Order-Extension Principle/Strict/Proof 2

== Theorem ==
=== Finite Set ===
== Proof ==
For the purposes of this proof, a relation $<_U$ on a subset $U$ of $S$ will be considered compatible with $\prec$ :
:$\forall a, b \in U: a \prec b \implies a < b$
Let $M$ be the set of partial mappings $f$ from $S \times S$ to $\left\{ {0, 1}\right\}$ such that for all $x, y, z \in S$:
:$(a): \quad \left({x, y}\right), \left({y, z}\right), \left({x, z}\right) \in \operatorname{Dom} \left({f}\right) \implies \left({f \left({x, y}\right) = 1 \land f \left({y, z}\right) = 1 \implies f \left({x, z}\right) = 1}\right)$
:$(b): \quad \left({x, y}\right), \left({y, x}\right) \in \operatorname{Dom} \left({f}\right) \implies \left({f \left({x, y}\right) = 1 \iff f \left({y, x}\right) = 0}\right)$
:$(c): \quad \left({x,y}\right) \in \operatorname{Dom} \left({f}\right) \implies \left({x \prec y \implies f \left({x, y}\right) = 1}\right)$
We will first show that for some $\phi \in M$, the domain of $f$ is $S \times S$.
We will then show that $\phi$ is the characteristic function of a strict total ordering on $S$ that extends $\prec$.
We will apply the Cowen-Engeler Lemma to show that $\phi$ exists, so we must show that $M$ satisfies the premises of that theorem.
Specifically, we must show that:
:$(1): \quad \left\{{f \left({x, y}\right): f \in M}\right\}$ is finite for each $\left({x, y}\right) \in S \times S$
:$(2): \quad$ For each finite subset $F$ of $S \times S$, there exists an element $f \in M$ such that the domain of $f$ is $F$
:$(3): \quad M$ has finite character.
=== $M$ satisfies premise $(1)$ ===
For each $g \in M$ and each $x \in \operatorname{Dom} \left({g}\right)$, $g \left({x}\right) \in \left\{{0, 1}\right\}$, so requirement $(1)$ is trivially satisfied.
=== $M$ satisfies premise $(2)$ ===
Let $F$ be a finite subset of $S \times S$.
Let $S_F = \operatorname{Dom} \left({F}\right) \cup \operatorname{Img} \left({F}\right)$.
Since $F$ is finite, so is $S_F$.
Thus by the Order-Extension Principle for Finite Sets there is a strict total ordering $<_F$ on $S_F$ which is compatible with $\prec$.
Let $f_F$ be the restriction of the characteristic function of $<_F$ to $F$.
Then since $F \subseteq S_F \times S_F$, $\operatorname{Dom} \left({f_F}\right) = F$ by the definition of restriction.
Then $f_F \in M$:
Let:
: $\left({x, y}\right), \left({y, z}\right), \left({x, z}\right) \in \operatorname{Dom} \left({f_F}\right) = F$
: $f_F \left({x, y}\right) = 1 \land f_F \left({y, z}\right) = 1$
Then $x <_F y \land y <_F z$.
Thus since $<_F$ is transitive:
:$x <_F z$
Thus:
:$f_F \left({x, z}\right) = 1$
Similarly, let $\left({x, y}\right), \left({y, x}\right) \in \operatorname{Dom} \left({f_F}\right) = F$.
Then since $<_F$ is asymmetric and connected:
: $x <_F y \iff \neg (y <_F x)$
Thus:
:$f_F \left({x, y}\right) = 1 \iff f_F \left({y, x}\right) = 0$.
If $\left({x, y}\right) \in \operatorname{Dom}(f_F)$, then since $<_F$ is compatible with $\prec$:
:$x \prec y \implies f \left({x, y}\right) = 1$
So $f_F \in M$.
As such an $f_F$ exists for each such $F$, $M$ satisfies premise $(2)$.
=== $M$ satisfies premise $(3)$ ===
We must show that $M$ has finite character.
That is, a mapping $f$ from some subset of $S \times S$ to $\left\{{0, 1}\right\}$ is an element of $M$ :
:its restriction to each finite subset of $\operatorname{Dom} \left({f}\right)$ is in $M$.
Let $f$ be a partial mapping from $S \times S$ to $\left\{{0, 1}\right\}$.
Suppose that $f \in M$.
Let $F$ be a finite subset of $\operatorname{Dom} \left({f}\right)$.
Let $x, y, z \in S$.
Let $\left({x, y}\right), \left({y, z}\right), \left({x, z}\right) \in \operatorname{Dom}(f \restriction F)$.
Then:
:$\left({x, y}\right), \left({y, z}\right), \left({x, z}\right) \in \operatorname{Dom} \left({f}\right)$
Thus since $f \in M$:
:$f \left({x, y}\right) = 1 \land f \left({y, z}\right) = 1 \implies f \left({x, z}\right) = 1$
But by the definition of restriction:
:$f \restriction F \left({x, y}\right) = f \left({x, y}\right)$
:$f \restriction F \left({y, z}\right) = f \left({y, z}\right)$
:$f \restriction F \left({x, z}\right) = f \left({x, z}\right)$
Thus:
:$f \restriction F \left({x, y}\right) = 1 \land f \restriction F \left({y, z}\right) = 1 \implies f \restriction F \left({x, z}\right) = 1$
So $f \restriction F$ satisfies $(a)$.
A similar argument shows that $f \restriction F$ satisfies $(b)$ and $(c)$, so $f \restriction F \in M$.
Suppose instead that for every finite subset $F$ of $\operatorname{Dom} \left({f}\right)$, $f \restriction F \in M$.
Let $x, y \in S$.
Suppose that $\left({x, y}\right), \left({y, x}\right) \in \operatorname{Dom} \left({f}\right)$.
Then $K = \left\{ {\left({x, y}\right), \left({y, x}\right)}\right\}$ is a finite subset of $\operatorname{Dom} \left({f}\right)$.
Thus $f \restriction K \in M$.
So:
:$f \restriction K \left({x, y}\right) = 1 \iff f \restriction K \left({y, x}\right) = 0$
Since $f \restriction K \left({x, y}\right) = f \left({x, y}\right)$ and $f \restriction K \left({y, x}\right) = f \left({y, x}\right)$:
:$f \left({x, y}\right) = 1 \iff f \left({y, x}\right) = 0$
Thus $f$ satisfies $(b)$ for all pairs.
A similar argument shows that $f$ satisfies $(a)$ for all pairs.
Thus $f \in M$.
Since $M$ satisfies $(1)$, $(2)$, and $(3)$, by the Cowen-Engeler Lemma there is a $\phi \in M$ whose domain is $S \times S$.
=== $\phi$ is Characteristic Function which is Compatible with $\prec$ ===
Since $\operatorname{Dom} \left({\phi}\right) = S \times S$ and $\phi \in M$, for all $x, y, z \in S$:
:$(a): \quad \phi \left({x, y}\right) = 1 \land \phi \left({y, z}\right) = 1 \implies \phi \left({x, z}\right) = 1$
:$(b): \quad \phi \left({x, y}\right) = 1 \iff \phi \left({y, x}\right) = 0$
:$(c): \quad x \prec y \implies \phi \left({x, y}\right) = 1$
By an argument similar to that given for premise $(2)$, above, $\phi$ is the characteristic function of a strict total ordering on $S$ compatible with $\prec$.
Category:Set Theory
Category:Order Theory