# Contents

## Definition

A simplicial set $X$ is (sometimes) called reduced if it has a single vertex, $X_0 \simeq *$.

More generally, for $n \in \mathbb{N}$ a simplicial set is $n$-reduced if its $n$-skeleton is the point, $sk_n X = \Delta[0]$.

## Properties

### Reflection

Write $sSet_0 \hookrightarrow$ sSet for the full subcategory inclusion of the reduced simplicial sets into all of them.

This is a reflective subcategory. The reflector

$red : sSet \to sSet_0$

identifies all vertices of a simplicial set.

Write $sSet^{*/}$ for the category of pointed simplicial sets. There is also a full inclusion $sSet_0 \hookrightarrow sSet^{*/}$. This has a right adjoint $red : sSet^{*/} \to sSet_0$ which sends a pointed simplicial set to the subobject all whose $n$-cells have as 0-faces the given point.

### Coreflection

The inclusion $sSet_0 \hookrightarrow sSet^{*/}$ into pointed simplicial sets is coreflective. The coreflector is the Eilenberg subcomplex construction in degree 1.

### Model structure

There is a model structure on reduced simplicial sets (see there) which serves as a presentation of the (∞,1)-category of pointed connected ∞-groupoids.

Revised on April 19, 2012 07:43:37 by Urs Schreiber (82.169.65.155)