How to make the citation beside the equation

Question

I want to make the citation beside the equation at the same line contrary to this image, has any one any idea about that?

\documentclass[twocolumn]{revtex4-1}% http://ctan.org/pkg/revtex4-1
\vspace{500px}
\addtocontents{toc}{\vspace{1em}}  % Add a gap in the Contents, 
\begin{document}
where $\rho_{ij}$ is the stoichiometric coefficient of species $S_i$ which 
reacting in the system.
From the deterministic reaction rate $K_j$, the reaction rate $C_j$ can be 
obtained. For more details about calculating $C_j$ 
\cite{wolkenhauer04}\cite{gillespie07}.\\
 Assuming that the reaction has the form
\ce{S_1+4S_2 ->[c_j] P1+ P2} , $a_j(X)$ deduced as next: 
\[
a_j(X)=c_j.\binom{X_1}{1}.\binom{X_2}{4} = (c_1 \centerdot x_1).(\frac{1}
{4}).X_2.(X_2-1).(X_2-2).(X_2-3) 
\]~\cite{herajy13}.
\end{document}

Answer 1

There is nothing wrong with calling \cite within a displayed equation. If, however, you want the citation flushed right, you may abuse the \tag* macro from amsmath

\documentclass{article}

\usepackage{amsmath}

\begin{document}

Consider
\[
a=b \tag*{from \cite{foo}}
\]
or
\begin{equation}
c=d \qquad \text{(see \cite{foo})}
\end{equation}

\begin{thebibliography}{9}
\bibitem{foo} Foo, by Me Myself.
\end{thebibliography}

\end{document}

IMNSHO the first form should be discouraged, as it may easily be mistaken for an equation number. At the very least some word (like "see", "cf." or "from" as in my example above) should be used.

If you prefer the citation on the left, you could use the solution proposed in this answer.