Hardy hierarchy

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In computability theory, computational complexity theory and proof theory, the Hardy hierarchy, named after G. H. Hardy, is a hierarchy of sets of numerical functions generated from an ordinal-indexed family of functions hα: N  N (where N is the set of natural numbers, {0, 1, ...}) called Hardy functions. It is related to the fast-growing hierarchy and slow-growing hierarchy.

The Hardy hierarchy was introduced by Stanley S. Wainer in 1972,[1][2] but the idea of its definition comes from Hardy's 1904 paper,[2][3] in which Hardy exhibits a set of reals with cardinality .

Definition

Summarize
Perspective

Let μ be a large countable ordinal such that a fundamental sequence is assigned to every limit ordinal less than μ. The Hardy functions hα: N  N, for α < μ, is then defined as follows:

  • if α is a limit ordinal.

Here α[n] denotes the nth element of the fundamental sequence assigned to the limit ordinal α. A standardized choice of fundamental sequence for all α  ε0 is described in the article on the fast-growing hierarchy.

The Hardy hierarchy is a family of numerical functions. For each ordinal α, a set is defined as the smallest class of functions containing Hα, zero, successor and projection functions, and closed under limited primitive recursion and limited substitution[2] (similar to Grzegorczyk hierarchy).

Caicedo (2007) defines a modified Hardy hierarchy of functions by using the standard fundamental sequences, but with α[n+1] (instead of α[n]) in the third line of the above definition.

Relation to fast-growing hierarchy

The Wainer hierarchy of functions fα and the Hardy hierarchy of functions Hα are related by fα = Hωα for all α < ε0. Thus, for any α < ε0, Hα grows much more slowly than does fα. However, the Hardy hierarchy "catches up" to the Wainer hierarchy at α = ε0, such that fε0 and Hε0 have the same growth rate, in the sense that fε0(n-1) ≤ Hε0(n) ≤ fε0(n+1) for all n ≥ 1. (Gallier 1991)

Notes

References

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