This is a follow up on a comment to one of my previous questions. What's the definition of $\omega$?
Are the following equivalent definition of $\omega$:
$\omega$ is the initial ordinal of $\aleph_0$.
$\omega$ is the least/first infinite ordinal.
$\omega$ is the set of all finite ordinals.
$\omega$ is the first non-zero limit ordinal
If yes, are there any more equivalent definitions, not on this list?
$\endgroup$4 Answers
$\begingroup$$\omega$ is defined to be the set of all finite ordinals.
This is provably equivalent to the assertion "the least infinite ordinal" or "the least limit ordinal" (note that $0$ is not a limit ordinal. It is $0$).
It can be stated as the smallest inductive set. Or the set of all ordinals whose rank is finite.
Indeed it is also defined to be $\aleph_0$.
$\endgroup$ 10 $\begingroup$Here is a definition that works even without the axiom of infinity, in which case $\omega$ can be a proper class. Namely, $\omega$ is the class of finite ordinals. An ordinal $\alpha$ is finite if $\alpha=0=\emptyset$ or $\alpha$ is a successor ordinal that has only $0$ and other succesor ordinals as predecessors.
$\endgroup$ 2 $\begingroup$I prefer to say: $\omega$ is the order type of the natural numbers with its usual order. All those others are theorems or common identifications.
$\endgroup$ 3 $\begingroup$I'd like to summarise what I have learnt from this question:
Point (1) is circular since $\aleph_0$ is defined to be the cardinality of $\omega$.
Let's assume that we define $\omega$ to be the first ordinal of infinite cardinality. Then it must contain all finite ordinals since the ordinals are a linear order with respect to $\subseteq$. From this it is immediately clear that (2) and (3) are equivalent. It is similarly easy to see that (4) is equivalent to (3).
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