- Source: Universal coefficient theorem
In algebraic topology, universal coefficient theorems establish relationships between homology groups (or cohomology groups) with different coefficients. For instance, for every topological space X, its integral homology groups:
Hi(X; Z)
completely determine its homology groups with coefficients in A, for any abelian group A:
Hi(X; A)
Here Hi might be the simplicial homology, or more generally the singular homology. The usual proof of this result is a pure piece of homological algebra about chain complexes of free abelian groups. The form of the result is that other coefficients A may be used, at the cost of using a Tor functor.
For example it is common to take A to be Z/2Z, so that coefficients are modulo 2. This becomes straightforward in the absence of 2-torsion in the homology. Quite generally, the result indicates the relationship that holds between the Betti numbers bi of X and the Betti numbers bi,F with coefficients in a field F. These can differ, but only when the characteristic of F is a prime number p for which there is some p-torsion in the homology.
Statement of the homology case
Consider the tensor product of modules Hi(X; Z) ⊗ A. The theorem states there is a short exact sequence involving the Tor functor
0
→
H
i
(
X
;
Z
)
⊗
A
→
μ
H
i
(
X
;
A
)
→
Tor
1
(
H
i
−
1
(
X
;
Z
)
,
A
)
→
0.
{\displaystyle 0\to H_{i}(X;\mathbf {Z} )\otimes A\,{\overset {\mu }{\to }}\,H_{i}(X;A)\to \operatorname {Tor} _{1}(H_{i-1}(X;\mathbf {Z} ),A)\to 0.}
Furthermore, this sequence splits, though not naturally. Here μ is the map induced by the bilinear map Hi(X; Z) × A → Hi(X; A).
If the coefficient ring A is Z/pZ, this is a special case of the Bockstein spectral sequence.
Universal coefficient theorem for cohomology
Let G be a module over a principal ideal domain R (e.g., Z or a field.)
There is also a universal coefficient theorem for cohomology involving the Ext functor, which asserts that there is a natural short exact sequence
0
→
Ext
R
1
(
H
i
−
1
(
X
;
R
)
,
G
)
→
H
i
(
X
;
G
)
→
h
Hom
R
(
H
i
(
X
;
R
)
,
G
)
→
0.
{\displaystyle 0\to \operatorname {Ext} _{R}^{1}(H_{i-1}(X;R),G)\to H^{i}(X;G)\,{\overset {h}{\to }}\,\operatorname {Hom} _{R}(H_{i}(X;R),G)\to 0.}
As in the homology case, the sequence splits, though not naturally.
In fact, suppose
H
i
(
X
;
G
)
=
ker
∂
i
⊗
G
/
im
∂
i
+
1
⊗
G
{\displaystyle H_{i}(X;G)=\ker \partial _{i}\otimes G/\operatorname {im} \partial _{i+1}\otimes G}
and define:
H
∗
(
X
;
G
)
=
ker
(
Hom
(
∂
,
G
)
)
/
im
(
Hom
(
∂
,
G
)
)
.
{\displaystyle H^{*}(X;G)=\ker(\operatorname {Hom} (\partial ,G))/\operatorname {im} (\operatorname {Hom} (\partial ,G)).}
Then h above is the canonical map:
h
(
[
f
]
)
(
[
x
]
)
=
f
(
x
)
.
{\displaystyle h([f])([x])=f(x).}
An alternative point-of-view can be based on representing cohomology via Eilenberg–MacLane space where the map h takes a homotopy class of maps from X to K(G, i) to the corresponding homomorphism induced in homology. Thus, the Eilenberg–MacLane space is a weak right adjoint to the homology functor.
Example: mod 2 cohomology of the real projective space
Let X = Pn(R), the real projective space. We compute the singular cohomology of X with coefficients in G = Z/2Z using the integral homology, i.e. R = Z.
Knowing that the integer homology is given by:
H
i
(
X
;
Z
)
=
{
Z
i
=
0
or
i
=
n
odd,
Z
/
2
Z
0
<
i
<
n
,
i
odd,
0
otherwise.
{\displaystyle H_{i}(X;\mathbf {Z} )={\begin{cases}\mathbf {Z} &i=0{\text{ or }}i=n{\text{ odd,}}\\\mathbf {Z} /2\mathbf {Z} &0
We have Ext(G, G) = G, Ext(R, G) = 0, so that the above exact sequences yield
∀
i
=
0
,
…
,
n
:
H
i
(
X
;
G
)
=
G
.
{\displaystyle \forall i=0,\ldots ,n:\qquad \ H^{i}(X;G)=G.}
In fact the total cohomology ring structure is
H
∗
(
X
;
G
)
=
G
[
w
]
/
⟨
w
n
+
1
⟩
.
