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\bibitem{Chen_etal2003} {\textquotedblleft}\textit{Optical bistability and multistability in four-level systems}{\textquotedblright}. Chen Z, Feng X, Xu Y, Yu MY, Physica scripta \textbf{68}, 199 (2003).   http://doi.org/10.1238/Physica.Regular.068a00199

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