Definition
σ
\sigma
-field generated by a map
If
X
∈
ℬ
/
ℬ
′
X \in \mathcal{B}/\mathcal{B}'
then we define
σ
(
X
)
=
X
−
1
(
ℬ
′
)
\sigma(X) = X^{-1}(\mathcal{B}')
.