Solution
= Solution
The <total variation distance> is
$$
\boxed{d_{\mathrm{TV}}(\mu,\nu)
=\sup_{B\in\mathcal B_X}|\mu(B)-\nu(B)|}.
$$
If both measures have densities $p,q$ with respect to a common <dominating measure>, then $d_{\mathrm{TV}}(\mu,\nu)=\frac12\int|p-q|$.