Transitive Class/Examples/Ordinal 3
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Example of Transitive Class
Let $\O$ denote the empty class.
Consider the ordinal $3$, defined as:
- $\mathcal 3 := \set {\O, \set \O, \set {\O, \set \O} }$
$\mathcal 3$ is transitive.
Proof
Consider the $3$ elements of $\mathcal 3$ in turn.
There are no elements of $\O$.
There is exactly one elements of $\set \O$, and that is $\O$.
We see that $\O$ is itself an element of $\set {\O, \set \O, \set {\O, \set \O} }$.
The elements of $\set {\O, \set \O}$, are $\O$ and $\set \O$.
We see that both are element of $\mathcal 3$.
Thus all elements of elements of $\mathcal 3$ are themselves elements of $\mathcal 3$.
Hence by definition $\mathcal 3$ is transitive.
$\blacksquare$
Sources
- 2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.) ... (previous) ... (next): Chapter $2$: Some Basics of Class-Set Theory: $\S 10$ Some useful facts about transitivity