Cohomology Theory |

**In General**

* __Idea__: A
framework, invented by H Whitney, for constructing algebraic quantities
that provide a partial classification of topological spaces, like homology
and homotopy, in which the structures are dual to homology classes; The
way the duality is defined may differ, giving rise to different cohomology theories.

* __Advantages__: It is
a more powerful and easier to use tool than homology theory, and its nice
extra algebraic structure permits in some cases to tell that two
topological spaces are not homeomorphic even if they have the same
cohomology groups, from the different ring structures [e.g., S^{2}
× S^{4} and \(\mathbb C\)P^{3},
below], and it uses a local operator (d), instead of a global one (∂).

$ __Def__: A cohomology
theory (*H**, d) consists of (a) A contravariant functor *H*
from differentiable manifolds and smooth maps to ... ; (b) A
transformation d ...

> __Online resources__:
see Wikipedia page.

**Related Concepts** > s.a. Schubert Calculus.

* __Cup product__: A
map ∪: *H*^{p}(*M*;
*X*) × *H*^{q}(*M*;
*X*) → *H*^{p+q}(*M*;
*X*), or ∪: *H**(*M*; *X*) × *H**(*M*;
*X*) → *H**(*M*; *X*), defined by [*c*]
∪ [*c*']:= [*cc*'], where \(\langle\)*cc*', *σ*\(\rangle\):=
(−1) \(\langle\)*c*, front *m*-face of *σ*\(\rangle\)
\(\langle\)*c*', back *n*-face of *σ*\(\rangle\);
It satisfies δ(*cc*') = (δ*c*) *c*' + (−1) *c*
(δ*c*'); __Examples__: For forms, [*ω*] ∪ [*η*]:=
[*ω* ∧ *η*].

* __Ring structure__:
The space *H**(*X*; Λ):= ⊕_{p>0}
*H*^{p}(*X*; Λ) is a
ring, with the cup product.

* __Kronecker index__:
Given a cohomology class *v* ∈ *H*^{n}(*M*;
\(\mathbb Z\)/2) for a manifold *M*, its Kronecker index is *v*[*M*]:=
\(\langle\)*v*, *ν*\(\rangle\) ∈ \(\mathbb Z\)/2, where *μ*_{M}
is the fundamental homology class of *M*.

* __Poincaré duality__:
If *M* is a compact, oriented *n*-manifold, then *H*^{i}(*M*)
is isomorphic to *H*_{n−i}(*M*)
under *a* → *a* ∩ *μ*_{M},
where *μ*_{M} is the
fundamental homology class of *M*.

> __Other related
concepts__:
see Cap Product; yang-mills
theories [operator complexes].

**Examples** > s.a. BRST
transformations; lie algebras; quantum
groups; tilings; types
of cohomology [including generalizations].

** *__Connected,
simply connected M__: In this case,

*

*

**References** > s.a. algebraic
topology; crystals.

@ __General__: De Rham 60; in
Nash & Sen 83.

@ __For groups__: Weiss 69; Brown 82;
Totaro 14; > s.a. group theory.

@ __Non-Abelian__: Eilenberg & MacLane AM(47);
Andersson 86.

@ __Quantum__: Ruan & Tian JDG(95).

**In Physics** > s.a. BRST
cohomology; topology in physics.

@ __Field theory on curved spacetime__: Khavkine a1404 [De Rham cohomology with causally restricted supports].

@ __Other applications__: Azcárraga & Izquierdo 95;
Gross JMP(96);
Forrest et al mp/00-proc [quasicrystals];
Alexandradinata et al PRX(16) [topological insulators].

> __Related topics__:
see classical particles; crystals;
formulations of general relativity; lagrangian dynamics;
renormalization theory.

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send feedback and suggestions to bombelli at olemiss.edu – modified 30 jul 2016