The Lascar groups and the first homology groups in model theory

Jan Dobrowolski, Byunghan Kim, Junguk Lee

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)


Let p be a strong type of an algebraically closed tuple over B=acleq(B) in any theory T. Depending on a ternary relation ⫝ satisfying some basic axioms (there is at least one such, namely the trivial independence in T), the first homology group H1 (p) can be introduced, similarly to [3]. We show that there is a canonical surjective homomorphism from the Lascar group over B to H1 (p). We also notice that the map factors naturally via a surjection from the ‘relativised’ Lascar group of the type (which we define in analogy with the Lascar group of the theory) onto the homology group, and we give an explicit description of its kernel. Due to this characterization, it follows that the first homology group of p is independent from the choice of ⫝, and can be written simply as H1(p). As consequences, in any T, we show that |H1(p)|≥20 unless H1(p) is trivial, and we give a criterion for the equality of stp and Lstp of algebraically closed tuples using the notions of the first homology group and a relativised Lascar group. We also argue how any abelian connected compact group can appear as the first homology group of the type of a model.

Original languageEnglish
Pages (from-to)2129-2151
Number of pages23
JournalAnnals of Pure and Applied Logic
Issue number12
Publication statusPublished - 2017 Dec

Bibliographical note

Funding Information:
All authors were supported by Samsung Science Technology Foundation under Project Number SSTF-BA1301-03. The third author was also supported by NRF of Korea grant 2013R1A1A2073702 and the Yonsei University Research Fund (Post Doc. Researcher Supporting Program) of 2016 (Project No. 2016-12-0004).

Publisher Copyright:
© 2017 Elsevier B.V.

All Science Journal Classification (ASJC) codes

  • Logic


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