Nerve complex

In topology, the nerve complex of a set family is an abstract complex that records the pattern of intersections between the sets in the family. It was introduced by Pavel Alexandrov and now has many variants and generalisations, among them the Čech nerve of a cover, which in turn is generalised by hypercoverings. It captures many of the interesting topological properties in an algorithmic or combinatorial way.

Basic definition

Let be a set of indices and be a family of sets . The nerve of is a set of finite subsets of the index set '. It contains all finite subsets such that the intersection of the whose subindices are in is non-empty:'

In Alexandrov's original definition, the sets are open subsets of some topological space .

The set may contain singletons (elements such that is non-empty), pairs (pairs of elements such that ), triplets, and so on. If , then any subset of is also in , making an abstract simplicial complex. Hence N(C) is often called the nerve complex of .

Examples

1. Let X be the circle and , where is an arc covering the upper half of and is an arc covering its lower half, with some overlap at both sides (they must overlap at both sides in order to cover all of ). Then , which is an abstract 1-simplex.
2. Let X be the circle and , where each is an arc covering one third of , with some overlap with the adjacent . Then . Note that {1,2,3} is not in since the common intersection of all three sets is empty; so is an unfilled triangle.
3. Let be a finite set of points in general position (no four points are cocircular) in , and let be the set of cells of the Voronoi diagram of . Then the nerve is the Delaunay triangulation of . Note that the Voronoi cells are closed sets, so this is a nerve of a closed cover rather than an open one; however, the definition of the nerve applies to arbitrary set families.

The Čech nerve

Given an open cover of a topological space , or more generally a cover in a site, we can consider the pairwise fibre products , which in the case of a topological space are precisely the intersections . The collection of all such intersections can be referred to as and the triple intersections as .

By considering the natural maps and , we can construct a simplicial object defined by , n-fold fibre product. This is the Čech nerve.

By taking connected components we get a simplicial set, which we can realise topologically: .

Nerve theorems

The nerve complex is a simple combinatorial object. Often, it is much simpler than the underlying topological space (the union of the sets in ). Therefore, a natural question is whether the topology of is equivalent to the topology of .

In general, this need not be the case. For example, one can cover any n-sphere with two contractible sets and that have a non-empty intersection, as in example 1 above. In this case, is an abstract 1-simplex, which is similar to a line but not to a sphere.

However, in some cases does reflect the topology of X. For example, if a circle is covered by three open arcs, intersecting in pairs as in Example 2 above, then is a 2-simplex (without its interior) and it is homotopy-equivalent to the original circle.

A nerve theorem (or nerve lemma) is a theorem that gives sufficient conditions on C guaranteeing that reflects, in some sense, the topology of '. A functorial nerve theorem is a nerve theorem that is functorial in an appropriate sense, which is, for example, crucial in topological data analysis.

The basic nerve theorem of Jean Leray says that, if any intersection of sets in is contractible (equivalently: for each finite the set is either empty or contractible; equivalently: C is a good open cover), then is homotopy-equivalent to '.

Borsuk's nerve theorem

There is a discrete version, which is attributed to Borsuk.' Let K₁,...,K_n be abstract simplicial complexes, and denote their union by K. Let U_i = ||K_i|| = the geometric realization of K_i, and denote the nerve of {U₁, ... , U_n } by N.