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\end{equation}  Our postulated result. Amazingly enough it is correct! (when verified elsewhere). We can try another result, potentially as \begin{equation}  \int_0^\infty \frac{x^3}{e^x-1} dx = \frac{\pi^4}{15}  \end{equation}  We could differentiate the integrand and times by x, then flip the sign and gain the result \begin{equation}  \int_0^\infty \frac{e^x(x-3)+3)x^3}{e^x-1)^2 dx = \frac{\pi^4}{15}  \end{equation}