Abstract simplicial complex

thumb|200px|Geometric realization of a 3-dimensional abstract simplicial complex

In combinatorics, an abstract simplicial complex (ASC), often called an abstract complex or just a complex, is a family of sets that is closed under taking subsets, i.e., every subset of a set in the family is also in the family. It is a purely combinatorial description of the geometric notion of a simplicial complex. For example, in a 2-dimensional simplicial complex, the sets in the family are the triangles (sets of size 3), their edges (sets of size 2), and their vertices (sets of size 1).

In the context of matroids and greedoids, abstract simplicial complexes are also called independence systems.

An abstract simplex can be studied algebraically by forming its Stanley–Reisner ring; this sets up a powerful relation between combinatorics and commutative algebra.

Definitions

A collection of non-empty finite subsets of a set S is called a set-family.

A set-family is called an abstract simplicial complex if, for every set in , and every non-empty subset , the set also belongs to .

The finite sets that belong to are called faces of the complex, and a face is said to belong to another face if , so the definition of an abstract simplicial complex can be restated as saying that every face of a face of a complex is itself a face of . The vertex set of is defined as , the union of all faces of . The elements of the vertex set are called the vertices of the complex. For every vertex v of , the set {v} is a face of the complex, and every face of the complex is a finite subset of the vertex set.

The maximal faces of (i.e., faces that are not subsets of any other faces) are called facets of the complex. The dimension of a face in is defined as : faces consisting of a single element are zero-dimensional, faces consisting of two elements are one-dimensional, etc. The dimension of the complex is defined as the largest dimension of any of its faces, or infinity if there is no finite bound on the dimension of the faces.

The complex is said to be finite if it has finitely many faces, or equivalently if its vertex set is finite. Also, is said to be pure if it is finite-dimensional (but not necessarily finite) and every facet has the same dimension. In other words, is pure if is finite and every face is contained in a facet of dimension .

One-dimensional abstract simplicial complexes are mathematically equivalent to simple undirected graphs: the vertex set of the complex can be viewed as the vertex set of a graph, and the two-element facets of the complex correspond to undirected edges of a graph. In this view, one-element facets of a complex correspond to isolated vertices that do not have any incident edges.

A subcomplex of is an abstract simplicial complex L such that every face of L belongs to ; that is, and L is an abstract simplicial complex. A subcomplex that consists of all of the subsets of a single face of is often called a simplex of . (However, some authors use the term "simplex" for a face or, rather ambiguously, for both a face and the subcomplex associated with a face, by analogy with the non-abstract (geometric) simplicial complex terminology. To avoid ambiguity, we do not use in this article the term "simplex" for a face in the context of abstract complexes).

The d-skeleton of is the subcomplex of consisting of all of the faces of that have dimension at most d. In particular, the 1-skeleton is called the underlying graph of . The 0-skeleton of can be identified with its vertex set, although formally it is not quite the same thing (the vertex set is a single set of all of the vertices, while the 0-skeleton is a family of single-element sets).

The link of a face in , often denoted or , is the subcomplex of defined by

:<math> \Delta/Y := \{ X\in \Delta \mid X\cap Y = \varnothing,\, X\cup Y \in \Delta \}. </math>

Note that the link of the empty set is itself.

Simplicial maps

Given two abstract simplicial complexes, and , a simplicial map is a function that maps the vertices of to the vertices of and that has the property that for any face of , the image is a face of . There is a category SCpx with abstract simplicial complexes as objects and simplicial maps as morphisms. This is equivalent to a suitable category defined using non-abstract simplicial complexes.

Moreover, the categorical point of view allows us to tighten the relation between the underlying set S of an abstract simplicial complex and the vertex set of : for the purposes of defining a category of abstract simplicial complexes, the elements of S not lying in are irrelevant. More precisely, SCpx is equivalent to the category where:

an object is a set S equipped with a collection of non-empty finite subsets that contains all singletons and such that if is in and is non-empty, then also belongs to .
a morphism from to is a function such that the image of any element of is an element of .

Geometric realization

We can associate to any abstract simplicial complex (ASC) K a topological space <math>|K|</math>, called its geometric realization. There are several ways to define <math>|K|</math>.

Geometric definition

Every geometric simplicial complex (GSC) determines an ASC:' the vertices of the ASC are the vertices of the GSC, and the faces of the ASC are the vertex-sets of the faces of the GSC. For example, consider a GSC with 4 vertices {1,2,3,4}, where the maximal faces are the triangle between {1,2,3} and the lines between {2,4} and {3,4}. Then, the corresponding ASC contains the sets {1,2,3}, {2,4}, {3,4}, and all their subsets. We say that the GSC is the geometric realization of the ASC.

Every ASC has a geometric realization. This is easy to see for a finite ASC.

Relation to other concepts

An abstract simplicial complex with an additional property called the augmentation property or the exchange property yields a matroid. The following expression shows the relations between the terms:

HYPERGRAPHS = SET-FAMILIES ⊃ INDEPENDENCE-SYSTEMS = ABSTRACT-SIMPLICIAL-COMPLEXES ⊃ MATROIDS.