Bachmann–Howard ordinal

In mathematics, the Bachmann–Howard ordinal (also known as the Howard ordinal, or Howard-Bachmann ordinal^[1]) is a large countable ordinal. It is the proof-theoretic ordinal of several mathematical theories, such as Kripke–Platek set theory (with the axiom of infinity) and the system CZF of constructive set theory. It was introduced by Heinz Bachmann (1950) and William Alvin Howard (1972).

Definition

The Bachmann–Howard ordinal is defined using an ordinal collapsing function:

• ε_α enumerates the epsilon numbers, the ordinals ε such that ω^ε = ε.
• Ω = ω₁ is the first uncountable ordinal.
• ε_Ω+1 is the first epsilon number after Ω = ε_Ω.
• ψ(α) is defined to be the smallest ordinal that cannot be constructed by starting with 0, 1, ω and Ω, and repeatedly applying ordinal addition, multiplication and exponentiation, and ψ to previously constructed ordinals (except that ψ can only be applied to arguments less than α, to ensure that it is well defined).
• The Bachmann–Howard ordinal is ψ(ε_Ω+1).

The Bachmann–Howard ordinal can also be defined as φ_εΩ+1(0) for an extension of the Veblen functions φ_α to certain functions α of ordinals; this extension was carried out by Heinz Bachmann and is not completely straightforward.^[2][3]