n-skeleton

In mathematics, particularly in algebraic topology, the n-skeleton of a topological space X presented as a simplicial complex (resp.

CW complex) refers to the subspace Xn that is the union of the simplices of X (resp.

cells of X) of dimensions m ≤ n. In other words, given an inductive definition of a complex, the n-skeleton is obtained by stopping at the n-th step.

These subspaces increase with n. The 0-skeleton is a discrete space, and the 1-skeleton a topological graph.

The skeletons of a space are used in obstruction theory, to construct spectral sequences by means of filtrations, and generally to make inductive arguments.

They are particularly important when X has infinite dimension, in the sense that the Xn do not become constant as n → ∞.

In geometry, a k-skeleton of n-polytope P (functionally represented as skelk(P)) consists of all i-polytope elements of dimension up to k.[1] For example: The above definition of the skeleton of a simplicial complex is a particular case of the notion of skeleton of a simplicial set.

Briefly speaking, a simplicial set

, together with face and degeneracy maps between them satisfying a number of equations.

to the "smallest possible" simplicial set so that the resulting simplicial set contains no non-degenerate simplices in degrees

More precisely, the restriction functor has a left adjoint, denoted

are comparable with the one of image functors for sheaves.)

The 0-coskeleton is given by the Cech nerve (The boundary and degeneracy morphisms are given by various projections and diagonal embeddings, respectively.)

The coskeleton is needed to define the concept of hypercovering in homotopical algebra and algebraic geometry.