# Motivations for Homology or Cohomology Theory

In some standard books on Algebraic Topology (except Hatcher's), the motivation for homology or cohomology theory is stated with the help of Cauchy's theorem, Green's theorem, Stoke's theorem etc (for example, one may see in books by Rotman, Massey, Fulton.)

In short, this motivation involves a relationship with "integration".

My question is, is there any other motivation for the study of homology/cohomology?

• Yes, lots. For example, group homology and cohomology has various purely algebraic applications, such as to the study of group extensions. There's lots more to say here depending on what you're interested in. – Qiaochu Yuan Nov 8 '15 at 6:06
• There certainly are. Quite generally, classifying spaces by deetmining numeric invariants which capture their structure, e.g. genus, number of "handles" that are attached to a sphere, "holes" in the space. – BrianO Nov 8 '15 at 6:09
• By first paragraph, I would say, "Homology helped to make integration theory more transparent in some direction". How this theory helped to solve basic question in say "Geometric objects in 2 and 3 dimensional euclidean space i.e. geometry of curves and surfaces in 3-dimension?" (Here "basic question" means- very fundamental, understable by "12th standard student") – Groups Nov 8 '15 at 6:14

• Homology and cohomology are invariant under homotopy equivalence. Hence if two spaces have different homology or cohomology, they're not equivalent. Closed $2$-manifolds, for example, are completely classified by their cohomology.
• We can also talk about the cohomology of groups acting on a module $M$. In addition to the abstract construction, various groups in low-dimensions have particular significance; for example, $H^2(\pi, \mathbb{Z})$ classifies central extensions. By abstract nonsense, $H^*(\pi, \mathbb{Z})$ is just the (say, singular) cohomology of the space $K(\pi, 1)$.
• We have a natural identification of $[X, K(\pi, n)]$ with $H^n(X, \pi)$, given by $f \to f^*[K(\pi, n)]$. That allows us to convert statements about homotopy into ones about homology, which is generally much easier to compute. As an example of this sort of procedure, real vector bundles over a compact space $X$ are classified by maps $X \to \mathbb{RP}^\infty$, and $\mathbb{RP}^\infty$ happens to be a $K(\mathbb{Z}_2, 1)$. We therefore get a natural bijective correspondence between line bundles over $X$ and $H^1(X, \mathbb{Z}_2)$, and more generally the Stiefel-Whitney class $w(\xi)\in H^*(X, \mathbb{Z}_2)$ of a bundle $\xi \to X$.