Simplicial Set
A simplicial set is a contravariant functor from the simplex category to sets. Read the order-preserving maps as operations on simplices, keeping degenerate simplices as part of the data.
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Where to read
Start here: Kerodon — Simplicial Sets
Jacob Lurie · Online reference · English · Free full text
§1.1, Definitions 1.1.0.2 (simplex category), 1.1.0.4 (simplicial object), and 1.1.0.6 (simplicial set; tag 000H)
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Category Theory in the library · Preparation and skip guidance