4-manifolds: 1-connected
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w_2(TS_d) = d h \ (\text{mod} \ 2) , | w_2(TS_d) = d h \ (\text{mod} \ 2) , | ||
$$ | $$ | ||
− | and because the inclusion $S_d \hookrightarrow \mathbb{CP}^3$ yields an injective restriction map in second cohomology with $\Z/2$ coefficients because of the hypersection theorem. | + | and because the inclusion $S_d \hookrightarrow \mathbb{CP}^3$ yields an injective restriction map in second cohomology with $\mathbb{Z}/2$ coefficients because of the hypersection theorem. |
A particularly interesting special case is that of $d=4$. The surface $S_4$ is a K3 surface. It is spin, has signature $-16$, and has $b_2=22$. By the classification results of indefinite [[Intersection_forms|intersection forms]] we know that the intersection form of $S_4$ is given by | A particularly interesting special case is that of $d=4$. The surface $S_4$ is a K3 surface. It is spin, has signature $-16$, and has $b_2=22$. By the classification results of indefinite [[Intersection_forms|intersection forms]] we know that the intersection form of $S_4$ is given by | ||
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$$ | $$ | ||
+ | Blowing up the surface $S_4$ yields the manifold $S_4 \# \overline{\mathbb{CP}^2}$ which now has an odd intersection form given by | ||
+ | $$ | ||
+ | q_{S_4 \# \overline{\mathbb{CP}^2}} = 3 (\ +1\ ) \oplus 20 (\ -1\ ), | ||
+ | $$ | ||
+ | the same form as that of the 4-manifold $3\mathbb{CP}^2} \# 20 \overline{\mathbb{CP}^2}$. Below we shall see that these two 4-manifolds are not diffeomorphic because they have different Seiberg-Witten invariants. | ||
</wikitex> | </wikitex> | ||
=== Elliptic surfaces === | === Elliptic surfaces === |
Revision as of 16:40, 8 June 2010
This page has not been refereed. The information given here might be incomplete or provisional. |
Contents |
1 Introduction
Any finitely presentable group may occur as the fundamental group of a smooth closed 4-manifold. On the other hand, the class of simply connected (topological or smooth) 4-manifolds still appears to be quite rich, so it appears reasonable to consider the classification of simply connected 4-manifolds in particular.
It appears that the intersection form is the main algebro-topological invariant of simply-connected 4-manifolds.
Technical remark: When we mention the term 4-manifold without the explicit mention of topological or smooth we shall mean the larger class of topological 4-manifolds.
2 Construction and examples, their intersection forms
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2.1 First examples
The first examples that come to one's mind are the 4-sphere , the complex projective space , the complex projective space with its opposite (non-complex) orientation , the product , various connected sums of these, and in particular .
The intersection form of the 4-sphere is the "empty form" of rank 0. The intersection forms of the others are given by
The manifolds and both have indefinite intersection forms of same rank and signature, but of different type. Therefore they are not homotopy-equvialent.
2.2 Hypersurfaces in CP3
For an integer we define a subset of by the formula
It is easy to check that in each chart of the is cut out transversally by the homogeneous polynomial of degree . Therefore, is a submanifold, in fact, an algebraic hypersurface. This is a special case of a complete intersection.
By the Lefschetz hyperplane section theorem the hypersurface is simply connected. Its intersection form may be computed as follows: First one computes the Chern classes of . Evaluating the second Chern class on the fundamental class yields the Euler characteristic and therefore the rank of . Likewise, by computing the Pontryagin class and using the Hirzebruch signature theorem, which states that the signature of a 4-manifold is given by one third of the evaluation of the Pontryagin class on the fundamental cycle, one computes the signature of . Whether the intersection form is even or odd may be seen from the second Stiefel-Whitney class .
There are three facts that we need to use:
- The normal bundle of in is given by , where is the line bundle dual to the hyperplane . Its first Chern class generates the cohomology ring of .
- The hypersurface is Poincaré dual to the class , or equivalently .
- The total Chern class of is given by
We can now apply the Whitney sum formula for the total Chern class to the splitting ,
which we can invert to obtain the formula
and in particular
We compute the Euler characteristic by the above mentioned fact. The first Pontryagin class yields the signature . We summarise
Furthermore is spin if and only if is even. This is because we have
and because the inclusion yields an injective restriction map in second cohomology with coefficients because of the hypersection theorem.
A particularly interesting special case is that of . The surface is a K3 surface. It is spin, has signature , and has . By the classification results of indefinite intersection forms we know that the intersection form of is given by
Blowing up the surface yields the manifold which now has an odd intersection form given by
Tex syntax error. Below we shall see that these two 4-manifolds are not diffeomorphic because they have different Seiberg-Witten invariants.
2.3 Elliptic surfaces
2.4 Branched coverings
2.5 The E8 manifold
3 Invariants
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4 Topological classification
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5 Non-existence results for smooth 4-manifolds
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6 The Seiberg-Witten invariants
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7 Failure of the h-cobordism theorem
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8 Further discussion
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