## Quadrics in 4D (again)

Higher-dimensional geometry (previously "Polyshapes").

### Quadrics in 4D (again)

I was looking for a classification of quadrics in 4D and I found this thread from 9 years ago.
http://hddb.teamikaria.com/forum/viewtopic.php?f=3&t=352
It didn't go very far, but this could be really useful for PDEs and coordinate systems.

I'd really like to have all the quadrics in 4D listed and named. This Wiki article is a good place to start:

Here I've listed all the quadrics up to 4D with the constants removed.

Ellipse: EE : x^2 + y^2 = 1
Hyperbola: EH : x^2 - y^2 = 1
Parabola: EP : x^2 - y= 0
Rectangle: II

Ellipsoid: EEE : x^2 + y^2 + z^2 = 1
Hyperboloid with one sheet: EEH : x^2 + y^2 - w^2 = 1
Hyperboloid with two sheets: EHH: x^2 + y^2 - z^2 - w^2 = 1

Elliptic paraboloid: EEP: x^2 + y^2 - z = 0
Hyperbolic paraboloid: EHP: x^2 - y^2 - z = 0

Elliptic cylinder: EEI : x^2 + y^2 = 1
Hyperbolic cylinder: EHI : x^2 - y^2 = 1
Parabolic cylinder: EPI : x^2 - y= 0
Cuboid: III

I was tempted to name x^2 - y - z = 0, but this is actually just a rotated parabolic cylinder. So we'll never find a P^2. Also note that the equation for hyperboloid with two sheets is the negative of the equation in Wikipedia, to match the pattern better.

EEEE: x^2 + y^2 + z^2 + w^2 = 1
EEEH: x^2 + y^2 + z^2 - w^2 = 1
EEHH: x^2 + y^2 - z^2 - w^2 = 1
EHHH: x^2 + y^2 - z^2 - w^2 = 1

EEEP : x^2 + y^2 + z^2 - w = 0
EEHP : x^2 + y^2 - z^2 - w = 0
EHHP : x^2 - y^2 - z^2 - w = 0

There are some simple cylinders:
EEEI
EEHI
EHHI
EEPI
EHPI
EEII
EHII
EPII
IIII

And finally some double cylinders:
EE x EE
EE x EH
EE x EP
EH x EH
EH x EP
EP x EP

It seems like in n dimensions there are n pure quadrics, n - 1 parabolic quadrics, and then a bunch of cylinders, which will relate to the number of rotatopes in some way.
Last edited by PWrong on Wed Oct 29, 2014 10:02 am, edited 1 time in total. PWrong
Pentonian

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### Re: Quadrics in 4D (again)

Yeah, I remember when I enumerated those Marek14
Pentonian

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### Re: Quadrics in 4D (again)

Here's some code to generate quadrics:

Code: Select all
`A[n_] := Apply[q,   Table[Join[ConstantArray[1, k], ConstantArray[-1, n - k]], {k, 1, n}], 1]B[n_] := Join[A[n], Append[#, 0] & /@ A[n - 1]]g[list_] := Flatten[Outer @@ Join[{Dot}, list, {1}]]Quadrics[n_] :=  DeleteDuplicates[  Map[Sort, Flatten[Map[g, Map[B, IntegerPartitions[n], {2}], {1}]], {-3}]]`

A "." represents the Cartesian product, but we can replace Dot with another function later.

Code: Select all
`In:= Quadrics//TableFormOut//TableForm= q[1,-1,-1,-1,-1]q[1,1,-1,-1,-1]q[1,1,1,-1,-1]q[1,1,1,1,-1]q[1,1,1,1,1]q[1,-1,-1,-1,0]q[1,1,-1,-1,0]q[1,1,1,-1,0]q[1,1,1,1,0]q.q[1,-1,-1,-1]q.q[1,1,-1,-1]q.q[1,1,1,-1]q.q[1,1,1,1]q.q[1,-1,-1,0]q.q[1,1,-1,0]q.q[1,1,1,0]q[1,-1].q[1,-1,-1]q[1,1].q[1,-1,-1]q[1,0].q[1,-1,-1]q[1,-1].q[1,1,-1]q[1,1].q[1,1,-1]q[1,0].q[1,1,-1]q[1,-1].q[1,1,1]q[1,1].q[1,1,1]q[1,0].q[1,1,1]q[1,-1].q[1,-1,0]q[1,1].q[1,-1,0]q[1,0].q[1,-1,0]q[1,-1].q[1,1,0]q[1,1].q[1,1,0]q[1,0].q[1,1,0]q.q.q[1,-1,-1]q.q.q[1,1,-1]q.q.q[1,1,1]q.q.q[1,-1,0]q.q.q[1,1,0]q.q[1,-1].q[1,-1]q.q[1,-1].q[1,1]q.q[1,-1].q[1,0]q.q[1,1].q[1,1]q.q[1,0].q[1,1]q.q[1,0].q[1,0]q.q.q.q[1,-1]q.q.q.q[1,1]q.q.q.q[1,0]q.q.q.q.q` PWrong
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