Exterior Algebra and Calculus |

**Exterior Algebra / Product** > s.a. forms.

$ __Def__: The associative,
bilinear composition law for differential forms on a manifold ∧ :
Ω^{ p}(*M*) ×
Ω^{ q}(*M*) →
Ω^{ p+q}(*M*)
given by

\[ (\omega\wedge\theta)_{a...bc...d} = {(p+q)!\over p!\,q!}\,\omega_{[a...b}\,\theta_{c...d]}\;.\]

* __Properties__:

– Under permutation, *ω* ∧ *θ*
= (−1)^{pq }*θ* ∧ *ω*.

– Contraction with a vector
field, *v* · (*ω* ∧ *θ*)
= (*v* · *ω*) ∧ *θ*
+ (−1)^{p} *ω*
∧ (*v* · *θ*).

> __Online resources__:
see MathWorld page;
Wikipedia page.

**Exterior Calculus / Derivatives** > s.a. cohomology;
differential forms; lie derivatives.

$ __Def__: An operator
d: Ω^{ p}(M) →
Ω^{ p+1}(*M*)
on the graded algebra of differential forms on a manifold, defined by

(1) Action on scalars, d*f*(*X*):= *X*(*f*),
for all 0-forms *f* and vector fields *X*;

(2) Linearity, d(*αω* + *βη*) = *α*
d*ω* + *β* d*η*, for all *p*-forms
*ω*, *η* and numbers *α*, *β*;

(3) Relation with exterior product, d(*ω* ∧ *θ*):=
d*ω* ∧ *θ* +
(−1)^{p} *ω* ∧ d*θ*,
for all *p*-forms *ω* and *q*-forms *θ*;

(4) Square, d^{2}*ω* = d(d*ω*)
= 0 for all *p*-forms *ω*.

* __Remark__: It does
not need a metric to be defined (it is a concomitant).

* __Notation__: In abstract index
and coordinate notation, respectively, for a *p*-form *ω*
= *ω*_{i...j}
d*x*^{ i} ∧ ...
∧ d*x*^{ j},

(d*ω*)_{ma... b} = (*p*+1)
∇_{[m }*ω*_{a...
b]}
, d*ω* = ∂_{k
}*ω*_{i...
j} d*x*^{k}
∧ dx^{ i} ∧ ...
∧ d*x*^{ j} .

* __Properties__:
It commutes with taking the Lie derivative with respect to some
vector field *v*^{a},
d(\(\cal L\)_{v} *ω*)
= \(\cal L\)_{v}(d*ω*).

@ __General references__: Colombaro et al Math(19)-a2002 [introduction].

@ __Discrete__: Harrison mp/06 [unified with continuum];
Arnold et al BAMS(10)
[finite-element exterior calculus, cohomology and Hodge theory];
Boom et al a2104 [applied to linear elasticity].

@ __Other generalized__: Okumura PTP(96)ht [in non-commutative geometry];
Gozzi & Reuter IJMPA(94)ht/03 [quantum deformed, on phase space];
Tarasov JPA(05) [of fractional order];
Yang a1507-wd
[in non-commutative geometry, nilpotent matrix representation].

> __Online resources__:
see Wikipedia page.

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