Completeness and Cocompleteness Transfer for Internal Group Objects with Geometric Obstructions

Jian-Gang Tang (), Nueraminaimu Maihemuti, Jia-Yin Peng, Yimamujiang Aisan and Ai-Li Song
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Jian-Gang Tang: Department of Mathematics, Sichuan University Jinjiang College, Meishan 620860, China
Nueraminaimu Maihemuti: School of Mathematics and Statistics, Kashi University, Kashi 844000, China
Jia-Yin Peng: School of Mathematics and Statistics, Kashi University, Kashi 844000, China
Yimamujiang Aisan: School of Mathematics and Statistics, Kashi University, Kashi 844000, China
Ai-Li Song: School of Mathematics and Statistics, Kashi University, Kashi 844000, China

Mathematics, 2025, vol. 13, issue 19, 1-28

Abstract: This work establishes definitive conditions for the inheritance of categorical completeness and cocompleteness by categories of internal group objects. We prove that while the completeness of Grp ( C ) follows unconditionally from the completeness of the base category C , cocompleteness requires C to be regular, cocomplete, and admit a free group functor left adjoint to the forgetful functor. Explicit limit and colimit constructions are provided, with colimits realized via coequalizers of relations induced by group axioms over free group objects. Applications demonstrate cocompleteness in topological groups, ordered groups, and group sheaves, while Lie groups serve as counterexamples revealing necessary analytic constraints—particularly the impossibility of equipping free groups on non-discrete manifolds with smooth structures. Further results include the inheritance of regularity when the free group functor preserves finite products, the existence of internal hom-objects in locally Cartesian closed settings, monadicity for locally presentable C , and homotopical extensions where model structures on Grp ( M ) reflect those of M . This framework unifies classical category theory with geometric obstruction theory, resolving fundamental questions on exactness transfer and enabling new constructions in homotopical algebra and internal representation theory.

Keywords: internal group objects; cocompleteness transfer; free group functor; regular categories; monadicity; Tannakian duality; homotopical algebra (search for similar items in EconPapers)
JEL-codes: C (search for similar items in EconPapers)
Date: 2025
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