Solution
= Solution
For a reversible transition matrix $P$ on $L^2(\pi)$, the <spectral gap> is
$$
\gamma=1-\max\{\lambda:\lambda<1\text{ is an eigenvalue of }P\}.
$$
Equivalently for a lazy chain,
$$
\gamma=\inf_{\operatorname{Var}_\pi f>0}
\frac{\mathcal E_P(f,f)}{\operatorname{Var}_\pi f}.
$$