$$latex
\documentclass{article}
\begin{document}
$
f(x) = \int_{-\infty}^\infty \hat f(\xi),e^{2 \pi i \xi x} , d\xi
$
\end{document}
$$
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From @test2391191
$$latex
\documentclass{article}
\begin{document}
$
f(x) = \int_{-\infty}^\infty \hat f(\xi),e^{2 \pi i \xi x} , d\xi
$
\end{document}
$$