Alexander duality


In mathematics, Alexander duality refers to a duality theory presaged by a result of 1915 by J. W. Alexander, and subsequently further developed, particularly by Pavel Alexandrov and Lev Pontryagin. It applies to the homology theory properties of the complement of a subspace X in Euclidean space, a sphere, or other manifold. It is generalized by Spanier–Whitehead duality.

Modern statement

Let be a compact, locally contractible subspace of the sphere of dimension n. Let be the complement of in. Then if stands for reduced homology or reduced cohomology, with coefficients in a given abelian group, there is an isomorphism
for all. Note that we can drop local contractibility as part of the hypothesis, if we use Čech cohomology, which is designed to deal with local pathologies.

Alexander's 1915 result

To go back to Alexander's original work, it is assumed that X is a simplicial complex.
Alexander had little of the modern apparatus, and his result was only for the Betti numbers, with coefficients taken modulo 2. What to expect comes from examples. For example the Clifford torus construction in the 3-sphere shows that the complement of a solid torus is another solid torus; which will be open if the other is closed, but this does not affect its homology. Each of the solid tori is from the homotopy point of view a circle. If we just write down the Betti numbers
of the circle, then reverse as
and then shift one to the left to get
there is a difficulty, since we are not getting what we started with. On the other hand the same procedure applied to the reduced Betti numbers, for which the initial Betti number is decremented by 1, starts with
and gives
whence
This does work out, predicting the complement's reduced Betti numbers.
The prototype here is the Jordan curve theorem, which topologically concerns the complement of a circle in the Riemann sphere. It also tells the same story. We have the honest Betti numbers
of the circle, and therefore
by flipping over and
by shifting to the left. This gives back something different from what the Jordan theorem states, which is that there are two components, each contractible. That is, the correct answer in honest Betti numbers is
Once more, it is the reduced Betti numbers that work out. With those, we begin with
to finish with
From these two examples, therefore, Alexander's formulation can be inferred: reduced Betti numbers are related in complements by