Overleaf/services/clsi/test/acceptance/fixtures/examples/makeindex/main.tex

\documentclass{article}

\usepackage{makeidx}
\makeindex

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To solve various problems in Physics \index{Physics} it can useful to express any arbitrary piecewise-smooth function as a Fourier Series \index{Fourier Series} composed of multiple sine and cosine funcions.

\printindex

\end{document}