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^[1] 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.^[2]

Constructing the nerve of an open good cover containing 3 sets in the plane.

Basic definition

Let ${\displaystyle I}$  be a set of indices and ${\displaystyle C}$  be a family of sets ${\displaystyle (U_{i})_{i\in I}}$ . The nerve of ${\displaystyle C}$  is a set of finite subsets of the index set ${\displaystyle I}$ . It contains all finite subsets ${\displaystyle J\subseteq I}$  such that the intersection of the ${\displaystyle U_{i}}$  whose subindices are in ${\displaystyle J}$  is non-empty:^[3]: 81

${\displaystyle N(C):={\bigg \{}J\subseteq I:\bigcap _{j\in J}U_{j}\neq \varnothing ,J{\text{ finite set}}{\bigg \}}.}$ 

In Alexandrov's original definition, the sets ${\displaystyle (U_{i})_{i\in I}}$  are open subsets of some topological space ${\displaystyle X}$ .

The set ${\displaystyle N(C)}$  may contain singletons (elements ${\displaystyle i\in I}$  such that ${\displaystyle U_{i}}$  is non-empty), pairs (pairs of elements ${\displaystyle i,j\in I}$  such that ${\displaystyle U_{i}\cap U_{j}\neq \emptyset }$ ), triplets, and so on. If ${\displaystyle J\in N(C)}$ , then any subset of ${\displaystyle J}$  is also in ${\displaystyle N(C)}$ , making ${\displaystyle N(C)}$  an abstract simplicial complex. Hence N(C) is often called the nerve complex of ${\displaystyle C}$ .

Examples

1. Let X be the circle ${\displaystyle S^{1}}$  and ${\displaystyle C=\{U_{1},U_{2}\}}$ , where ${\displaystyle U_{1}}$  is an arc covering the upper half of ${\displaystyle S^{1}}$  and ${\displaystyle U_{2}}$  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 ${\displaystyle S^{1}}$ ). Then ${\displaystyle N(C)=\{\{1\},\{2\},\{1,2\}\}}$ , which is an abstract 1-simplex.
2. Let X be the circle ${\displaystyle S^{1}}$  and ${\displaystyle C=\{U_{1},U_{2},U_{3}\}}$ , where each ${\displaystyle U_{i}}$  is an arc covering one third of ${\displaystyle S^{1}}$ , with some overlap with the adjacent ${\displaystyle U_{i}}$ . Then ${\displaystyle N(C)=\{\{1\},\{2\},\{3\},\{1,2\},\{2,3\},\{3,1\}\}}$ . Note that {1,2,3} is not in ${\displaystyle N(C)}$  since the common intersection of all three sets is empty; so ${\displaystyle N(C)}$  is an unfilled triangle.

The Čech nerve

Given an open cover ${\displaystyle C=\{U_{i}:i\in I\}}$  of a topological space ${\displaystyle X}$ , or more generally a cover in a site, we can consider the pairwise fibre products ${\displaystyle U_{ij}=U_{i}\times _{X}U_{j}}$ , which in the case of a topological space are precisely the intersections ${\displaystyle U_{i}\cap U_{j}}$ . The collection of all such intersections can be referred to as ${\displaystyle C\times _{X}C}$  and the triple intersections as ${\displaystyle C\times _{X}C\times _{X}C}$ .

By considering the natural maps ${\displaystyle U_{ij}\to U_{i}}$  and ${\displaystyle U_{i}\to U_{ii}}$ , we can construct a simplicial object ${\displaystyle S(C)_{\bullet }}$  defined by ${\displaystyle S(C)_{n}=C\times _{X}\cdots \times _{X}C}$ , n-fold fibre product. This is the Čech nerve.^[4]

By taking connected components we get a simplicial set, which we can realise topologically: ${\displaystyle |S(\pi _{0}(C))|}$ .

Nerve theorems

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

In general, this need not be the case. For example, one can cover any n-sphere with two contractible sets ${\displaystyle U_{1}}$  and ${\displaystyle U_{2}}$  that have a non-empty intersection, as in example 1 above. In this case, ${\displaystyle N(C)}$  is an abstract 1-simplex, which is similar to a line but not to a sphere.

However, in some cases ${\displaystyle N(C)}$  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 ${\displaystyle N(C)}$  is a 2-simplex (without its interior) and it is homotopy-equivalent to the original circle.^[5]

A nerve theorem (or nerve lemma) is a theorem that gives sufficient conditions on C guaranteeing that ${\displaystyle N(C)}$  reflects, in some sense, the topology of ${\displaystyle \bigcup C}$ . A functorial nerve theorem is a nerve theorem that is functorial in an approriate sense, which is, for example, crucial in topological data analysis.^[6]

Leray's nerve theorem

The basic nerve theorem of Jean Leray says that, if any intersection of sets in ${\displaystyle N(C)}$  is contractible (equivalently: for each finite ${\displaystyle J\subset I}$  the set ${\displaystyle \bigcap _{i\in J}U_{i}}$  is either empty or contractible; equivalently: C is a good open cover), then ${\displaystyle N(C)}$  is homotopy-equivalent to ${\displaystyle \bigcup C}$ .

