# nLab semi-simplicial set

Semi-simplicial sets

# Semi-simplicial sets

## Idea

A semi-simplicial set is like a simplicial set, but without the degeneracy maps: it is a sequence $\{X_n\}_{n \in \mathbb{N}}$ of sets together with functions called face maps between them which encode that an element in $X_{n+1}$ has $(n+1)$ “faces” (boundary segments) which are elements in $X_{n}$.

The semi-simplicial set version of a simplicial complex is also called a Delta set.

## Definition

Let $\Delta$ denote the simplex category, which is a skeleton of the category of inhabited finite totally ordered sets. Let $\Delta_+$ denote the wide subcategory of $\Delta$ containing only the injective functions. Thus, $\Delta_+$ is equivalent to the category of inhabited finite totally ordered sets and order-preserving injections.

Recall that a simplicial set is a presheaf $X\colon \Delta^{op}\to Set$. Similarly, a semi-simplicial set is a presheaf $X\colon \Delta_+^{op} \to Set$.

More generally, for $\mathcal{C}$ any other category, a functor $\Delta_+^{op} \to \mathcal{C}$ is a semi-simplicial object in $\mathcal{C}$.

## Properties

The forgetful functor from SimplicialSets to the category of semi-simplicial sets is given by precomposition with the opposite functor of the non-full wide subcategory inclusion

$\Delta_{inj} \xrightarrow{\;j\;} \Delta$

into the simplex category and hence has both a left adjoint as well as a right adjoint given by left/right Kan extension, respectively:

$X \xrightarrow{ \;\eta_X\; } j^\ast j_! X$

is a weak homotopy equivalence in the sense that its geometric realization is so (Rourke & Sanderson 71, Rem. 5.8).

Notice that

1. for $X \in$ SimplicialSets, the geometric realization of the underlying semi-simplicial set $j^\ast X$ is the fat geometric realization of $X$ (see there):

$\left\Vert X \right\Vert \;\coloneqq\; \left\vert j^\ast X \right \vert$
2. There is (by this Prop.) a natural weak homotopy equivalence from the fat to the ordinay geometric realization of a simplicial set (which is always “good” when regarded as a simplicial space):

$\left\vert j^\ast X \right \vert \;\simeq_{whe}\; \left\vert X \right \vert \,.$

$j_! j^\ast X \xrightarrow{\; \epsilon_X \;} X$

### Relation to semi-categories

The nerve of a semicategory is a semi-simplicial set (satisfying the Segal conditions) just as the nerve of a category is a simplicial set.

### Model category structure

There is a model structure on semi-simplicial sets, transferred along the right adjoint to the forgetful functor from the model structure on simplicial sets.

## Historical and terminological remarks

The original paper Eilenberg & Zilber 50 defined both (what we now call) semi-simplicial sets, under the name semi-simplicial complexes, and (what we now call) simplicial sets, under the name complete semi-simplicial complexes. The motivation for the name “semi-simplicial” was that a semi-simplicial set is like a simplicial complex, but lacks the property that a simplex is uniquely determined by its vertices. Then they added the degeneracies and a corresponding adjective “complete.”

Over time it became clear that “complete semi-simplicial complexes” were much more important and useful than the non-complete ones. This seems to have led first to the omission of the adjective “complete,” and then the omission of the prefix “semi” (and at some point the replacement of “complex” by “set”), resulting in the current name simplicial sets.

The concept is essentially the same as that of $\Delta$-set, as used by Rourke & Sanderson 71. Their motivation was from geometric topology.

On the other hand, in other contexts the prefix “semi-” is used to denote absence of identities (such as a semigroup (which is, admittedly, missing more than identities relative to a group) or a semicategory), thus if we start from the modern name “simplicial sets” it makes independent sense to refer to their degeneracy-less variant as “semi-simplicial sets.” This is coincidentally in line with the original terminology of Eilenberg and Zilber, but not of course with the intermediate usage of “semi-simplicial set” for what we now call a “simplicial set.”