Package natbib Warning: Citation `a9' on page 5 un...

That warning is La TeX’s way of saying, “I can’t find a reference with the key a9.”
Here’s what it’s telling you and how to clear it up:

---

1  Why the warning appears

• You’ve written \cite{a9} (line 453), but there is no BibTeX/\bibitem entry whose key is exactly a9 when La TeX builds the document.
• During the first La TeX pass the citation is marked as “undefined,” so natbib sends the warning. If it still can’t find the key after the BibTeX run and a second La TeX pass, the warning remains and the citation prints as “??” in the PDF.

2  How to fix it

Situation	What to do
You meant to cite an existing source but mistyped the key	Correct the key in \cite{…} so it matches the entry in your .bib file or your manual \bibitem.
The source isn’t in your bibliography yet	Add a BibTeX entry (or \bibitem) for it, using a9 as the key. Example for BibTeX:

bibtex
@inproceedings{a9,
  author    = {First A. Author and Second B. Author},
  title     = {Title of the work},
  booktitle = {Proceedings …},
  year      = {2024},
  pages     = {1--10}
}
``` |
| You no longer need that citation | Delete or comment out the `\cite{a9}` command. |

After fixing one of the above, run:

1. **latex/pdflatex**  
2. **bibtex** (or biber)  
3. **latex/pdflatex** twice more  

to resolve all cross‑references.

### 3  Camera‑ready formatting reminders (per the note you quoted)

- **References section** comes *after* acknowledgments.  
- Use an **unnumbered first‑level heading**:  
  ```latex
  \section*{References}

• Any consistent citation style is fine; natbib plus BibTeX is common.
• You may set the reference list to \small (9 pt) to save space:

  latex
  \begingroup
  \small
  \bibliography{yourbibfile}
  \endgroup

• The reference section does not count toward the page limit.

Quick checklist

1. Is there a @…{a9, …} entry in your .bib?
2. Does the key in \cite{…} match exactly (case‑sensitive)?
3. Have you rerun La TeX → BibTeX → La TeX → La TeX after the change?

Once the key is defined and the document is recompiled in the proper sequence, the natbib warning will disappear and your citation will show up correctly.

下面用中文说明如何让 natbib 把引用显示成数字序号（如 [1]、[2]…）。

---

1 载入 natbib 时切换成数字模式

latex
\usepackage[numbers]{natbib}            % 仅数字
% 或者
\usepackage[numbers,sort&compress]{natbib} % 数字并按大小排序、连号压缩

要点
‑ 如果省略了 numbers，natbib 默认用作者‑年份格式。
‑ sort&compress 可把 [3,2,5,4] 自动排成 [2‑5]。

---

2 选择支持数字的参考文献样式

你已经用 ieeenat_fullname，它本身就是数字样式的 BibTeX 文件；也可以用
plainnat、unsrtnat、abbrvnat、IEEEtranN 等。示例：

latex
\bibliographystyle{ieeenat_fullname}
\bibliography{main}   % .bib 文件名

---

3 正常写 \cite{键} 就会显示数字

latex
……这是正文中的一句话\cite{a9}。

编译顺序保持 LaTeX → BibTeX → LaTeX → LaTeX，PDF 里就会出现 [1] 形式。

---

4 不要再手写 [1] …

你在代码里手动列了

latex
[1] Alexander, J.A. ...

如果继续用 BibTeX + natbib，请把这段删掉，改用标准环境：

latex
\begingroup
\small        % 9pt 字号
\bibliography{main}
\endgroup

BibTeX 会自动把 .bib 里的条目排成 [1]、[2]、[3]…。

---

5 常见问题排查

现象	解决办法
PDF 中显示 “??”	.bib 里没有对应 key；或忘记跑 BibTeX；或 key 拼写大小写不一致。
仍然是作者‑年份	① 确认 \usepackage[numbers]{natbib}；② 检查是否有别的包（如 apalike）覆盖了样式。
想在文中用 “[作者, 年份]” 但参考文献仍用数字	不可同时；natbib 要么全数字，要么全作者‑年份。

---

最小工作示例

latex
\documentclass{article}
\usepackage[numbers,sort&compress]{natbib}
\begin{document}

这是一次引用\cite{a9}。

\section*{References}
\begingroup
\small
\bibliographystyle{ieeenat_fullname}
\bibliography{main}
\endgroup

\end{document}

确保 main.bib 里有 @…{a9, …} 条目并按前述顺序编译，即可得到数字序号引用。