Differential Forms |

**In General** > s.a. exterior
algebra and calculus; integration on manifolds.

* __Idea__: Antisymmetric
tensors, for which Cartan developed a special formalism, terminology and notation.

$ __Def__: A *p*-form
is a totally antisymmetric covariant tensor field of order *p*.

* __Notation__: The space of one-forms on *M* is usually denoted by
T**M*, that of *p*-forms by Λ^{p} T**M*,
Λ^{p}*M*
or Ω^{p}*M*
(or the same symbols with parentheses around *M*).

* __And other structure__:
Forms are a submodule of the algebra of all tensor fields on *M*; Together
with the exterior product, this forms the exterior algebra
or grassmann algebra of *M*; dim Λ^{p}_{x}(*M*)
= *n*!/*p*!(*n*–*p*)!

@ __General references__: Flanders 63;
Cartan 67;
Von Westenholz 81;
Bott & Tu 82;
Darling 94;
Jancewicz gq/98 [visualization];
Morita 01 [geometry];
Ivancevic & Ivancevic a0807-ln;
Lessig a1206 [primer].

@ __Related topics__: in de Rham 60 [of odd kind];
Hitchin m.DG/00 [3-forms in 6D and 7D];
> s.a. grassmann structures; types of cohomology theories.

@ __Chiral p-forms__: Bengtsson & Kleppe IJMPA(97)ht/96;
Pasti et al PRD(97) [covariant actions];
Bonetti et al PLB(13) [Kaluza-Klein inspired action].

@

**(Hodge) Dual**

$ __Def__: Given a *p*-form *f* on
an *n*-manifold *M* with volume form *ε* and metric *g* (with
*p* ≤ *n*), its dual is the (*n*–*p*)-form

**f*_{c.. d} :=
(1/*p*!) *f*^{ a.. b} ε_{a.. bc.. d} .

* __Properties__: It follows that
***f* = (–1)^{s+p(n–p) }*f*.

* __For a curvature tensor__: The left and right duals are, respectively,

**R*_{abcd} := \(1\over2\)ε_{ab}^{mn} *R*_{mncd} and *R**_{abcd}:=
\(1\over2\)*R*_{abmn }ε^{mn}_{cd} .

* __And physics__: The duality operation applied to the electromagnetic (Faraday) field tensor (2-form) interchanges the electric and magnetic fields; > s.a. duality.

@ __And conformal structure__: Dray et al JMP(89); Harnett JMP(91).

@ __Related topics__: Saa JGP(95) [for affine manifold with torsion]; Klinker JGP(11) [generalized definition].

**Volume Form** > s.a. types of field theories.

$ __Def__: For an *n*-dimensional manifold *M*, a nowhere-vanishing *n*-form *ω*.

* __And connections__: A volume
form is compatible with a connection if for all *X* ∈ T*M*, \(\cal L\)_{X }*ω* =
(*D*_{a} *X*^{a})
*ω* (for the divergence theorem to apply).

* __Example__: The volume
form ε defined by a metric *g* is ε =
|*g*|^{1/2} d*x*^{1} ∧ ...
∧ d*x*^{n};
It is compatible with the Christoffel symbols, but not with a Riemann-Cartan
connection, \(\cal L\)_{X} *ω* =
(*D*_{a}* X*^{a} –
2 *T*_{a}* X*^{a})
*ω*, where *T*_{a}:= *T*_{ba}^{b}.

@ __General references__: Cartier et al in(01)mp/00 [characterization].

@ __As a dynamical variable__: Guendelman & Kaganovich a0811-conf;
Guendelman et al a1505-conf [in gravity and cosmology].

**Lie-Algebra Valued Forms**

* __Canonical form__: Given
a Lie group *G*, the canonical (Maurer-Cartan)
form on *G* is the Lie-algebra-valued 1-form *ω*: T*G* →
\(\cal G\) =
T_{e}*G*
defined by *ω*(*v*_{g})
= L_{g}^{–1}*'*
*v*_{g}; __Theorem__:
R_{g}* *ω* =
Ad(*g*^{–1}) · *ω*.

* __Maurer-Cartan structure
equation__: If {*θ*^{I}} is a basis for
the dual \(\cal G\)* of the Lie
algebra of a group *G*, i.e. for the left-invariant 1-forms on *G*, then

d*θ*^{I}
= –\(1\over2\)*C*^{I}_{JK} *θ*^{J}
∧ *θ*^{K} .

**Other Special Types and Generalizations** > s.a. deformation
quantization; Star Product; superspace.

* __Closed form__: A form
*ω* whose exterior derivative vanishes, d*ω* = 0.

* __Exact form__: A form
*ω* which can be expressed as the exterior derivative of
another form, *ω* = d*θ*.

* __Poincaré's lemma__: On
a contractible space, any closed form is exact
(e.g., not in \(\mathbb R\)^{2}\{0}, non-contractible).

@ __General references__: in Flanders 63;
in Nash & Sen 83.

@ __Closed forms__: Torre CQG(95)gq/94 [classification];
Farber 04;
Farber & Schütz Top(06) [closed 1-forms with at most one zero].

@ __Fractional order__:
Cottrill-Shepherd & Naber JMP(01)mp/03,
mp/03;
Tarasov JPA(05).

@ __Discrete__:
Richter et al CQG(07)gq/06 [and spherical symmetry in general relativity];
Dowker JGP(07) [on sphere tessellations];
Dolotin et al TMP(08)-a0704 [based on simplicial complexes];
Richter & Frauendiener JSC(10)-a0805 [Gowdy solutions, numerical].

@ __Other generalizations__:
Nurowski & Robinson CQG(01) [and spacetime geometry];
Robinson IJTP(03) [and gauge theories],
JMP(03) [and general relativity],
JPA(07) [rev, and applications],
CQG(09)
[integral calculus, Stokes' theorem, Chern-Simons and Einstein-Yang-Mills theories];
Chatterjee et al IJGMP(08)-a0706 [negative forms, and path-space forms];
Gallego a1207 [higher-order, and applications to electrodynamics];
Robinson a1312 [and gravitation];
> s.a. clifford calculus;
formulations of electromagnetism [pair and impair].

@ __Related topics__: Frauendiener talk(03).

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