math

# Differential forms

## ¶[2018-11-24] my thoughts on that math

so basically, treat dx, dy as formal symbols? you can multiply them by scalar, add them up
NOTE in agda it would be just n-form with ordered requirement? or just a subset?


## ¶DONE diffforms.pdf mathstudy

CREATED: [2018-11-10]

### ¶total differential is an example of diff. form: mathstudy

df = pf/px dx + pf/py dy

### ¶closed – if pF/py = pG/px mathstudy

exact impllies closed, but not in reverse

### ¶wedge space: given vector space V: mathstudy

wedge rules: vu = -uv; u ^ u = 0; and linearity
build the space: \Wedge2 V = {Sumi ui ^ vi | ui ^ vi | ui, vi in V} are imposed

### ¶rules for derivative: mathstudy

linearity
d(f alpha) = df ^ alpha + f d alpha
d (dx) = d (dy) = d(dz) = 0 # TODO hmm what does that one mean??

### ¶orientation: for a curve, direction; for a surface – normal direction mathstudy

basically, continuous normal field. If it exists, there are two of them: n and minus n
parameterise the surface by two coordinates u, v so that du ^ dv is the direction of normal

#### ¶so basically, by definition: integral of 2-form would be mathstudy

int F dxdy + G dydz + H dz ^dx = int int [ F p(x, y)/p(u, v) + G p(y, z)/p(u, v) + H p(z, x)/p(u, v)] du dv

#### ¶NOTE arclength is NOT a differential form (always > 0) mathstudy

but what we can do is to define a metric tensor

## ¶https://en.wikipedia.org/wiki/Differential_form#Intrinsic_definitionsmath

### ¶TODO By the universal property of exterior powers, this is equivalently an alternating multilinear map math

mm, I guess this bit is crucial to my understanding of alternating property

## ¶[2018-11-06] differential forms ^ : wedge/exterior product math

f(x, y, z) dx ^ dy ^ dx – integrated over volume, gives volume
d operation – exterior derivative, results in k+1 form

ok so treat dx, dy, dz formally in form expression

## ¶[2018-11-25] ok, so diff forms are special types of tensors. Not all tensors are diff forms mathtensor

https://en.wikipedia.org/wiki/Antisymmetric_tensor
a completely antisymmetric covariant tensor – p-form; contravarian – p-vector
https://en.wikipedia.org/wiki/Volume_form#Riemannian_volume_form

## ¶DONE[2018-11-14] calculus - What does $$dx$$ mean in differential form? - Mathematics Stack Exchange math

https://math.stackexchange.com/questions/664648/what-does-dx-mean-in-differential-form/664674#664674
dx1 is a differential 1-form, linear map acting on tangent space. Action is: (1, 0, …. 0)

dxi is a covector field. {dxi}i span all covector fields. For f ∈ Rn -> R, you can write df = [f1(x) f2(x) .. fn(x)] as f1(x) dx1 + … + fn(x) dxn
in that sense, dx1 is the derivative of coordinate function f(x1…xn) = x1

### ¶right, here it's a 1-form! so no concerns about antisymmetry etc.. math

so, to compute the 1-form along the path, we:
split the path gamma(t); t from 0 to 1 into k parts
each part can be considered a tangent vector at gammai, and plugged into the form. then sum up the numbers

## ¶TODO what are the other examples of 1-forms? math

### ¶gradient – ok, definitely pretty intuitive math

#### ¶TODO hmm. gradient is a vector and total derivative is covector?? https://math.stackexchange.com/questions/47618/definition-of-the-gradient-for-non-cartesian-coordinatesmath

ok, so I guess definition might involve coordinates; but otherwise they are coordinate free once we prove they transform properly

## ¶TODO some intuitive stuff from NotesOnVectors.pdf math

Examples of how you can picture contravariant and covariant vectors. A contravari-ant vector is a “stick” with a direction to it.  Its “worth” (or “magnitude”) is proportional tothe length of the stick.  A covariant vector is like “lasagna.”  Its worth is proportional to thedensity of noodles; that is, the closer together are the sheets, the larger is the magnitude ofthe covector.  These and other pictorial examples of visualizing contravariant and covariantvectors are discussed in Am.J.Phys.65(1997)1037.Figure  3:   Pictorial  representation  of  the  innerproduct between a contravariant vector and a co-variant  vector.   The  “stick”  is  imbedded  in  the“lasagna”  and  the  inner  product  is  equal  to  thenumber of noodles pierced by the stick.  The in-ner product shown has the value 5.


## ¶[2018-11-10] Дифференциал (дифференциальная геометрия) — Википедия https://ru.wikipedia.org/wiki/%D0%94%D0%B8%D1%84%D1%84%D0%B5%D1%80%D0%B5%D0%BD%D1%86%D0%B8%D0%B0%D0%BB_(%D0%B4%D0%B8%D1%84%D1%84%D0%B5%D1%80%D0%B5%D0%BD%D1%86%D0%B8%D0%B0%D0%BB%D1%8C%D0%BD%D0%B0%D1%8F_%D0%B3%D0%B5%D0%BE%D0%BC%D0%B5%D1%82%D1%80%D0%B8%D1%8F)math

Пусть {\displaystyle M} M — гладкое многообразие и {\displaystyle f\colon M\to \mathbb {R} } {\displaystyle f\colon M\to \mathbb {R} } гладкая функция. Дифференциал {\displaystyle f} f представляет собой 1-форму на {\displaystyle M} M, обычно обозначается {\displaystyle df} df и определяется соотношением
{\displaystyle df(X)=d_{p}f(X)=Xf,} {\displaystyle df(X)=d_{p}f(X)=Xf,}
где {\displaystyle Xf} Xf обозначает производную {\displaystyle f} f по направлению касательного вектора {\displaystyle X} X в точке {\displaystyle p\in M} p\in M.


hmm, that actually makes sense too!

## ¶[2018-11-10] differentiation in nLab https://ncatlab.org/nlab/show/differentiationmath

1. Idea
Differentiation is the process that assigns to a function f:X→Y f : X \to Y its derivative, sometimes denoted df d f. The derivative is a function that, roughly speaking, assigns to each point x∈X x \in X the linear transformation dfx d f_x that maps infinitesimal differences y−x y - x (for points y y infinitesimally close to x x) to infinitesimal differences f(y)−f(x) f(y) - f(x).


## ¶[2018-11-24] Closed and exact differential forms - Wikipedia math

 In mathematics, especially vector calculus and differential topology, a closed form is a differential form α whose exterior derivative is zero (dα = 0), and an exact form is a differential form, α, that is the exterior derivative of another differential form β. Thus, an exact form is in the image of d, and a closed form is in the kernel of d.

For an exact form α, α = dβ for some differential form β of degree one less than that of α. The form β is called a "potential form" or "primitive" for α. Since the exterior derivative of a closed form is zero, β is not unique, but can be modified by the addition of any closed form of degree one less than that of α.

Because d2 = 0, any exact form is necessarily closed. The question of whether every closed form is exact depends on the topology of the domain of interest. On a contractible domain, every closed form is exact by the Poincaré lemma. More general questions of this kind on an arbitrary differentiable manifold are the subject of de Rham cohomology, which allows one to obtain purely topological information using differential methods.


## ¶START[B][2019-01-24] A Visual Introduction to Differential Forms and Calculus on Manifolds | Jon Pierre Fortney | Springer mathviz

• State "START" from "TODO" [2019-02-27]