TheoremBase

One Threshold for Finitely Many Monotone Conditions and for Tails

If a monotone condition P(p, N), expressed by a predicative formula, has a threshold N for each p in a finite set I, then one N serves for every p in I; in particular finitely many tail conditions have a common threshold.

Statement

In the setting of Routine Facts Used Without Citation: Arithmetic, Maps, Predicativity and Indices, let II be a finite set, and let N\mathbb{N} and its order ≤\le be as in Arithmetic and Order of the Natural Numbers. Both clauses below are statements of the metatheory: they concern conditions expressed by formulas.

Let P(p,N)P(p,N) be a condition on the set variables pp and NN, possibly with parameters, that is expressed by a predicative formula, and that is monotone: for all p∈Ip\in I and N,N′∈NN,N'\in\mathbb{N} with N≤N′N\le N', P(p,N)P(p,N) implies P(p,N′)P(p,N'). If for every p∈Ip\in I there is N∈NN\in\mathbb{N} with P(p,N)P(p,N), then there is one N∈NN\in\mathbb{N} with P(p,N)P(p,N) for every p∈Ip\in I.

Let Q(p,n)Q(p,n) be a condition on the set variables pp and nn, possibly with parameters, that is expressed by a predicative formula. If for every p∈Ip\in I there is N∈NN\in\mathbb{N} such that Q(p,n)Q(p,n) holds for every n∈Nn\in\mathbb{N} with N≤nN\le n, then there is one N∈NN\in\mathbb{N} such that Q(p,n)Q(p,n) holds for every p∈Ip\in I and every n∈Nn\in\mathbb{N} with N≤nN\le n.

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