\documentclass{article}
\usepackage{tipa}
\begin{document}
\begin{equation}
R_P(\tau)=\frac{1}{T}\int_{-\infty}^{+\infty}P_{T}(t)P_{T}(t+\tau)\,dt=\textturnv_T(\tau)
\end{equation}
\end{document}