Solution
= Solution
The contraction in the PDF is $I_{iijj}$. By <Einstein notation>, its integrand is $(\sum_i x_i^2)(\sum_jx_j^2)=r^4$. In <spherical coordinates>,
$$
\boxed{I_{iijj}=\int_Br^4\,dV=4\pi\int_0^1r^6\,dr=\frac{4\pi}{7}.}
$$
= Solution
The contraction in the PDF is $I_{iijj}$. By <Einstein notation>, its integrand is $(\sum_i x_i^2)(\sum_jx_j^2)=r^4$. In <spherical coordinates>,
$$
\boxed{I_{iijj}=\int_Br^4\,dV=4\pi\int_0^1r^6\,dr=\frac{4\pi}{7}.}
$$