In mathematics, Alexander duality refers to a duality theory initiated by a result of J. W. Alexander in 1915, 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.
Let
X
Sn
Sn\setminusX
X
Sn
\tilde{H}
| n\setminus | |
| \tilde{H} | |
| q(S |
X)\cong\tilde{H}n-q-1(X)
for all
q\ge0
This is useful for computing the cohomology of knot and link complements in
S3
K\colonS1\hookrightarrowS3
L
| 3\setminus | |
| \tilde{H} | |
| q(S |
L)\cong\tilde{H}3-q-1(L)
L
| 3 | |
| \begin{align} \tilde{H} | |
| 0(S |
\setminusL)&\cong\tilde{H}2(L)=0
| 3 | |
| \\ \tilde{H} | |
| 1(S |
\setminusL)&\cong\tilde{H}1(L)=\Z ⊕
| 3 | |
| \\ \tilde{H} | |
| 2(S |
\setminusL)&\cong\tilde{H}0(L)=\Z ⊕
| 3 | |
| \\ \tilde{H} | |
| 3(S |
\setminusL)&\cong0\\ \end{align}
Let
X
V
n
X*
X
V
X
X*=\{\sigma \colon V\setminus\sigma\not\inX\}
(X*)*=X
Alexander duality implies the following combinatorial analog (for reduced homology and cohomology, with coefficients in any given abelian group):
| *) | |
| \tilde{H} | |
| q(X |
\cong\tilde{H}n-q-3(X)
q\ge0
Y\simeqSn-2
(n-2)
V
Y
n-1
|X*|
|Y|\setminus|X|
For smooth manifolds, Alexander duality is a formal consequence of Verdier duality for sheaves of abelian groups. More precisely, if we let
X
Y\subsetX
i\colonY\hookrightarrowX
k
l{F}\in\operatorname{Sh}k(Y)
k
| \vee | |
| H | |
| c(Y,l{F}) |
\cong
| n-s | |
| \operatorname{Ext} | |
| k |
(i*l{F},\omegaX[n-s])
l{F}=\underline{k}
Y
| n-s | |
| \operatorname{Ext} | |
| k |
(i*l{F},\omegaX[n-r])\cong
| n-s | |
| H | |
| Y(X,\omega |
X)
Y
X\setminusY
Y
d
Referring 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
1, 1, 0, 0
of the circle (up to
H3
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
\tilde{b}i
\tilde{b}i\to\tilde{b}n-i-1
. Allen Hatcher. 2002. Algebraic Topology. Cambridge. Cambridge University Press. 254. 0-521-79540-0.