Stable classification of 4-manifolds
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In this page we report about the stable classification of closed oriented -manifolds. We will begin with a special class of closed oriented -manifolds, namely those, where the universal covering is not spinnable.
2 Construction and examples
We begin with the construction of manifolds which give many stable diffeomorphism types of -manifolds:
- , the Kummer surface.
Let be the presentation of a group . Then we build a -dimensional complex by taking a wedge of circles and attaching a -cell via each relation . Then we thicken to a smooth compact manifold with boundary in and consider its boundary denoted by . For details and why this is well defined see Thickenings. is a smooth -manifold with fundamental group and we add it to our list:
Let be a finitely presentable group. Then for each element in there is a smooth, closed, oriented manifold with signature zero and a map mapping the fundamental class to . The existence follows (for example using the Atiyah-Hirzebruch spectral sequence) from the fact that the oriented bordism groups are zero in degree , and : see Oriented bordism. This manifold is, of course, not unique. But we will see that its stable diffeomorphism class is unique, if we require that the universal covering is non spinnable. We add it to our list:
The following is a complete list of invariants for the stable classification of closed, smooth oriented -manifolds whose universal covering is not spinnable:
- The Euler characteristic
- The signature
- The fundamanetal group
- The image of the fundamental class of .
Here is a classifying map of the universal covering and is the outer automorphism group which acts on the homology of .
Theorem 4.1. Let and be -dimensional compact smooth manifolds with non spinnable universal covering. Then and are stably diffeomorphic if and only if the invariants above agree.
The different stable diffeomorphism classes of manifolds with fundamental group are given by .
The proof of this result is an easy consequence of the general stable classification theorem ([Kreck1999], Stable classification of manifolds). Namely, the normal -type is , see Stable classification of manfifolds. Thus the -bordism group is , which by the Atiyah-Hirzebruch spectral sequence is ismorphic to under the signature and the image of the fundamental class. Now the statement follows from Theorem 3.1 of Stable classification of manifolds.
5 Further discussion