We prove that the Benjamin-Ono equation is locally well-posed in
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\begin{document}$$H^{1/2}(\mathbb{T})$$\end{document}. This leads to a global well-posedeness result in
\documentclass[12pt]{minimal}
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\setlength{\oddsidemargin}{-69pt}
\begin{document}$$H^{1/2}(\mathbb{T})$$\end{document} thanks to the energy conservation.