ThmDex – An index of mathematical definitions, results, and conjectures.
Result R4267 on D76: Set intersection
Set intersection with empty set is empty
Formulation 0
Let $\emptyset$ be the D13: Empty set.
Let $X_j$ be a D11: Set for each $j \in J$ such that
(i) \begin{equation} \exists \, j \in J : X_j = \emptyset \end{equation}
Then \begin{equation} \bigcap_{j \in J} X_j = \emptyset \end{equation}
Proofs
Proof 0
Let $\emptyset$ be the D13: Empty set.
Let $X_j$ be a D11: Set for each $j \in J$ such that
(i) \begin{equation} \exists \, j \in J : X_j = \emptyset \end{equation}
In search of contradiction, assume that $\bigcap_{j \in J} X_j \neq \emptyset$ and fix $x \in \bigcap_{j \in J} X_j$. By definition of set intersection, we have that $x \in X_j$ for every $j \in J$. By hypothesis, there now exists an index $i \in J$ such that $x \in X_i = \emptyset$. That is, $x \in \emptyset$, which is a contradiction and hence the equality $\bigcap_{j \in J} X_j = \emptyset$ must hold. $\square$