Part of the Progress in Mathematics book series (PM, volume 263)


This book is devoted to a particular recursive method of constructing mathematical structures that live on a given ordinal θ, using a single transformation ξCξ which assigns to every ordinal ξ < θ a set Cξ of smaller ordinals that is closed and unbounded in the set of ordinals < ξ. The transfinite sequence
$$ C_\xi \left( {\xi < \theta } \right) $$
which we call a ‘C-sequence’ and on which we base our recursive constructions may have a number of ‘coherence properties’ and we shall give a detailed study of them and the way they influence these constructions. Here, ‘coherence’ usually means that the Cξ’s are chosen in some canonical way, beyond the already mentioned and natural requirement that Cξ is closed and unbounded in ξ for all ξ. For example, choosing a canonical ‘fundamental sequence’ of sets Cξξ for ξ < ε0, relying on the specific properties of the Cantor normal form for ordinals below the first ordinal satisfying the equation x = ωx, is a basis for a number of important results in proof theory. In set theory, one is interested in longer sequences as well and usually has a different perspective in applications, so one is naturally led to use some other tools besides the Cantor normal form. It turns out that the sets Cξ can not only be used as ‘ladders’ for climbing up in recursive constructions but also as tools for ‘walking’ from an ordinal β to a smaller one α, β = β0 > β1 > ... > βn−1 > βn = α where the ‘step’βiβi+1 is defined by letting βi+1 be the minimal point of Cβi that is bigger than or equal to α.


Oscillation Mapping Force Notion Stationary Subset Singular Cardinal Unbounded Subset 
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© Birkhäuser Verlag AG 2007

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