{\displaystyle H^{*}(X;G)=G[w]/\left\langle w^{n+1}\right\rangle .}
Corollaries
A special case of the theorem is computing integral cohomology. For a finite CW complex X, Hi(X; Z) is finitely generated, and so we have the following decomposition.
H
i
(
X
;
Z
)
≅
Z
β
i
(
X
)
⊕
T
i
,
{\displaystyle H_{i}(X;\mathbf {Z} )\cong \mathbf {Z} ^{\beta _{i}(X)}\oplus T_{i},}
where βi(X) are the Betti numbers of X and
T
i
{\displaystyle T_{i}}
is the torsion part of
H
i
{\displaystyle H_{i}}
. One may check that
Hom
(
H
i
(
X
)
,
Z
)
≅
Hom
(
Z
β
i
(
X
)
,
Z
)
⊕
Hom
(
T
i
,
Z
)
≅
Z
β
i
(
X
)
,
{\displaystyle \operatorname {Hom} (H_{i}(X),\mathbf {Z} )\cong \operatorname {Hom} (\mathbf {Z} ^{\beta _{i}(X)},\mathbf {Z} )\oplus \operatorname {Hom} (T_{i},\mathbf {Z} )\cong \mathbf {Z} ^{\beta _{i}(X)},}
and
Ext
(
H
i
(
X
)
,
Z
)
≅
Ext
(
Z
β
i
(
X
)
,
Z
)
⊕
Ext
(
T
i
,
Z
)
≅
T
i
.
{\displaystyle \operatorname {Ext} (H_{i}(X),\mathbf {Z} )\cong \operatorname {Ext} (\mathbf {Z} ^{\beta _{i}(X)},\mathbf {Z} )\oplus \operatorname {Ext} (T_{i},\mathbf {Z} )\cong T_{i}.}
This gives the following statement for integral cohomology:
H
i
(
X
;
Z
)
≅
Z
β
i
(
X
)
⊕
T
i
−
1
.
{\displaystyle H^{i}(X;\mathbf {Z} )\cong \mathbf {Z} ^{\beta _{i}(X)}\oplus T_{i-1}.}
For X an orientable, closed, and connected n-manifold, this corollary coupled with Poincaré duality gives that βi(X) = βn−i(X).
Universal coefficient spectral sequence
There is a generalization of the universal coefficient theorem for (co)homology with twisted coefficients.
For cohomology we have
E
2
p
,
q
=
E
x
t
R
q
(
H
p
(
C
∗
)
,
G
)
⇒
H
p
+
q
(
C
∗
;
G
)
{\displaystyle E_{2}^{p,q}=Ext_{R}^{q}(H_{p}(C_{*}),G)\Rightarrow H^{p+q}(C_{*};G)}
Where
R
{\displaystyle R}
is a ring with unit,
C
∗
{\displaystyle C_{*}}
is a chain complex of free modules over
R
{\displaystyle R}
,
G
{\displaystyle G}
is any
(
R
,
S
)
{\displaystyle (R,S)}
-bimodule for some ring with a unit
S
{\displaystyle S}
,
E
x
t
{\displaystyle Ext}
is the Ext group. The differential
d
r
{\displaystyle d^{r}}
has degree
(
1
−
r
,
r
)
{\displaystyle (1-r,r)}
.
Similarly for homology
E
p
,
q
2
=
T
o
r
q
R
(
H
p
(
C
∗
)
,
G
)
⇒
H
∗
(
C
∗
;
G
)
{\displaystyle E_{p,q}^{2}=Tor_{q}^{R}(H_{p}(C_{*}),G)\Rightarrow H_{*}(C_{*};G)}
for Tor the Tor group and the differential
d
r
{\displaystyle d_{r}}
having degree
(
r
−
1
,
−
r
)
{\displaystyle (r-1,-r)}
.
Notes
References
Allen Hatcher, Algebraic Topology, Cambridge University Press, Cambridge, 2002. ISBN 0-521-79540-0. A modern, geometrically flavored introduction to algebraic topology. The book is available free in PDF and PostScript formats on the author's homepage.
Kainen, P. C. (1971). "Weak Adjoint Functors". Mathematische Zeitschrift. 122: 1–9. doi:10.1007/bf01113560. S2CID 122894881.
Jerome Levine. “Knot Modules. I.” Transactions of the American Mathematical Society 229 (1977): 1–50. https://doi.org/10.2307/1998498
External links
Universal coefficient theorem with ring coefficients
Kata Kunci Pencarian:
- Statistika
- Statistika matematika
- Daftar masalah matematika yang belum terpecahkan
- Universal coefficient theorem
- Betti number
- Tor functor
- Ext functor
- Universal approximation theorem
- List of algebraic topology topics
- Lefschetz hyperplane theorem
- List of theorems
- Algebraic topology
- Hodge cycle
