Kan Complex
A Kan complex is a simplicial set in which every horn admits a filler. Check outer as well as inner horns to distinguish this condition from the inner-horn condition for quasicategories.
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Where to read
Start here: Kerodon — Kan Complexes
Jacob Lurie · Online reference · English · Free full text
§1.2.5, Definition 1.2.5.1 (tag 002H), including all horns with n > 0 and 0 ≤ i ≤ n
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Category Theory in the library · Preparation and skip guidance