# Set Intersection is Self-Distributive/Families of Sets

## Theorem

Let $I$ be an indexing set.

Let $\family {A_\alpha}_{\alpha \mathop \in I}$ and $\family {B_\alpha}_{\alpha \mathop \in I}$ be indexed families of subsets of a set $S$.

Then:

$\ds \map {\bigcap_{\alpha \mathop \in I} } {A_\alpha \cap B_\alpha} = \paren {\bigcap_{\alpha \mathop \in I} A_\alpha} \cap \paren {\bigcap_{\alpha \mathop \in I} B_\alpha}$

where $\ds \bigcap_{\alpha \mathop \in I} A_i$ denotes the intersection of $\family {A_\alpha}$.

## Proof

 $\ds x$ $\in$ $\ds \map {\bigcap_{\alpha \mathop \in I} } {A_\alpha \cap B_\alpha}$ $\ds \leadsto \ \$ $\ds \forall \alpha \in I: \,$ $\ds x$ $\in$ $\ds A_\alpha \cap B_\alpha$ Definition of Intersection of Family $\ds \leadsto \ \$ $\ds \forall \alpha \in I: \,$ $\ds x$ $\in$ $\ds A_\alpha$ Definition of Set Intersection $\, \ds \land \,$ $\ds x$ $\in$ $\ds B_\alpha$ $\ds \leadsto \ \$ $\ds x$ $\in$ $\ds \bigcap_{\alpha \mathop \in I} A_\alpha$ Definition of Intersection of Family $\, \ds \land \,$ $\ds x$ $\in$ $\ds \bigcap_{\alpha \mathop \in I} B_\alpha$ Definition of Intersection of Family $\ds \leadsto \ \$ $\ds x$ $\in$ $\ds \paren {\bigcap_{\alpha \mathop \in I} A_\alpha} \cap \paren {\bigcap_{\alpha \mathop \in I} B_\alpha}$ Definition of Set Intersection

Thus by definition of subset:

$\ds \map {\bigcap_{\alpha \mathop \in I} } {A_\alpha \cap B_\alpha} \subseteq \paren {\bigcap_{\alpha \mathop \in I} A_\alpha} \cap \paren {\bigcap_{\alpha \mathop \in I} B_\alpha}$

$\Box$

 $\ds x$ $\in$ $\ds \paren {\bigcap_{\alpha \mathop \in I} A_\alpha} \cap \paren {\bigcap_{\alpha \mathop \in I} B_\alpha}$ $\ds \leadsto \ \$ $\ds x$ $\in$ $\ds \bigcap_{\alpha \mathop \in I} A_\alpha$ Definition of Set Intersection $\, \ds \land \,$ $\ds x$ $\in$ $\ds \bigcap_{\alpha \mathop \in I} B_\alpha$ $\ds \leadsto \ \$ $\ds \forall \alpha \in I: \,$ $\ds x$ $\in$ $\ds A_\alpha$ Definition of Intersection of Family $\, \ds \land \,$ $\ds x$ $\in$ $\ds B_\alpha$ $\ds \leadsto \ \$ $\ds \forall \alpha \in I: \,$ $\ds x$ $\in$ $\ds A_\alpha \cap B_\alpha$ Definition of Set Intersection $\ds \leadsto \ \$ $\ds x$ $\in$ $\ds \map {\bigcap_{\alpha \mathop \in I} } {A_\alpha \cap B_\alpha}$ Definition of Intersection of Family

Thus by definition of subset:

$\ds \paren {\bigcap_{\alpha \mathop \in I} A_\alpha} \cap \paren {\bigcap_{\alpha \mathop \in I} B_\alpha} \subseteq \map {\bigcap_{\alpha \mathop \in I} } {A_\alpha \cap B_\alpha}$

$\Box$

By definition of set equality:

$\ds \map {\bigcap_{\alpha \mathop \in I} } {A_\alpha \cap B_\alpha} = \paren {\bigcap_{\alpha \mathop \in I} A_\alpha} \cap \paren {\bigcap_{\alpha \mathop \in I} B_\alpha}$

$\blacksquare$