Hirzebruch surfaces
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The homology of $H_n$ is trivial except in degree $0$ and $4$, where it is $\mathbb Z$ and in degree $2$ where it is isomorphic to $\Zz^2$ with basis given by the homology class represented by a fibre and the homology class of the base considered as homology class of $H_n$ using the section of points at infinity. | The homology of $H_n$ is trivial except in degree $0$ and $4$, where it is $\mathbb Z$ and in degree $2$ where it is isomorphic to $\Zz^2$ with basis given by the homology class represented by a fibre and the homology class of the base considered as homology class of $H_n$ using the section of points at infinity. | ||
− | With respect to this basis the intersection form on $H_2(H_n)$ is given by the matrix | + | With respect to this basis the [[Wikipedia:Intersection_form_(4-manifold)|intersection form]] on $H_2(H_n)$ is given by the matrix |
$$ \left( \begin{array}{cc} 0~ & 1~\\ 1~ & -n~ \end{array} \right).$$ | $$ \left( \begin{array}{cc} 0~ & 1~\\ 1~ & -n~ \end{array} \right).$$ | ||
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Contents |
1 Introduction
Hirzebruch surfaces were introduced by Hirzebruch (without that name) in his thesis [Hirzebruch1951]. They are algebraic surfaces over the complex numbers. Here we consider them as smooth manifolds. The interest in them comes from Hirzebruch's result that as complex manifolds they are pairwise distinct whereas there are only two diffeomorphism types.
2 Construction and examples
Let for some be the tautological line bundle over . Recall that if is a vector bundle over (real or complex) taking fibrewise the projective space yields a bundle , where if and only if there is a in the ground field such that , with fibres P(E_x), the associated projective bundle. If is a smooth vector bundle over a smooth manifold, then P(E) is a smooth manifold, or similarly if and are holomorphic, the total space is a complex manifold.
For define as , where is the tensor product of copies of with itself. For the bundle is by definition the trivial bundle. For we define as the corresponding construction with , the complex conjugated bundle, instead of . These are the . They come with a complex structure but we consider them as smooth manifolds.
3 Invariants
The dimension as a smooth manifold is 4.
Since the Hirzebruch surfaces are, as smooth manifolds, -bundles over they are closed and, by the orientation coming from the complex structure, oriented -dimensional manifolds. The homotopy sequence of a fibration implies that they are simply connected. Since this fibration has a section of points at infinity the homotopy groups of are given by .
The signature of is zero since the Hirzebruch surfaces are the boundary of the associated -bundle (or derive this directly from the intersection form below).
The homology of is trivial except in degree and , where it is and in degree where it is isomorphic to with basis given by the homology class represented by a fibre and the homology class of the base considered as homology class of using the section of points at infinity.
With respect to this basis the intersection form on is given by the matrix
Here we use that the the self intersection number of a class represented by an embedded oriented submanifold is the evaluation of the Euler class of the normal bundle on the fundamental class.
The Euler characteristic is .
The first Pontrjagin class of is zero (its evaluation on the fundamental class is a bordism invariant) and the second Stiefel-Whitney class evaluates trivially on the first basis element and by mod on the second. In particular is a Spin-manifold if and only if is even. If we consider Hirzebruch survfaces as complex manifolds then we have the first Chern class , which evaluates trivially on the first base element and by on the second.
4 Classification
The intersection form implies that if is diffeomorphic to , then mod . On the other hand, considered as smooth manifolds, Hirzebruch surfaces are the total spaces of the 2-sphere bundle of a 3-dimensional vector bundle over . these bundles are classified by (note that is diffeomorphic to ). Thus there are at most two diffeomorphism types of Hirzebruch surfaces and so we conclude:
where means diffeomorphic. By construction and by an easy consideration # , where # is the connected sum and is with the opposite orientation.
For more information on Hirzebruch surfaces, in particular why they are pairwise distinct as complex manifolds, see [Hirzebruch1951].
