The Lascar-group upper-bound conjecture for countable tuples

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Let TT be a theory with monster model \EuFrakC{\EuFrak C}, let a‾\overline{a} be a countable tuple from \EuFrakC{\EuFrak C}, and let ELa‾E_L^{\overline a} and FLa‾F_L^{\overline a} denote respectively Lascar equivalence on the type space of a‾\overline a and its restriction to the Kim--Pillay class of a‾\overline a. Let GalL(T)Gal_L(T) be the Lascar group and Gal0(T)Gal_0(T) the kernel of its canonical map to GalKP(T)Gal_{KP}(T). Lascar-group upper-bound conjecture. The Borel cardinality of ELa‾E_L^{\overline a} is at most that of GalL(T)Gal_L(T), and the Borel cardinality of FLa‾F_L^{\overline a} is at most that of Gal0(T)Gal_0(T). The claim is motivated by placing any countable tuple inside a countable elementary substructure; its general validity is left open.

References

Primary source

Krzysztof Krupinski, Anand Pillay and Slawomir Solecki, “Borel equivalence relations and Lascar strong types”, arXiv:1204.3485 (2012).

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