# Cohomology

In mathematics, specifically in algebraic topology, **cohomology** is a general term for a sequence of abelian groups defined from a cochain complex. That is, cohomology is defined as the abstract study of cochains, cocycles and coboundaries. Cohomology can be viewed as a method of assigning algebraic invariants to a topological space that has a more refined algebraic structure than does homology. Cohomology arises from the algebraic dualization of the construction of homology.

From its beginning in topology, this idea became a dominant method in the mathematics of the second half of the twentieth century; from the initial idea of *homology* as a topologically invariant relation on *chains*, the range of applications of homology and cohomology theories has spread out over geometry and abstract algebra. The terminology tends to mask the fact that in many applications *cohomology*, a contravariant theory, is more natural than *homology*. At a basic level this has to do with functions and pullbacks in geometric situations: given spaces *X* and *Y*, and some kind of function *F* on *Y*, for any mapping *f* : *X* → *Y* composition with *f* gives rise to a function *Fof* on *X*.
Cohomology groups also have natural products, making calculation easier.

With hindsight, general homology theory should probably have been given an inclusive meaning covering both *homology* and *cohomology*: the direction of the arrows in a chain complex is not much more than a sign convention.

## Contents

## History

Although cohomology is fundamental to modern algebraic topology, its importance was not seen for some 40 years after the development of homology. The concept of *dual cell structure*, which Henri Poincaré used in his proof of his Poincaré duality theorem, contained the germ of the idea of cohomology, but this was not seen until later.

There were various precursors to cohomology. In the mid-1920s, J.W. Alexander and Lefschetz founded the intersection theory of cycles on manifolds. On an *n*-dimensional manifold *M*, a *p*-cycle and a *q*-cycle with nonempty intersection will, if in general position, have intersection a (*p+q−n*)-cycle. This enables us to define a multiplication of homology classes

*H*_{p}(*M*) ×*H*_{q}(*M*) →*H*_{p+q-n}(*M*).

Alexander had by 1930 defined a first cochain notion, based on a *p*-cochain on a space *X* having relevance to the small neighborhoods of the diagonal in *X*^{p+1}.

In 1931, De Rham related homology and exterior differential forms, proving De Rham's theorem. This result is now understood to be more naturally interpreted in terms of cohomology.

In 1934, Pontrjagin proved the Pontrjagin duality theorem; a result on topological groups. This (in rather special cases) provided an interpretation of Poincaré duality and Alexander duality in terms of group characters.

In a 1935 conference in Moscow, Kolmogorov and Alexander both introduced cohomology and tried to construct a cohomology product structure.

In 1936 Steenrod published a paper constructing Čech cohomology by dualizing Čech homology.

From 1936 to 1938, Hassler Whitney and Eduard Čech developed the cup product (making cohomology into a graded ring) and cap product, and realized that Poincaré duality can be stated in terms of the cap product. Their theory was still limited to finite cell complexes.

In 1944, Eilenberg overcame the technical limitations, and gave the modern definition of singular homology and cohomology.

In 1945, Eilenberg and Steenrod stated the axioms defining a homology or cohomology theory. In their 1952 book, *Foundations of Algebraic Topology*, they proved that the existing homology and cohomology theories did indeed satisfy their axioms.

In 1948 Spanier, building on work of Alexander and Kolmogorov, developed Alexander-Spanier cohomology.

## Cohomology theories

### Eilenberg-Steenrod theories

A *cohomology theory* is a family of contravariant functors from the category of pairs of topological spaces and continuous functions (or some subcategory thereof such as the category of CW complexes) to the category of Abelian groups and group homomorphisms that satisfies the Eilenberg-Steenrod axioms

Some cohomology theories in this sense are:

### Extraordinary cohomology theories

When one axiom (*dimension axiom*) is relaxed, one obtains the idea of *extraordinary cohomology theory*; this allows theories based on K-theory and cobordism theory. There are others, coming from stable homotopy theory.

### Other cohomology theories

Theories in a broader sense of *cohomology* include:

- Group cohomology
- Galois cohomology
- Lie algebra cohomology
- Coherent cohomology
- Local cohomology
- Étale cohomology
- Crystalline cohomology
- Flat cohomology
- Motivic cohomology
- Deligne cohomology
- Perverse cohomology
- Intersection cohomology
- Non-abelian cohomology
- Gel'fand-Fuks cohomology
- Spencer cohomology.