Weak Upper Closure in Restricted Ordering

Theorem
Let $\struct {S, \preccurlyeq}$ be an ordered set.

Let $T \subseteq S$ be a subset of $S$.

Let $\preccurlyeq \restriction_T$ be the restricted ordering on $T$.

Then for all $t \in T$:


 * $t^{\succcurlyeq T} = T \cap t^{\succcurlyeq S}$

where:
 * $t^{\succcurlyeq T}$ is the weak upper closure of $t$ in $\struct {T, \preccurlyeq \restriction_T}$
 * $t^{\succcurlyeq S}$ is the weak upper closure of $t$ in $\struct {S, \preccurlyeq}$.

Proof
Let $t \in T$.

Suppose that:
 * $t' \in t^{\succcurlyeq T}$

By definition of weak upper closure $t^{\succcurlyeq T}$, this is equivalent to:


 * $t \preccurlyeq \restriction_T t'$

By definition of $\preccurlyeq \restriction_T$, this comes down to:


 * $t \preccurlyeq t' \land t' \in T$

as it is assumed that $t \in T$.

The first conjunct precisely expresses that $t' \in t^{\succcurlyeq S}$.

By definition of set intersection, it also holds that:


 * $t' \in T \cap t^{\succcurlyeq S}$

$t' \in T$ and $t' \in t^{\succcurlyeq S}$.

Thus it follows that the following are equivalent:


 * $t' \in t^{\succcurlyeq T}$
 * $t' \in T \cap t^{\succcurlyeq S}$

and hence the result follows, by definition of set equality.