Borsuk's nerve theorem

There is a discrete version, which is attributed to Borsuk.^{[7][3]: 81, Thm.4.4.4} 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.

If, for each nonempty ${\displaystyle J\subset I}$ , the intersection ${\displaystyle \bigcap _{i\in J}U_{i}}$  is either empty or contractible, then N is homotopy-equivalent to K.

A stronger theorem was proved by Anders Bjorner.^[8] if, for each nonempty ${\displaystyle J\subset I}$ , the intersection ${\displaystyle \bigcap _{i\in J}U_{i}}$  is either empty or (k-|J|+1)-connected, then for every j ≤ k, the j-th homotopy group of N is isomorphic to the j-th homotopy group of K. In particular, N is k-connected if-and-only-if K is k-connected.

Čech nerve theorem

Another nerve theorem relates to the Čech nerve above: if ${\displaystyle X}$  is compact and all intersections of sets in C are contractible or empty, then the space ${\displaystyle |S(\pi _{0}(C))|}$  is homotopy-equivalent to ${\displaystyle X}$ .^[9]

Homological nerve theorem

The following nerve theorem uses the homology groups of intersections of sets in the cover.^[10] For each finite ${\displaystyle J\subset I}$ , denote ${\displaystyle H_{J,j}:={\tilde {H}}_{j}(\bigcap _{i\in J}U_{i})=}$  the j-th reduced homology group of ${\displaystyle \bigcap _{i\in J}U_{i}}$ .

If H_J,j is the trivial group for all J in the k-skeleton of N(C) and for all j in {0, ..., k-dim(J)}, then N(C) is "homology-equivalent" to X in the following sense:

• ${\displaystyle {\tilde {H}}_{j}(N(C))\cong {\tilde {H}}_{j}(X)}$  for all j in {0, ..., k};
• if ${\displaystyle {\tilde {H}}_{k+1}(N(C))\not \cong 0}$  then ${\displaystyle {\tilde {H}}_{k+1}(X)\not \cong 0}$  .

See also

• Hypercovering

References

1. Aleksandroff, P. S. (1928). "Über den allgemeinen Dimensionsbegriff und seine Beziehungen zur elementaren geometrischen Anschauung". Mathematische Annalen. 98: 617–635. doi:10.1007/BF01451612. S2CID 119590045.
2. Eilenberg, Samuel; Steenrod, Norman (1952-12-31). Foundations of Algebraic Topology. Princeton: Princeton University Press. doi:10.1515/9781400877492. ISBN 978-1-4008-7749-2.
3. Matoušek, Jiří (2007). Using the Borsuk-Ulam Theorem: Lectures on Topological Methods in Combinatorics and Geometry (2nd ed.). Berlin-Heidelberg: Springer-Verlag. ISBN 978-3-540-00362-5. Written in cooperation with Anders Björner and Günter M. Ziegler , Section 4.3
4. "Čech nerve in nLab". ncatlab.org. Retrieved 2020-08-07.
5. Artin, Michael; Mazur, Barry (1969). Etale Homotopy. Lecture Notes in Mathematics. Vol. 100. doi:10.1007/bfb0080957. ISBN 978-3-540-04619-6. ISSN 0075-8434.
6. Bauer, Ulrich; Kerber, Michael; Roll, Fabian; Rolle, Alexander (2023). "A unified view on the functorial nerve theorem and its variations". Expositiones Mathematicae. arXiv:2203.03571. doi:10.1016/j.exmath.2023.04.005.
7. Borsuk, Karol (1948). "On the imbedding of systems of compacta in simplicial complexes". Fundamenta Mathematicae. 35 (1): 217–234. doi:10.4064/fm-35-1-217-234. ISSN 0016-2736.
8. Björner, Anders (2003-04-01). "Nerves, fibers and homotopy groups". Journal of Combinatorial Theory. Series A. 102 (1): 88–93. doi:10.1016/S0097-3165(03)00015-3. ISSN 0097-3165.
9. Nerve theorem at the nLab
10. Meshulam, Roy (2001-01-01). "The Clique Complex and Hypergraph Matching". Combinatorica. 21 (1): 89–94. doi:10.1007/s004930170006. ISSN 1439-6912. S2CID 207006642.