# Universal coefficient theorem

In algebraic topology, universal coefficient theorems establish relationships between homology and cohomology theories. For instance, the integral homology theory of a topological space X, and its homology with coefficients in any abelian group A are related as follows: the integral homology groups

Hi(X; Z)

completely determine the groups

Hi(X; A)

Here Hi might be the simplicial homology or more general singular homology theory: the result itself 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

Furthermore, this sequence splits, though not naturally. Here μ is a map induced by the bilinear map Hi(X; Z) × AHi(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

As in the homology case, the sequence splits, though not naturally.

In fact, suppose

and define:

Then h above is the canonical map:

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.[1]

## 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 R = Z/2Z.

Knowing that the integer homology is given by:

We have Ext(R, R) = R, Ext(Z, R) = 0, so that the above exact sequences yield

In fact the total cohomology ring structure is

## 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.

where βi(X) are the Betti numbers of X and is the torsion part of . One may check that

and

This gives the following statement for integral cohomology:

For X an orientable, closed, and connected n-manifold, this corollary coupled with Poincaré duality gives that βi(X) = βni(X).

## 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.