Coanalytic ultrafilter bases
نویسندگان
چکیده
We study the definability of ultrafilter bases on $$\omega $$ in sense descriptive set theory. As a main result we show that there is no coanalytic base for Ramsey ultrafilter, while L can construct $$\Pi ^1_1$$ P-point and Q-point bases. also existence $${\varvec{\Delta }}^1_{n+1}$$ equivalent to $${\varvec{\Pi }}^1_n$$ base, $$n \in \omega . Moreover introduce Borel version classical number make some observations.
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ژورنال
عنوان ژورنال: Archive for Mathematical Logic
سال: 2021
ISSN: ['1432-0665', '0933-5846']
DOI: https://doi.org/10.1007/s00153-021-00801-7