Slow-growing hierarchy
Introduction In computability theory, computational complexity theory and proof theory, the '''slow-growing hierarchy''' is an ordinal-indexed family of slowly increasing functions ''g''α: '''N''' → '''N''' (where '''N''' is the set of natural numbers, {0, 1, ...}). It contrasts with the fast-growing hierarchy.
Definition Let μ be a large countable ordinal such that a fundamental sequence is assigned to every limit ordinal less than μ. The '''slow-growing hierarchy''' of functions ''g''α: '''N''' → '''N''', for α th element of the fundamental sequence assigned to the limit ordinal α.The article on the Fast-growing hierarchy describes a standardized choice for fundamental sequence for all α 0.
Relation to fast-growing hierarchy The slow-growing hierarchy grows much more slowly than the fast-growing hierarchy. Even ''g''ε0 is only equivalent to ''f''3 and ''g''α only attains the growth of ''f''ε0 (the first function that Peano arithmetic cannot prove total in the hierarchy) when α is the Bachmann–Howard ordinal.However, Girard proved that the slow-growing hierarchy eventually ''catches up'' with the fast-growing one. Specifically, that there exists an ordinal α such that for all integers ''n'':''g''α(''n'') α(''n'') α(''n'' + 1)where ''f''α are the functions in the fast-growing hierarchy. He further showed that the first α, this holds for, is the ordinal of the theory ''ID'' of arbitrary finite iterations of an inductive definition. However, for the assignment of fundamental sequences found in  the first match up occurs at the level ε0.For Buchholz style tree ordinals it could be shown that the first match up even occurs at .Extensions of the result proved to considerably larger ordinals show that there are very few ordinals below the ordinal of transfinitely iterated -comprehension where the slow- and fast-growing hierarchy match up.The slow-growing hierarchy depends extremely sensitively on the choice of the underlying fundamental sequences.
Relation to term rewriting Cichon provided an interesting connection between the slow-growing hierarchy and derivation length for term rewriting.
References * PDF's:  part 1  2  3. (In particular part 3, Section 12, pp. 59–64, "A Glimpse at Hierarchies of Fast and Slow Growing Functions".)
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