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.
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
- Hp(M) × Hq(M) → Hp+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 Xp+1.
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.
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 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.
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.