In equation \eqref{eq:sample}, we find the value of aninteresting integral:
$$
\require{ams}
\begin{equation}
\int_0^\infty \frac{x^3}{e^x-1}\,dx = \frac{\pi^4}{15}
\label{eq:sample}
\end{equation}
$$
fixed tag $(2)$
$$
\frac{\partial{f}}{\partial{g}} \equiv \left.\frac{\partial{f(\exp(\delta)\cdot g)\cdot f^{-1}(g)}}{\partial{\delta}}\right|_{\delta=0}\tag{2}
$$