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WEAK CANONICAL BASES IN NSOP$_1$ THEORIES

Part of: Model theory

Published online by Cambridge University Press:  11 June 2021

BYUNGHAN KIM
BYUNGHAN KIM*
Affiliation:
DEPARTMENT OF MATHEMATICS YONSEI UNIVERSITYSEOUL, SOUTH KOREAE-mail:bkim@yonsei.ac.kr
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Abstract

We study the notion of weak canonical bases in an NSOP $_{1}$ theory T with existence. Given $p(x)=\operatorname {tp}(c/B)$ where $B=\operatorname {acl}(B)$ in ${\mathcal M}^{\operatorname {eq}}\models T^{\operatorname {eq}}$ , the weak canonical base of p is the smallest algebraically closed subset of B over which p does not Kim-fork. With this aim we firstly show that the transitive closure $\approx $ of collinearity of an indiscernible sequence is type-definable. Secondly, we prove that given a total $\mathop {\smile \hskip -0.9em ^| \ }^K$ -Morley sequence I in p, the weak canonical base of $\operatorname {tp}(I/B)$ is $\operatorname {acl}(e)$ , if the hyperimaginary $I/\approx $ is eliminable to e, a sequence of imaginaries. We also supply a couple of criteria for when the weak canonical base of p exists. In particular the weak canonical base of p is (if exists) the intersection of the weak canonical bases of all total $\mathop {\smile \hskip -0.9em ^| \ }^K$ -Morley sequences in p over B. However, while we investigate some examples, we point out that given two weak canonical bases of total $\mathop {\smile \hskip -0.9em ^| \ }^K$ -Morley sequences in p need not be interalgebraic, contrary to the case of simple theories. Lastly we suggest an independence relation relying on weak canonical bases, when T has those. The relation, satisfying transitivity and base monotonicity, might be useful in further studies on NSOP $_1$ theories .

Keywords

weak canonical basesNSOP1 theoriescollinearity

MSC classification

Primary: 03C45: Classification theory, stability and related concepts
Secondary: 03C50: Models with special properties (saturated, rigid, etc.)
Type
Article
Information
The Journal of Symbolic Logic , Volume 86 , Issue 3 , September 2021 , pp. 1259 - 1281
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Copyright
© Association for Symbolic Logic 2021

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References

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