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\begin{document}
 \maketitle
 \section{ Prima }
 $$\int_a^b f(x) dx$$
 $$\Delta f(x)=0$$
  \subsection{ Seconda}
  $$\frac {\partial^2 }  {\partial x^2} f $$
  \begin{equation}\label{wave}
    u_{tt}=\Delta u
     \end{equation}
     \subsubsection{terza}
La formula \eqref{wave}....
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