Definition 7.1.1 (Ideal).label Let $X$ be a set and $\sigma \subset 2^{X}$, then $\sigma$ is an ideal over $X$ if:
- (I1)
For any $E \in \sigma$ and $F \subset E$, $F \in \sigma$.
- (I2)
For any $E, F \in \sigma$, $E \cup F \in \sigma$.
Definition 7.1.1 (Ideal).label Let $X$ be a set and $\sigma \subset 2^{X}$, then $\sigma$ is an ideal over $X$ if:
For any $E \in \sigma$ and $F \subset E$, $F \in \sigma$.
For any $E, F \in \sigma$, $E \cup F \in \sigma$.
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