Finite intersection property

Let $X$ be a set and $\mathcal{A}\subseteq \mathcal{P}(X)$ a family of subsets of $X$. A finite list in $\mathcal{A}$ is a function $s:n\to \mathcal{A}$ for some $n\in \mathbb{N}$, where $n=\{0,1,\dots ,n-1\}$ is its own set of predecessors in the von Neumann encoding (The natural numbers $\mathbb{N}$ (von Neumann)). Its intersection is the subset of $X$

${\bigcap }_{i\in n}s(i):=\{x\in X:x\in s(i)\text{ for every }i\in n\},$

a definition by Separation alone, with no recursion involved. For $n=0$, that is $n=\mathrm{\varnothing }$, the condition is vacuous and the empty intersection is $X$.

The family $\mathcal{A}$ has the finite intersection property, abbreviated FIP, when

${\bigcap }_{i\in n}s(i)\mathrm{\ne }\mathrm{\varnothing }\text{for every }n\in \mathbb{N}\text{ and every finite list }s:n\to \mathcal{A}\mathrm{.}$

Equivalently: no finitely many members of $\mathcal{A}$ have empty intersection.

• The empty intersection is $X$, and it is included above, so a family with the FIP forces $X\mathrm{\ne }\mathrm{\varnothing }$ (take $n=0$). Many texts state the condition only for $n\ge 1$ and add "$X\mathrm{\ne }\mathrm{\varnothing }$" or "$\mathcal{A}\mathrm{\ne }\mathrm{\varnothing }$" as a separate standing hypothesis. The two readings agree except when $\mathcal{A}=\mathrm{\varnothing }$, where the $n\ge 1$ reading is vacuous and this one still asks that $X$ be nonempty. Including $n=0$ is what makes A family lies in a filter exactly when it has the finite intersection property hold with no side condition, since the empty intersection is exactly the member $X$ that every filter must contain.
• Lists may repeat, and this is harmless: repeating a member does not change an intersection, so quantifying over lists is the same as quantifying over finite subfamilies. Lists are used rather than "finite subfamilies" because a function out of a natural number is available immediately from The natural numbers $\mathbb{N}$ (von Neumann), whereas a general theory of finite sets is not needed anywhere here.
• The FIP is exactly the condition for a family to sit inside some filter (A family lies in a filter exactly when it has the finite intersection property): the finite intersections of $\mathcal{A}$ form a filter base (Filter base and the filter it generates) precisely when none of them is empty, and conversely every family inside a filter inherits the property from properness (Filter on a set).
• The property is about finite intersections only. The family $\{\{n\in \mathbb{N}:N\le n\}:N\in \mathbb{N}\}$ of tails of $\mathbb{N}$ has the FIP, since the intersection of finitely many tails is the one with the largest starting index and no tail is empty, yet the intersection of all of them is empty, because no $n$ lies in the tail starting at $\sigma (n)$. That gap between finite and infinite intersections is the whole reason filters are worth having.