Definition ((n-$\mu$)-small)

A set $A\in\mathcal{B}(\mathbb{X})$ is ($n$-$\mu$)-small on $(\mathbb{X},\mathcal{B}(\mathbb{X}))$ if for some positive measure $\mu$ and $n\in\mathbb{N}$ $$P^{n}(x,B)\ge\mu(B), \ \ \forall x\in A\text{ and }B\in\mathcal{B}(\mathbb{X})$$