### Abstract

Let p be a strong type of an algebraically closed tuple over B=acl^{eq}(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 H_{1} ^{⁎}(p) can be introduced, similarly to [3]. We show that there is a canonical surjective homomorphism from the Lascar group over B to H_{1} ^{⁎}(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 H_{1}(p). As consequences, in any T, we show that |H_{1}(p)|≥2^{ℵ0} unless H_{1}(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 language | English |
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Pages (from-to) | 2129-2151 |

Number of pages | 23 |

Journal | Annals of Pure and Applied Logic |

Volume | 168 |

Issue number | 12 |

DOIs | |

Publication status | Published - 2017 Dec |

### All Science Journal Classification (ASJC) codes

- Logic

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## Cite this

*Annals of Pure and Applied Logic*,

*168*(12), 2129-2151. https://doi.org/10.1016/j.apal.2017.06.006