# Contents

## Idea

The horn $\Lambda_k[n] = \Lambda^n_k \hookrightarrow \Delta^n$ is the simplicial set obtained from the boundary of the n-simplex $\partial \Delta^n$ of the standard simplicial $n$-simplex $\Delta^n$ by discarding the $k$th face.

## Definition

Let

$\Delta[n] = \mathbf{\Delta}( -, [n]) \in Simp Set$

be the standard simplicial $n$-simplex in SimpSet.

###### Definition

For each $i$, $0 \leq i \leq n$, the $(n,i)$-horn or $(n,i)$-box is the subsimplicial set

$\Lambda^i[n] \hookrightarrow \Delta[n]$

which is the union of all faces except the $i^{th}$ one.

This is called an outer horn if $k = 0$ or $k = n$. Otherwise it is an inner horn.

###### Remark

Since sSet is a presheaf topos, unions of subobjects make sense and they are calculated objectwise, thus in this case dimensionwise. This way it becomes clear what the structure of a horn as a functor $\Lambda^k[n]: \Delta^{op} \to Set$ must therefore be: it takes $[m]$ to the collection of ordinal maps $f: [m] \to [n]$ which do not have the element $k$ in the image.

## Examples

###### Example

The inner horn of the 2-simplex

$\Delta^2 = \left\{ \array{ && 1 \\ & \nearrow &\Downarrow& \searrow \\ 0 &&\to&& 2 } \right\}$

with boundary

$\partial \Delta^2 = \left\{ \array{ && 1 \\ & \nearrow && \searrow \\ 0 &&\to&& 2 } \right\}$

looks like

$\Lambda^2_1 = \left\{ \array{ && 1 \\ & \nearrow && \searrow \\ 0 &&&& 2 } \right\} \,.$

The two outer horns look like

$\Lambda^2_0 = \left\{ \array{ && 1 \\ & \nearrow && \\ 0 &&\to&& 2 } \right\}$

and

$\Lambda^2_2 = \left\{ \array{ && 1 \\ & && \searrow \\ 0 &&\to&& 2 } \right\}$

respectively.

## Relation to other concepts

• A Kan fibration is a morphism of simplicial sets which has the right lifting property with respect to all horn inclusions $\Lambda^k[n] \hookrightarrow \Delta^n$.

• A Kan complex is a simplicial set in which “all horns have fillers“: a simplicial set for which the morphism to the point is a Kan fibration.

• A quasi-category is a simplicial set in which all inner horns have fillers.

• The boundary of a simplex is the union of its faces.

• The spine of a simplex is the union of all its generating 1-cells.

Revised on March 20, 2015 07:46:54 by Tim Porter (2.27.153.157)