Mapping Preserves Finite and Filtered Infima

Theorem
Let $\left({S_1, \preceq_1}\right)$, $\left({S_2, \preceq_2}\right)$ be meet semilattices.

Let $f: S_1 \to S_2$ be a mapping.

Let $f$ preserve finite infima and preserve filtered infima.

Then $f$ also preserves all infima

Proof
Assume that
 * $(1): \quad f$ preserves finite infima

and
 * $(2): \quad f$ preserves filtered infima.

Let $X$ be a subset of $S_1$.

Let $X$ admits an infimum in $\left({S_1, \preceq_1}\right)$

Define $Z := \left\{ {\inf A: A \in {\it Fin}\left({X}\right) \land A \ne \varnothing}\right\}$

where
 * $\inf A$ denotes the infimum of $A$ in $\left({S_1, \preceq_1}\right)$
 * ${\it Fin}\left({X}\right)$ denotes the set of all finite subsets of $X$

By Existence of Non-Empty Finite Infima in Meet Semilattice:
 * for every non-empty finite subset $A$ of $X$, $A$ admits an infimum in $\left({S_1, \preceq_1}\right)$

By Infimum of Infima:
 * $\inf Z = \inf\bigcup \left({ {\it Fin}\left({X}\right) \setminus \left\{ {\varnothing}\right\} }\right) = \inf X$
 * $Z$ admits an infimum in $\left({S_1, \preceq_1}\right)$

We will prove that
 * $Z$ is filtered

Let $x, y \in Z$.

By definition of $Z$:
 * $\exists A \in {\it Fin}\left({X}\right) \setminus \left\{ {\varnothing}\right\}: x = \inf A$

and
 * $\exists B \in {\it Fin}\left({X}\right) \setminus \left\{ {\varnothing}\right\}: y = \inf B$

By Finite Union of Finite Sets is Finite:
 * $A \cup B$ is finite
 * $A \cup B \ne \varnothing$

By definition of $Z$:
 * $\inf\left({A \cup B}\right) \in Z$

By Infimum of Infima:
 * $\left({\inf A}\right) \wedge \left({\inf B}\right) = \inf\left({A \cup B}\right)$

Thus by definition of infimum:
 * $\inf\left({A \cup B}\right) \preceq x \land \inf\left({A \cup B}\right) \preceq y$

Thus by definition
 * $Z$ is filtered

By $(2)$ and definitions of mapping preserves filtered infima and mapping preserves infimum on subset:
 * $f^\to\left({Z}\right)$ admits an infimum in $\left({S_2, \preceq_2}\right)$ and $\inf\left({f^\to\left({Z}\right)}\right) = f\left({\inf Z}\right)$

We will prove that
 * $X \subseteq Z$

Let $x \in X$.

By definition of ${\it Fin}\left({X}\right)$:
 * $\left\{ {x}\right\} \in {\it Fin}\left({X}\right)$ and $\left\{ {x}\right\} \ne \varnothing$

By definition of $Z$:
 * $\inf \left\{ {x}\right\} \in Z$

Thus by Infimum of Singleton:
 * $x \in Z$

By Image of Subset under Relation is Subset of Image/Corollary 2:
 * $f^\to\left({X}\right) \subseteq f^\to\left({Z}\right)$

By definition of infimum:
 * $\inf\left({f^\to\left({Z}\right)}\right)$ is a lower bound of $f^\to\left({Z}\right)$

By Lower Bound is Lower Bound for Subset
 * $\inf\left({f^\to\left({Z}\right)}\right)$ is a lower bound of $f^\to\left({X}\right)$