Finding the first appearance of citation

Question

Is it possible to define a LaTeX command that takes as input a citation key and gives as output the section of the article, in which this key appeared the first time (inside \cite{}), i.e. where this given reference was first referred to? (not using bibtex or biblatex)

Answer 1

I'm not sure about your use case, but here a starting point for you with package backref. At least, it prints the number of section(s) where each bib.-item was cited after the item itself.

\documentclass[a4paper]{article}

\usepackage[ref]{backref}
\renewcommand*{\backref}[1]{}
\renewcommand*\backrefalt[4]{%
  \ifcase #1 %
    No citations.%
  \or
    One citation in section #2.%
  \else
    #1 citations in sections #2.%
  \fi
}

\begin{document}

\section{First Section}
\cite{Rei91}

\section{Second Section}
\cite{CA}, \cite{MSW00}, \cite{Rei91}

\section{Third Section}
\cite{MSW00}, \cite{Rei91}

\section{Fourth Section}
\cite{CA}, \cite{MSW00}, \cite{Rei91}

\begin{thebibliography}{99}
\bibitem{HB98} Huynen, M.~A. and Bork, P. 1998. Measuring genome
  evolution. {\itshape Proceedings of the National Academy of Sciences
    USA} 95:5849--5856.

\bibitem{CA} Caprara, A. 1997. Sorting by reversals is difficult. In:
  {\itshape Proceedings of the First Annual International Conference
    on Computational Molecular Biology (RECOMB 97),} New York: ACM.
  pp. 75--83.

\bibitem{MSW00} McLysaght, A., Seoighe, C. and Wolfe, K.~H. 2000. High
  frequency of inversions during eukaryote gene order evolution.  In
  Sankoff, D. and Nadeau, J.~H., editors, {\itshape Comparative
    Genomics}, Dordrecht, NL: Kluwer Academic Press. pp. 47--58.

\bibitem{Rei91} Reinelt, G. 1991. {\itshape The Traveling Salesman -
    Computational Solutions for TSP Applications.} Berlin: Springer
  Verlag.

\end{thebibliography}

\end{document}

Answer 2

A colleague had a solution for the problem:

\documentclass[a4]{article}
\makeatletter
\renewcommand\@bibitem[1]{\item\if@filesw\immediate\write\@auxout
               {\string\bibcite{#1}{\theenumiv}}\fi\ignorespaces}

\renewenvironment{thebibliography}[1]
             {\section*{\refname}%  %% use \bibname instead of
                                 %  %% \refname if using the
                                 %  %% report or book document classes
               \@mkboth{\MakeUppercase\refname}{\MakeUppercase\refname}%
               \list{\@biblabel{\the\c@section.\@arabic\c@enumiv}}% 
               % %% ^^ added \the\c@section
                    {\settowidth\labelwidth{\@biblabel{#1}}%
                      \leftmargin\labelwidth
                      \advance\leftmargin\labelsep
                      \@openbib@code
                      \usecounter{enumiv}%
                      \let\p@enumiv\@empty`
                      \renewcommand\theenumiv{\the\c@section.\@arabic\c@enumiv}}% 
                    % %% ^^ added \the\c@section
                    \sloppy
                    \clubpenalty4000
                    \@clubpenalty \clubpenalty
                    \widowpenalty4000%
                    \sfcode`\.\@m}
             {\def\@noitemerr
               {\@latex@warning{Empty `thebibliography' environment}}%
               \endlist}
             \makeatother

\begin{document}

\section{Foo}

Blah blah blah~\cite{cite1}, and see later~\cite{cite2}.


\begin{thebibliography}{99}
\bibitem{cite1} Foo bar. 
\end{thebibliography}

\section{Bar}

Bleh bleh bleh~\cite{cite2}. And also~\cite{cite1}.

\begin{thebibliography}{99}
\bibitem{cite2} Baz bof.
\end{thebibliography}

\end{document}