Equation 3.4.67DDC45C194C3

\begin{equation} \frac{1}{|B|}\int _ Bu(y)\, \mathrm d y=\frac{d}{R^ d}\int _0^ Rr^{d-1}M_ u(x,r)\, \mathrm d r=d\int _0^1t^{d-1}M_ u(x,Rt)\, \mathrm d t, \end{equation}