Definition:Monotone Class Generated by Collection of Subsets

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Let $X$ be a set.

Let $\mathcal G \subseteq \mathcal P \left({X}\right)$ be a collection of subsets of $X$.

Then the monotone class generated by $\mathcal G$, $\mathfrak m \left({\mathcal G}\right)$, is the smallest monotone class on $X$ that contains $\mathcal G$.

That is, $\mathfrak m \left({\mathcal G}\right)$ is subject to:

$(1):\quad \mathcal G \subseteq \mathfrak m \left({\mathcal G}\right)$
$(2):\quad \mathcal G \subseteq \mathcal M \implies \mathfrak m \left({\mathcal G}\right) \subseteq \mathcal M$ for any monotone class $\mathcal M$ on $X$

In fact, $\mathfrak m \left({\mathcal G}\right)$ always exists, and is unique, as proved on Existence and Uniqueness of Monotone Class Generated by Collection of Subsets.


One says that $\mathcal G$ is a generator for $\mathfrak m \left({\mathcal G}\right)$.