Most systems of notation use symbols such as ψ(α), θ(α), ψα(β), some of which are modifications of the Veblen functions to produce countable ordinals even for uncountable arguments, and some of which are "collapsing functions".
The last one is an extension of the Veblen functions for more than 2 arguments.
The smaller Ackermann ordinal is the limit of a system of ordinal notations invented by Ackermann (1951), and is sometimes denoted by
, where Ω is the smallest uncountable ordinal.
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