Abstract
We prove a tree lemma on perfect trees to obtain an ultrafilter of definable subsets of ℕ, which is Ramsey but not conservative (see [Phi 2]).
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References
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Phillips, R. G.: A minimal extension that is not conservative. Mich. Math. J. vol.21, 27–32 (1974).
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Potthoff, K. A simple tree lemma and its application to a counterexample of philips. Arch math Logik 18, 67–71 (1977). https://doi.org/10.1007/BF02007258
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DOI: https://doi.org/10.1007/BF02007258