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 P. S. 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.

Modern statement

Let "X" be a compact, locally contractible subspace of Euclidean space "E" of dimension "n". Let "Y" be the complement of "X" in "E". Then if "H" stands for reduced homology or reduced cohomology, with coefficients in a given abelian group, there is an isomorphism between

:"H"_"q"("X")

and

:"H"^{"n" − "q" − 1}("Y").

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

The statement above is from Spanier, "Algebraic Topology" (p. 296). To go back to Alexander's original work, it is assumed first that "X" is a simplicial complex, and secondly that complements are taken in the "n"-sphere, i.e. the one-point compactification of "E". (Taking out one point from the complement of the compact set makes no difference to the homotopy type, as long as we remove it far enough away from "X".)

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 doesn't 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

:1, 1, 0, 0

of the circle (up to "H"₃, since we are in the 3-sphere), then reverse as

:0, 0, 1, 1

and then shift one to the left to get

:0, 1, 1, 0

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

:0, 1, 0, 0

and gives

:0, 0, 1, 0

whence

:0, 1, 0, 0.

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

:1, 1, 0

of the circle, and therefore

:0, 1, 1

by flipping over and

:1, 1, 0

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 (Schoenflies theorem, to be accurate about what is used here). That is, the correct answer in honest Betti numbers is

:2, 0, 0.

Once more, it is the reduced Betti numbers that work out. With those, we begin with

:0, 1, 0

to finish with

:1, 0, 0.

From these two examples, therefore, Alexander's formulation can be inferred: reduced Betti numbers "b"*_"i" are related in complements by

:"b"*_"i" → "b"*_{"n" − "i" − 1}.