The Standard n-simplex and The Simplex Category
Categorically, the standard n-simplex, denoted Δn, is the functor hom(-, n) where n denotes the string 0 → 1 → ... → n of the first (n + 1) nonnegative integers and the homset is taken in the category Δ. In many texts, it is written instead as hom(n,-) where the homset is understood to be in the opposite category Δop.
The geometric realization |Δn| is just defined to be the standard topological n-simplex in general position given by
By the Yoneda lemma, the n-simplices of a simplicial set X are classified by natural transformations in hom(Δn, X). The n-simplices of X are then collectively denoted by Xn. Furthermore, there is a simplex category, denoted by whose objects are maps (i.e. natural transformations) Δn → X and whose morphisms are natural transformations Δn → Δm over X arising from maps n → m in Δ. The following isomorphism shows that a simplicial set X is a colimit of its simplices:
where the colimit is taken over the simplex category of X.
Read more about this topic: Simplicial Set
Famous quotes containing the words standard and/or category:
“The urge for Chinese food is always unpredictable: famous for no occasion, standard fare for no holiday, and the constant as to demand is either whim, the needy plebiscite of instantly famished drunks, or pregnancy.”
—Alexander Theroux (b. 1940)
“I see no reason for calling my work poetry except that there is no other category in which to put it.”
—Marianne Moore (18871972)