the value of the \(\mathbf{R}_\mathbf{G}\) Carlson elliptic integral: \(\frac1{4\pi} \int_{0}^{2\pi}\int_{0}^{\pi}
\scriptstyle\sqrt{x\sin^2\theta\cos^2\phi
+y\sin^2\theta\sin^2\phi
+z\cos^2\theta} \scriptstyle\;\mathrm{d}\theta\;\mathrm{d}\phi\) is returned: