reproduces the warning

\documentclass{article}
\begin{document}
Transformers were introduced by Vaswani et al.~\cite{vaswani2017}.
\begin{thebibliography}{9}
\bibitem{vaswani2017attention} A. Vaswani et al.
  Attention is all you need. NeurIPS, 2017.
\end{thebibliography}
\end{document}

Fixed: compiles cleanly

\documentclass{article}
\begin{document}
Transformers were introduced by Vaswani et al.~\cite{vaswani2017attention}.
\begin{thebibliography}{9}
\bibitem{vaswani2017attention} A. Vaswani et al.
  Attention is all you need. NeurIPS, 2017.
\end{thebibliography}
\end{document}

Related errors

• Reference undefined – A \ref or \eqref has no matching \label, so the PDF shows ??.
• File not found – A package or class named in \usepackage or \documentclass is misspelled or not installed.
• Too many unprocessed floats – Figures or tables pile up behind one that LaTeX cannot place, until it runs out of float slots.
• All LaTeX errors and warnings – The full list, grouped by cause

Frequently Asked Questions

Why does my citation show as [?] in the PDF?

[?] is how LaTeX prints a citation it cannot resolve. Either the key does not match any bibliography entry, or BibTeX/Biber has not run since you added it. Fix the key, then compile LaTeX, BibTeX or Biber, and LaTeX twice.

How do I run BibTeX in the right order?

pdflatex main, bibtex main (or biber main for biblatex), pdflatex main, pdflatex main. latexmk -pdf main does this automatically, and online editors like Octree run it on every compile.

The key is correct but the citation is still undefined. Why?

Usually the .bib file is not being read: check the file name in \bibliography{} or \addbibresource{} (biblatex needs the .bib extension), and open the .blg file, which reports missing files and broken entries such as an unescaped & or a missing comma.

What does "There were undefined references" mean?

It is the summary at the end of the run. It appears whenever at least one \cite or \ref could not be resolved; scroll up for the individual "Citation ... undefined" or "Reference ... undefined" warnings.