Poincaré duality

From Wikipedia, the free encyclopedia

In mathematics, the Poincaré duality theorem, named after Henri Poincaré, is a basic result on the structure of the homology and cohomology groups of manifolds. It states that if M is an n-dimensional compact oriented manifold, then the kth cohomology group of M is isomorphic to the (n − k)th homology group of M, for all integers k. It further states that if mod 2 homology and cohomology is used, then the assumption of orientability can be dropped.

[edit] History

A form of Poincaré duality was first stated, without proof, by Henri Poincaré in 1893. It was stated in terms of Betti numbers: The kth and (n − k) th Betti numbers of a closed (i.e. compact and without boundary) orientable n-manifold are equal. The cohomology concept was at that time about 40 years from being clarified. In his 1895 paper Analysis Situs, Poincaré tried to prove the theorem using topological intersection theory, which he had invented. Criticism of his work by Poul Heegaard led him to realize that his proof was seriously flawed. In the first two complements to Analysis Situs, Poincaré gave a new proof in terms of dual triangulations.

Poincaré duality did not take on its modern form until the advent of cohomology in the 1930s, when Eduard Čech and Hassler Whitney invented the cup and cap products and formulated Poincaré duality in these new terms.

[edit] Dual cell structures

Given a triangulated manifold, there is a corresponding dual polyhedral decomposition. The dual polyhedral decomposition is a cell decomposition of the manifold such that the k-cells of the dual polyhedral decomposition are in bijective correspondence with the n-k-cells of the triangulation, generalising the notion of dual polyhedra.

$\cup_{S \in T} \Delta \cap DS$ -- a picture of all the dual-cells intersect a top-dimensional simplex.

Precisely, let T be a triangulation of an n-manifold M. Let S be a simplex of T. We denote the dual cell (to be defined precisely) corresponding to S by DS. Let $Δ$ be a top-dimensional simplex of T containing S. So we can think of S as a subset of the vertices of $Δ$ . Then $\Delta \cap DS$ is defined to be the convex hull (in $Δ$ ) of the barycentres of all subsets of the vertices of $Δ$ that contain $S$ . One can check that if S is i-dimensional, then DS is an n-i-dimensional cell. Moreover, the dual cells to T form a CW-decomposition of M, and the only n-i-dimensional dual cell that intersects an i-cell S is DS. This gives a natural map from the chain complex $C i M$ from the triangulation T to the dual cochain complex $C n - i M$ from the dual polyhedral/CW decomposition of the manifold, by considering how the cells intersect. The fact that this is an isomorphism of chain complexes is a proof of Poincare Duality.

[edit] Modern formulation

The modern statement of the Poincaré duality theorem is in terms of homology and cohomology: if M is a closed oriented n-manifold, and k is an integer, then there is a canonically defined isomorphism from the k-th homology group H_k(M) to the (n − k)th cohomology group H^n − k(M). (Here, homology and cohomology is taken with coefficients in the ring of integers, but the isomorphism holds for any coefficient ring.) Specifically, one maps an element of H^k(M) to its cap product with a fundamental class of M, which will exist for oriented M.

For non-compact oriented manifolds, one has to replace cohomology by cohomology with compact support.

Homology and cohomology groups are defined to be zero for negative degrees, so Poincaré duality in particular implies that the homology and cohomology groups of orientable closed n-manifolds are zero for degrees bigger than n.

[edit] Naturality

Note that H^k is a contravariant functor while H_n − k is covariant. The family of isomorphisms

D_M : H^k(M) → H_{n − k}(M)

is natural in the following sense: if

f : M → N

is a continuous map between two oriented n-manifolds which is compatible with orientation, i.e. which maps the fundamental class of M to the fundamental class of N, then

D_N = f_∗ D_M f^∗,

where f_∗ and f^∗ are the maps induced by f in homology and cohomology, respectively.

