Spin bordism
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Revision as of 19:43, 10 March 2010
Contents |
1 Introduction
By the Pontrjagin-Thom isomorphism the spin bordism groups of closed manifolds with spin structures are isomorphic to the homotopy groups of the Thom spectrum .
Preliminary results were by Novikov. The main calculation was achieved in [Anderson&Brown&Peterson1966] and [Anderson&Brown&Peterson1967] where it is shown that all torsion is of exponent 2, being of two types: that arising by products with a framed and that which maps monomorphically into unoriented cobordism.
2 Generators
, generated by a point.
, generated by , the circle with the "antiperiodic" spin structure.
, generated by .
.
, generated by , the Kummer surface.
.
, generated by quaternionic projective space and a generator which equals 1/4 of the square of the Kummer surface.
For a multi-index where all , we set and . By the theorem of Anderson, Brown and Peterson below there exist manifolds
- of dimension if is even,
- of dimension if is odd,
such that the characteristic numbers , and are odd. For odd, let be a spin nullbordism of , and let (using an orientation-reversing automorphism of ). If all are even, one can choose to be a product of quaternionic projective spaces.
Then a basis for is given by
- .
A basis for is given by
- with and even,
- with even,
- with odd,
- with and odd
- ,
- .
3 Invariants
The spin bordism class of a manifold is detected by -cohomology (Stiefel-Whitney) and KO-theory (Pontryagin) characteristic numbers.
A spin structure induces a KO-orientation , so that we can evaluate polynomials in the KO-Pontryagin classes to get characteristic numbersA feature of Spin manifolds is that they possess Dirac operators, . The (Clifford-linear) Dirac operator can be considered as a representative of the fundamental class , where is the dimension of , see [Atiyah1970] and [Higson&Roe2000]. The characteristic numbers above can then be defined as the indices of the (Clifford-linear) Dirac operators obtained by twisting with the corresponding vector bundles. The easiest case is the non-twisted one: is the trivial bundle, and taking the index of the Dirac operator defines an element of when is n-dimensional. This gives rises to a ring homomorphism often called the -invariant:
See [Lawson&Michelsohn1989].
4 Classification
After inverting 2 the map of Thom spectra becomes a homotopy equivalence. Thus there is no odd torsion in the spin cobordism groups, and all summands are in degrees divisible by 4.
Using the Thom isomorphism we get for each multi-index a map , for which Anderson,Brown and Peterson show that it factorizes through if is even and if is odd. Hence the corresponding characteristic numbers vanish for manifolds of smaller dimension.
Theorem [Anderson&Brown&Peterson1967] 4.1. There are classes such that there is a 2-local homotopy equivalence
From this one can compute the additive structure completely. Concerning the multiplicative structure, is the subring of an integral polynomial ring on classes (dimension 4i) consisting of all classes of dimension a multiple of 8 and twice the classes whose dimension is not a multiple of 8 [Stong1968]. Anderson, Brown and Peterson determine the structure of modulo the ideal consisting of torsion mapping monomorphically into unoriented cobordism. According to [Laures2003], the multiplicative structure of this ideal is still not completely known.
5 Further topics
5.1 Relationship with other bordism groups
- Framed bordism the image of is 0 except in unless or when it is and detected by the -invariant.
- Oriented bordism: the kernel of lies in dimensions and only and is the part generated by framed manifolds: it is the ideal generated by the non-trivial class of .
- Unoriented bordism: the image of is all bordism classes for which the characteristic numbers divisible by and are zero.
6 References
- [Anderson&Brown&Peterson1966] D. W. Anderson, E. H. Brown and F. P. Peterson, Spin cobordism, Bull. Amer. Math. Soc. 72 (1966), 256–260. MR0190939 (32 #8349) Zbl 0156.21605
- [Anderson&Brown&Peterson1967] D. W. Anderson, E. H. Brown and F. P. Peterson, The structure of the Spin cobordism ring, Ann. of Math. (2) 86 (1967), 271–298. MR0219077 (36 #2160) Zbl 0156.21605
- [Atiyah1970] M. F. Atiyah, Global theory of elliptic operators, (1970), 21–30. MR0266247 (42 #1154) Zbl 0193.43601
- [Higson&Roe2000] N. Higson and J. Roe, Analytic -homology, Oxford University Press, Oxford, 2000. MR1817560 (2002c:58036) Zbl 1146.19004
- [Laures2003] G. Laures, An splitting of spin bordism, Amer. J. Math. 125 (2003), no.5, 977–1027. MR2004426 (2004g:55007) Zbl 1058.55001
- [Lawson&Michelsohn1989] H. B. Lawson and M. Michelsohn, Spin geometry, Princeton University Press, Princeton, NJ, 1989. MR1031992 (91g:53001) Zbl 0801.58017
- [Stong1968] R. E. Stong, Notes on cobordism theory, Princeton University Press, Princeton, N.J., 1968. MR0248858 (40 #2108) Zbl 0277.57010
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