Climb into Cantor’s Attic, where you will find infinities large and small. We aim to provide a comprehensive resource of information about all notions of mathematical infinity.
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Cantor's Attic (original site)
Joel David Hamkins blog post about the Attic
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Zero, or $0$, is the smallest ordinal (and cardinal) number. It is usually represented by the empty set, $\varnothing$ (or $\{\}$).
Given an ordinal (or cardinal) $x$:
$x + 0 = 0 + x = x$.
$x \cdot 0 = 0 \cdot x = 0$
$x^0 = 1$
$0^x = 0$ (if $x > 0$)
Most set theorists classify zero as the only ordinal which is neither a limit ordinal nor the successor of any ordinal.
Some set theorists (e.g. Thomas Jech) instead classify zero as a limit ordinal, because like other limit ordinals, it is the limit of all ordinals less than it (of which there are none).