Definition:Filter on Set

Definition
Let $X$ be a set, and $\mathcal P \left({X}\right)$ be the power set of $X$.

A filter on $X$ (or filter of $X$) is a set $\mathcal F \subset \mathcal P \left({X}\right)$ which satisfies the following conditions:


 * $X \in \mathcal F$


 * $\varnothing \notin \mathcal F$


 * $U, V \in \mathcal F \implies U \cap V \in \mathcal F$


 * $\forall U \in \mathcal F: U \subseteq V \subseteq X \implies V \in \mathcal F$

Filtered Set
The set $X$ on which the filter has been applied is called a set filtered by $\mathcal F$, or just filtered set.

Trivial Filter
A filter $\mathcal F$ on a set $X$ as defined above specifically does not include the empty set $\varnothing$.

If a filter $\mathcal F$ were to include $\varnothing$, then from Empty Set Subset of All it would follow that every subset of $X$ would have to be in $\mathcal F$, and so $\mathcal F = \mathcal P \left({X}\right)$.

Such a filter is called the trivial filter on $X$.

Finite Intersection
It follows directly by Principle of Mathematical Induction from:
 * $U, V \in \mathcal F \implies U \cap V \in \mathcal F$

that the intersection of any finite number of sets of $\mathcal F$ is also an element of $\mathcal F$.

Some treatments of this subject start with this as an axiom.

Also see

 * See Filter on a Set is a Proper Filter.