## Abstract

Given an arbitrary connected groupoid G with its vertex group Ga, if G_{a} is a central subgroup of a group F, then there is a canonical extension F = G ⊗ F of G in the sense that Ob(G) = Ob(F), Mor(G) ⊆ Mor(F), and F is isomorphic to all the vertex groups of F. From the failure of 3-uniqueness of a strong type p over A = acleq(A) in a stable theory T, a canonical finitary connected commutative groupoid G with the binding group G was A-type-definably constructed by John Goodrick and Alexei Kolesnikov (2012). In this paper we take a certain (possibly non-commutative) automorphism group F where G is embedded centrally (so inducing ι_{a}: G_{a} → Z(F)), and show that the abstract groupoid G ⊗ F lives A-invariantly in models of T. More precisely, we A-invariantly construct a connected groupoid F, isomorphic to G ⊗ F as abstract groupoids, satisfying the following: (1) Ob(F) = Ob(G), and Mor(F) and composition maps are A-invariant (i.e., described by infinite disjunctions of conjunctions of formulas over A), so that an A-automorphism of a model of T induces a groupoid automorphism of F. (2) There is an A-invariant faithful functor I: G → F which is the identity on the objects, and I(Ga) = i_{a} o ι_{a}, where i_{a} is a canonical group isomorphism from F onto a vertex group F_{a} of F. An automorphism group approximated by the vertex groups of the non-commutative groupoids is suggested as a "fundamental group" of the strong type p.

Original language | English |
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Pages (from-to) | 47-70 |

Number of pages | 24 |

Journal | Fundamenta Mathematicae |

Volume | 249 |

Issue number | 1 |

DOIs | |

State | Published - 2020 |

### Bibliographical note

Publisher Copyright:© 2020 Institute of Mathematics. Polish Academy of Sciences. All rights reserved.

## Keywords

- 3-uniqueness
- Fundamental group
- Generic groupoid