Yang, Y., and F.J. Sigworth. The Journal of General Physiology. Volume 112, No. 6, December 1998. 665–678.
Page 668
\begin{equation*}\hspace{1em}{\sigma}^{2}={\int _{0}^{{\infty}}}S \left (f) \right{\mid} H \left (f) \right{\mid} ^{2}d\hspace{.167em}f\end{equation*}
and
\begin{equation*}i_{s}(f)=\bar {I}^{-1}{\int _{0}^{f}}S(f^{\prime})df^{\prime}\end{equation*}
1999
