Epsilon nought

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This article is about an ordinal in mathematics. For the physics constant ε0, see permittivity.

In mathematics, ε0 is the smallest transfinite ordinal number which cannot be reached from ω (the smallest transfinite ordinal) with a finite number of the ordinal operations of addition, multiplication and exponentiation. As such it is a limit ordinal. It is given by

\epsilon_0 = \omega^{\omega^{\omega^{\cdots}}},

or in Cantor normal form by

\epsilon_0 = \omega^{\epsilon_0}.

The ordinal ε0 is still countable (there exist uncountable ordinals). This ordinal is very important in many induction proofs, because for many purposes, transfinite induction is only required up to ε0.

This was created by the Russian-born mathematician Georg Cantor. It is also frequently cited by the Argentine-American mathematician and computer scientist Gregory Chaitin in his lectures and papers. This ordinal is also called "epsilon zero".

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