Bordism
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Contents |
1 Introduction
The theory of bordism is is one of the most deep and influential parts of the algebraic topology, which experienced a spectacular development in the 1960s. The main introductory reference is the monograph [Stong1968].
Basic geometric constructions of bordisms and cobordisms, as well as homotopical definitions are summarised here. For the more specific information, see B-Bordism and pages on specific bordism theories, such as unoriented, oriented and complex.
2 The bordism relation
All manifolds here are assumed to be smooth, compact and closed (without boundary), unless otherwise specified. Given two -dimensional manifolds and , a bordism between them is an -dimensional manifold with boundary, whose boundary is the disjoint union of and , that is, . If such exists, and are called bordant. The bordism relation splits manifolds into equivalence classes (see Figure), which are called bordism classes.
3 Unoriented bordism
We denote the bordism class of by , and denote by the set of bordism classes of -dimensional manifolds. Then is an abelian group with respect to the disjoint union operation: . Zero is represented by the bordism class of an empty set (which is counted as a manifold in any dimension), or by the bordism class of any manifold which bounds. We also have , so that is a 2-torsion group.
Set . The product of bordism classes, namely , makes a graded commutative ring known as the unoriented bordism ring.
For any (good) space the bordism relation can be extended to maps of -dimensional manifolds to : two maps and are bordant if there is a bordism between and and the map extends to a map . The set of bordism classes of maps forms an abelian group called the group of -dimensional unoriented bordisms of and denoted (other notations: , ).
The assignment defines a generalised homology theory, that is, satisfies the homotopy invariance, has the excision property and exact sequences of pairs. For this theory we have , and is an -module.
The Pontrjagin--Thom construction reduces the calculation of the bordism groups to a homotopical problem:
where , and is the Thom space of the universal vector -plane bundle . The cobordism groups are defined dually:
where denotes the set of homotopy classes of maps from to . The resulting generalised cohomology theory is multiplicative, which implies that is a graded commutative ring. It follows from the definitions that . The graded ring with is called the unoriented cobordism ring. It has nonzero elements only in nonpositively graded components. The bordism ring and the cobordism ring differ only by their gradings, so the notions of the "bordism class" and "cobordism class" of a manifold are interchangeable. The difference between bordism and cobordism appears only for nontrivial spaces .
4 Oriented and complex bordism
The most important examples of bordism theories arise from extending the bordism relation to manifolds endowed with some additional structure. To take account of this structure in the definition of bordism one requires that , where the structure on is induced from that on , and denotes the manifold with the opposite structure. The universal homotopical framework for geometric bordisms with additional structure is provided by the theory of B-bordisms.
The simplest additional structure is an orientation. The oriented bordism relation arises accordingly. The oriented bordism ring is defined similarly to , with the only difference that . Elements of generally do not have order 2.
Complex structure gives another important example of an additional structure on manifolds. However, a direct attempt to define the bordism relation on complex manifolds fails because the manifold is odd-dimensional and therefore cannot be complex. This can be remedied by considering stably complex (also known as weakly almost complex, stably almost complex or quasicomplex) structures.
Let denote the tangent bundle of , and the product vector bundle over . A tangential stably complex structure on is determined by a choice of an isomorphism
between the "stable" tangent bundle and a complex vector bundle over . Some of the choices of such isomorphisms are deemed to be equivalent, i.e. determining the same stably complex structures (see details in Chapters II and VII of [Stong1968]). In particular, two stably complex structures are equivalent if they differ by a trivial complex summand. A normal stably complex structure on is determined by a choice of a complex bundle structure in the normal bundle of an embedding . A tangential and normal stably complex structures on determine each other by means of the canonical isomorphism . We therefore may restrict our attention to tangential structures only.
A stably complex manifold is a pair consisting of a manifold and a stably complex structure on it. This is a generalisation to a complex and almost complex manifold (where the latter means a manifold with a choice of a complex structure on , i.e. a stably complex structure with ).
Example 4.1. Let . The standard complex structure on is equivalent to a stably complex structure determined by the isomorphism
where is the Hopf line bundle. On the other hand, the isomorphism
determines a trivial stably complex structure on~.
The bordism relation can be defined between stably complex manifolds. Like the case of unoriented bordisms, the set of bordism classes of stably complex manifolds is an Abelian group with respect to the disjoint union. This group is called the group of -dimensional complex bordisms and denoted . A zero is represented by the bordism class of any manifold which bounds and whose stable tangent bundle is trivial (and therefore isomorphic to a product complex vector bundle ). The sphere provides an example of such a manifold. The opposite element to the bordism class in the group may be represented by the same manifold with the stably complex structure determined by the isomorphism
where is given by .
An abbreviated notation for the complex bordism class will be used whenever the stably complex structure is clear from the context.
The groups of complex bordisms and cobordisms of a space are defined similarly to the [unoriented] case:
where is the Thom space of the universal complex -plane bundle . These groups are -modules and give rise to a multiplicative (co)homology theory. In particular, is a graded ring. The graded ring with is called the \emph{complex cobordism ring}; it has nontrivial elements only in nonpositively graded components.
When is a manifold the groups~ and have geometric interpretation as groups of bordisms of maps of manifolds into with certain additional structure~[quil71]. The simplest situation when certain cobordism classes can be represented by maps of manifolds is described in the following construction.
References
- [Stong1968] R. E. Stong, Notes on cobordism theory, Princeton University Press, Princeton, N.J., 1968. MR0248858 (40 #2108) Zbl 0277.57010
- [quil71] Template:Quil71
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