Zero

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Definition

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).