Solution (source code)

= Solution

A <probability measure> $\mu$ is $T$-invariant when
$$
\boxed{\mu(T^{-1}A)=\mu(A)\quad\text{for every }A\in\mathcal A}.
$$
Equivalently, the <pushforward measure> satisfies $T_*\mu=\mu$, so $T$ is a <measure-preserving transformation>.