5 Further remarks
The Hirzebruch surfaces are the second stage of the so called Bott towers, which are inductively constructed starting from a point as the total space of a projective bundle associated to , where is a line bundle over a lower Bott tower (for more details see [Choi&Masuda&Suh2008]). The classification of the Bott towers up to homeomorphism or diffeomorphism is an interesting open problem. In particular one can ask wether the integral cohomology ring determines the homeomorphism or diffeomorphism type.
6 References
- [Choi&Masuda&Suh2008] Template:Choi&Masuda&Suh2008
- [Hirzebruch1951] F. Hirzebruch, Über eine Klasse von einfachzusammenhängenden komplexen Mannigfaltigkeiten, Math. Ann. 124 (1951), 77–86. MR0045384 (13,574e) Zbl 0043.30302
Please modify these headings or choose other headings according to your needs.
This page has not been refereed. The information given here might be incomplete or provisional. |
For define as , where is the tensor product of copies of with itself. For the bundle is by definition the trivial bundle. For we define as the corresponding construction with , the complex conjugated bundle, instead of . These are the . They come with a complex structure but we consider them as smooth manifolds.
3 Invariants
The dimension as a smooth manifold is 4.
Since the Hirzebruch surfaces are, as smooth manifolds, -bundles over they are closed and, by the orientation coming from the complex structure, oriented -dimensional manifolds. The homotopy sequence of a fibration implies that they are simply connected. Since this fibration has a section of points at infinity the homotopy groups of are given by .
The signature of is zero since the Hirzebruch surfaces are the boundary of the associated -bundle (or derive this directly from the intersection form below).
The homology of is trivial except in degree and , where it is and in degree where it is isomorphic to with basis given by the homology class represented by a fibre and the homology class of the base considered as homology class of using the section of points at infinity.
With respect to this basis the intersection form on is given by the matrix
Here we use that the the self intersection number of a class represented by an embedded oriented submanifold is the evaluation of the Euler class of the normal bundle on the fundamental class.
The Euler characteristic is .
The first Pontrjagin class of is zero (its evaluation on the fundamental class is a bordism invariant) and the second Stiefel-Whitney class evaluates trivially on the first basis element and by mod on the second. In particular is a Spin-manifold if and only if is even. If we consider Hirzebruch survfaces as complex manifolds then we have the first Chern class , which evaluates trivially on the first base element and by on the second.
4 Classification
The intersection form implies that if is diffeomorphic to , then mod . On the other hand, considered as smooth manifolds, Hirzebruch surfaces are the total spaces of the 2-sphere bundle of a 3-dimensional vector bundle over . these bundles are classified by (note that is diffeomorphic to ). Thus there are at most two diffeomorphism types of Hirzebruch surfaces and so we conclude:
where means diffeomorphic. By construction and by an easy consideration # , where # is the connected sum and is with the opposite orientation.
For more information on Hirzebruch surfaces, in particular why they are pairwise distinct as complex manifolds, see [Hirzebruch1951].
5 Further remarks
The Hirzebruch surfaces are the second stage of the so called Bott towers, which are inductively constructed starting from a point as the total space of a projective bundle associated to , where is a line bundle over a lower Bott tower (for more details see [Choi&Masuda&Suh2008]). The classification of the Bott towers up to homeomorphism or diffeomorphism is an interesting open problem. In particular one can ask wether the integral cohomology ring determines the homeomorphism or diffeomorphism type.
6 References
- [Choi&Masuda&Suh2008] Template:Choi&Masuda&Suh2008
- [Hirzebruch1951] F. Hirzebruch, Über eine Klasse von einfachzusammenhängenden komplexen Mannigfaltigkeiten, Math. Ann. 124 (1951), 77–86. MR0045384 (13,574e) Zbl 0043.30302
Please modify these headings or choose other headings according to your needs.
This page has not been refereed. The information given here might be incomplete or provisional. |