# Cartesian Product of Intersections

## Theorem

$\paren {S_1 \cap S_2} \times \paren {T_1 \cap T_2} = \paren {S_1 \times T_1} \cap \paren {S_2 \times T_2}$

where $S_1, S_2, T_1, T_2$ are sets.

### Corollary 1

$A \times \paren {B \cap C} = \paren {A \times B} \cap \paren {A \times C}$

### Corollary 2

$\paren {A \times B} \cap \paren {B \times A} = \paren {A \cap B} \times \paren {A \cap B}$

### General Case

$\displaystyle \left({ \prod_{i \mathop = 1}^n S_i }\right) \cap \left({ \prod_{i \mathop = 1}^n T_i }\right) = \prod_{i \mathop = 1}^n \left({S_i \cap T_i}\right)$

## Proof

 $\displaystyle$  $\displaystyle \tuple {x, y} \in \paren {S_1 \cap S_2} \times \paren {T_1 \cap T_2}$ $\displaystyle$ $\leadstoandfrom$ $\displaystyle \paren {x \in S_1 \land x \in S_2} \land \paren {y \in T_1 \land y \in T_2}$ Definition of Cartesian Product and Definition of Set Intersection $\displaystyle$ $\leadstoandfrom$ $\displaystyle \paren {x \in S_1 \land y \in T_1} \land \paren {x \in S_2 \land y \in T_2}$ Rule of Commutation, Rule of Association $\displaystyle$ $\leadstoandfrom$ $\displaystyle \tuple {x, y} \in S_1 \times T_1 \land \tuple {x, y} \in S_2 \times T_2$ Definition of Cartesian Product $\displaystyle$ $\leadstoandfrom$ $\displaystyle \tuple {x, y} \in \paren {S_1 \times T_1} \cap \paren {S_2 \times T_2}$ Definition of Set Intersection

$\blacksquare$