Membership is Left Compatible with Ordinal Multiplication
Theorem
Let $x$, $y$, and $z$ be ordinals.
Then:
- $\paren {x < y \land z > 0} \iff \paren {z \cdot x} < \paren {z \cdot y}$
Proof
Sufficient Condition
The proof of the sufficient condition shall proceed by Transfinite Induction on $y$.
$\Box$
Basis for the Induction
Both $x < 0$ and $\paren {x \cdot z} < \paren {0 \cdot z}$ are contradictory, so the if and only if statement holds for the condition that $y = 0$.
This proves the basis for the induction.
$\Box$
Induction Step
Suppose the biconditional statement holds for $y$. Then:
\(\ds x < y^+\) | \(\leadsto\) | \(\ds x < y \lor x = y\) | Definition of Successor Set | |||||||||||
\(\ds x < y \land z > 0\) | \(\leadsto\) | \(\ds \paren {z \cdot x} < \paren {z \cdot y}\) | Inductive Hypothesis | |||||||||||
\(\ds x = y\) | \(\leadsto\) | \(\ds \paren {z \cdot x} = \paren {z \cdot y}\) | Leibniz's law | |||||||||||
\(\ds z > 0\) | \(\leadsto\) | \(\ds \paren {z \cdot y} < \paren {\paren {z \cdot y} + z}\) | Membership is Left Compatible with Ordinal Addition | |||||||||||
\(\ds \) | \(\leadsto\) | \(\ds \paren {z \cdot y} < \paren {z \cdot y^+}\) | Definition of Ordinal Multiplication | |||||||||||
\(\ds \) | \(\leadsto\) | \(\ds \paren {z \cdot x} < \paren {z \cdot y^+}\) | Transitivity of $<$ and Leibniz's law |
In either case:
- $\paren {z \cdot x} < \paren {z \cdot y^+}$
This proves the induction step.
$\Box$
Limit Case
Suppose $y$ is a limit ordinal:
\(\ds \) | \(\) | \(\ds \forall w \in y: \paren {\paren {x < w \land z > 0} \iff \paren {z \cdot x} < \paren {z \cdot w} }\) | by hypothesis | |||||||||||
\(\ds \) | \(\leadsto\) | \(\ds \paren {\exists w \in y: \paren {x < w \land z > 0} \iff \exists w \in y: \paren {z \cdot x} < \paren {z \cdot w} }\) | Predicate Logic Manipulation | |||||||||||
\(\ds \) | \(\leadsto\) | \(\ds \paren {\paren {x < \bigcup y \land z > 0} \iff \paren {z \cdot x} < \bigcup_{w \mathop \in y} \paren {z \cdot w} }\) | Definition of Set Union | |||||||||||
\(\ds \) | \(\leadsto\) | \(\ds \paren {\paren {x < y \land z > 0} \iff \paren {z \cdot x} < \bigcup_{w \mathop \in y} \paren {z \cdot w} }\) | Limit Ordinal Equals its Union | |||||||||||
\(\ds \) | \(\leadsto\) | \(\ds \paren {\paren {x < y \land z > 0} \iff \paren {z \cdot x} < \paren {z \cdot y} }\) | Definition of Ordinal Multiplication |
This proves the limit case.
$\Box$
Necessary Condition
Conversely, suppose $\paren {z \cdot x} < \paren {z \cdot y^+}$.
Then $z \ne 0$ because if it were equal, both sides of the inequality would be $0$.
So $z > 0$.
Furthermore:
- $y < x \implies \paren {z \cdot y} < \paren {z \cdot x}$
- $y = x \implies \paren {z \cdot y} = \paren {z \cdot x}$
So if $\paren {z \cdot x} < \paren {z \cdot y}$, then $y \ne x$ and $y \not < x$, so $x < y$ by Ordinal Membership is Trichotomy.
$\blacksquare$
Sources
- 1971: Gaisi Takeuti and Wilson M. Zaring: Introduction to Axiomatic Set Theory: $\S 8.19$