[edit] Bilinear pairings formulation

Assuming M is compact boundaryless and orientable, let $τ H i M$ denote the torsion subgroup of $H i M$ and let $f H i M = H i M / τ H i M$ be the free part -- all homology groups taken with integer coefficients in this section. Then there are bilinear maps which are duality pairings

$fH_i M \otimes fH_{n-i} M \to \Bbb Z$

and

$\tau H_i M \otimes \tau H_{n-i-1} M \to \Bbb Q / \Bbb Z.$

(Here $\Bbb Q / \Bbb Z$ is the quotient of the rationals by the integers, taken as an additive group.

The first form is typically called the intersection product and the 2nd the torsion linking form. Assuming the manifold M is smooth, the intersection product is computed by perturbing the homology classes to be transverse and computing their oriented intersection number. For the torsion linking form, one computes the pairing of x and y by realizing nx as the boundary of some class z. The form is the fraction with numerator the transverse intersection number of z with y and denominator n.

The statement that the pairings are duality pairings means that the adjoint maps

$fH_i M \to Hom_{\Bbb Z}(fH_{n-i} M,\Bbb Z)$

and

$\tau H_i M \to Hom_{\Bbb Z}(\tau H_{n-i-1} M, \Bbb Q/\Bbb Z)$

are isomorphisms of groups.

This result is an application of Poincare Duality $H_i M \simeq H^{n-i} M$ together with the Universal coefficient theorem which gives an identification $fH^{n-i} M \equiv Hom(H_{n-i} M; \mathbb Z)$ and $\tau H^{n-i} M \equiv Ext(H_{n-i-1} M; \mathbb Z) \equiv Hom(\tau H_{n-i-1} M; \mathbb Q/\mathbb Z)$ . Thus, Poincaré duality says that $f H i M$ and $f H n - i M$ are isomorphic, although there is no natural map giving the isomorphism, and similarly $τ H i M$ and $τ H n - i - 1 M$ are also isomorphic, though not naturally.

This approach to Poincaré duality was used by Przytycki and Yasuhara to give an elementary homotopy and diffeomorphism classification of 3-dimensional lens spaces. ^[1]

[edit] Generalizations and related results

The Poincaré-Lefschetz duality theorem is a generalisation for manifolds with boundary. In the non-orientable case, taking into account the sheaf of local orientations, one can give a statement that is independent of orientability.

Blanchfield duality is a version of Poincaré duality which provides an isomorphism between the homology of an abelian covering space of a manifold and the corresponding cohomology with compact supports. It is used to get basic structural results about the Alexander module and can be used to define the signatures of a knot.

With the development of homology theory to include K-theory and other extraordinary theories from about 1955, it was realised that the homology H_* could be replaced by other theories, once the products on manifolds were constructed; and there are now textbook treatments in generality.

Verdier duality is the appropriate generalization to (possibly singular) geometric objects, such as analytic spaces or schemes, while intersection homology was developed R. MacPherson and M. Goresky for stratified spaces, such as real or complex algebraic varieties, precisely so as to generalise Poincaré duality to such stratified spaces.

There are many other forms of geometric duality in algebraic topology, including Lefschetz duality, Alexander duality, Hodge duality, and S-duality (homotopy theory).

[edit] See also

[edit] References

^ Przytycki, Yasuhara. Symmetry of Links and Classification of Lens Spaces. Geom. Ded. Vol 98. No. 1. (2003)

[edit] Bibliography

R.C. Blanchfield, Intersection theory of manifolds with operators with applications to knot theory, Annals of Math, 65 (1957), 340--356.

Griffiths, Phillip; Harris, Joseph (1994), Principles of algebraic geometry, Wiley Classics Library, New York: Wiley, MR 1288523, ISBN 978-0-471-05059-9

[0] Przytycki, Yasuhara. Symmetry of Links and Classification of Lens Spaces. Geom. Ded. Vol 98. No. 1. (2003)

[1]

Poincaré duality

From Wikipedia, the free encyclopedia

Contents

[edit] History

[edit] Dual cell structures

[edit] Modern formulation

[edit] Naturality

[edit] Bilinear pairings formulation

[edit] Generalizations and related results

[edit] See also

[edit] References

[edit] Bibliography

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