Up-Complete Product/Lemma 1

Theorem
Let $X$ be a directed subset of $S$.

Let $Y$ be a directed subset of $T$.

Then $X \times Y$ is also a directed subset of $S \times T$.

Proof
Let $\left({s_1, t_1}\right)$, $\left({s_2, t_2}\right) \in X \times Y$.

By definition of Cartesian product:
 * $s_1, s_2 \in X$ and $t_1, t_2 \in Y$

By definition of directed subset:
 * $\exists h_1 \in X: s_1 \preceq_1 h_1 \land s_2 \preceq_1 h_1$

and
 * $\exists h_2 \in X: t_1 \preceq_2 h_2 \land t_2 \preceq_2 h_2$

By definition of Cartesian product of ordered sets:
 * $\exists \left({h_1, h_2}\right) \in X \times Y: \left({s_1, t_1}\right) \preceq \left({h_1, h_2}\right) \land \left({s_2, t_2}\right) \preceq \left({h_1, h_2}\right)$

Thus by definition:
 * $X \times Y$ is a directed subset of $S \times T$.