Borel–Moore homology
In topology, Borel−Moore homology or homology with closed support is a homology theory for locally compact spaces, introduced by Borel and Moore (1960).
For reasonable compact spaces, Borel−Moore homology coincides with the usual singular homology. For non-compact spaces, each theory has its own advantages. In particular, a closed oriented submanifold defines a class in Borel–Moore homology, but not in ordinary homology unless the submanifold is compact.
Note: Borel equivariant cohomology is an invariant of spaces with an action of a group G; it is defined as H*G(X) = H*((EG × X)/G). That is not related to the subject of this article.
Definition
There are several ways to define Borel−Moore homology. They all coincide for reasonable spaces such as manifolds and locally finite CW complexes.
Definition via sheaf cohomology
For any locally compact space X, Borel–Moore homology with integral coefficients is defined as the cohomology of the dual of the chain complex which computes sheaf cohomology with compact support.[1] As a result, there is a short exact sequence analogous to the universal coefficient theorem:
In what follows, the coefficients Z are not written.
Definition via locally finite chains
The singular homology of a topological space X is defined as the homology of the chain complex of singular chains, that is, finite linear combinations of continuous maps from the simplex to X. The Borel−Moore homology of a reasonable locally compact space X, on the other hand, is isomorphic to the homology of the chain complex of locally finite singular chains. "Reasonable" means here that X is locally contractible, σ-compact, and of finite dimension.[2]
In more detail, let CiBM(X) be the abelian group of formal (infinite) sums
where σ runs over the set of all continuous maps from the standard i-simplex Δi to X and each aσ is an integer, such that for each compact subset S of X, only finitely many maps σ whose image meets S have nonzero coefficient in u. Then the usual definition of the boundary ∂ of a singular chain makes these abelian groups into a chain complex:
The Borel−Moore homology groups HiBM(X) are the homology groups of this chain complex. That is,
If X is compact, then every locally finite chain is in fact finite. So, given that X is "reasonable" in the sense above, Borel−Moore homology HiBM(X) coincides with the usual singular homology Hi(X) for X compact.
Definition via compactifications
Suppose that X is homeomorphic to the complement of a closed subcomplex S in a finite CW complex Y. Then Borel–Moore homology HiBM(X) is isomorphic to the relative homology Hi(Y, S). Under the same assumption on X, the one-point compactification of X is homeomorphic to a finite CW complex. As a result, Borel–Moore homology can be viewed as the relative homology of the one-point compactification with respect to the added point.
Definition via Poincaré duality
Let X be any locally compact space with a closed embedding into an oriented manifold M of dimension m. Then
where in the right hand side, relative cohomology is meant.[3]
Definition via the dualizing complex
For any locally compact space X of finite dimension, let DX be the dualizing complex of X. Then
where in the right hand side, hypercohomology is meant.[4]
Properties
- Borel−Moore homology is a covariant functor with respect to proper maps. That is, a proper map f: X → Y induces a pushforward homomorphism HiBM(X) → HiBM(Y) for all integers i. In contrast to ordinary homology, there is no pushforward on Borel−Moore homology for an arbitrary continuous map f. As a counterexample, one can consider the non-proper inclusion R2 − 0 → R2.
- Borel−Moore homology is a contravariant functor with respect to inclusions of open subsets. That is, for U open in X, there is a natural pullback or restriction homomorphism HiBM(X) → HiBM(U).
- For any locally compact space X and any closed subset F, with U = X − F the complement, there is a long exact localization sequence:[5]
- Borel−Moore homology is homotopy invariant in the sense that for any space X, there is an isomorphism HiBM(X) → Hi+1BM(X × R). The shift in dimension means that Borel−Moore homology is not homotopy invariant in the naive sense. For example, the Borel−Moore homology of Euclidean space Rn is isomorphic to Z in degree n and is otherwise zero.
- Poincaré duality extends to non-compact manifolds using Borel–Moore homology. Namely, for an oriented n-manifold X, Poincaré duality is an isomorphism from singular cohomology to Borel−Moore homology,
- for all integers i. A different version of Poincaré duality for non-compact manifolds is the isomorphism from cohomology with compact support to usual homology:
- A key advantage of Borel−Moore homology is that every oriented manifold M of dimension n (in particular, every smooth complex algebraic variety), not necessarily compact, has a fundamental class [M] ∈ HnBM(M). If the manifold M has a triangulation, then its fundamental class is represented by the sum of all the top dimensional simplices. In fact, in Borel−Moore homology, one can define a fundamental class for arbitrary (possibly singular) complex varieties. In this case the set of smooth points Mreg ⊂ M has complement of (real) codimension at least 2, and by the long exact sequence above the top dimensional homologies of M and Mreg are canonically isomorphic. The fundamental class of M is then defined to be the fundamental class of Mreg.[6]
Examples
Compact Spaces
Given a compact topological space its Borel-Moore homology agrees with its standard homology; that is,
Real line
The first non-trivial calculation of Borel-Moore homology is of the real line. First observe that any -chain is cohomologous to . Since this reduces to the case of a point , notice that we can take the Borel-Moore chain
since the boundary of this chain is and the non-existent point at infinity, the point is cohomologous to zero. Now, we can take the Borel-Moore chain
which has not boundary, hence is a homology class. This shows that
Real n-space
The previous computation can be generalized to the case . We get
Infinite Cylinder
Using the Kunneth decomposition, we can see that the infinite cylinder has homology
Plane with Points Removed
Let have -distinct points removed. Notice the previous computation with the fact that Borel-Moore homology is an isomorphism invariant gives this computation for the case . In general, we will find a -class corresponding to a loop around a point, and the fundamental class in .
Double Cone
Consider the double cone . If we take then the long exact sequence shows
Notes
- ↑ B. Iversen. Cohomology of sheaves. Section IX.1.
- ↑ G. Bredon. Sheaf theory. Corollary V.12.21.
- ↑ B. Iversen. Cohomology of sheaves. Theorem IX.4.7.
- ↑ B. Iversen. Cohomology of sheaves. Equation IX.4.1.
- ↑ B. Iversen. Cohomology of sheaves. Equation IX.2.1.
- ↑ W. Fulton. Intersection theory. Lemma 19.1.1.
References
- Borel, Armand; Moore, John C. (1960), "Homology theory for locally compact spaces", The Michigan Mathematical Journal, 7: 137–159, ISSN 0026-2285, MR 0131271, doi:10.1307/mmj/1028998385
- Bredon, Glen E. (1997), Sheaf Theory (2nd ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-387-94905-5, MR 1481706
- Fulton, William (1998), Intersection Theory (2nd ed.), Berlin, New York: Springer-Verlag, ISBN 978-3-540-62046-4, MR 1644323
- Iversen, Birger (1986), Cohomology of Sheaves, Berlin: Springer-Verlag, ISBN 3-540-16389-1, MR 0842190