notes/inversive.md

@@ -5,17 +5,17 @@
 These coordinates are of form $I=(c, r, x, y, z)$ where we think of $c$ as the co-radius, $r$ as the radius, and $x, y, z$ as the "Euclidean" part, which we abbreviate $E_I$.  There is an underlying basic quadratic form $Q(I_1,I_2) = (c_1r_2+c_2r_1)/2 - x_1x_2 -y_1y_2-z_1z_2$ which aids in calculation/verification of coordinates in this representation. We have:
 
 | Entity or Relationship                                                          | Representation                                                                                                                                                                                                                                                                                                                                                                                                                       | Comments/questions                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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 | Sphere s with radius r>0 centered on P = (x,y,z)                                | $I_s = (1/c, 1/r, x/r, y/r, z/r)$ satisfying $Q(I_s,I_s) = -1$, i.e., $c = r/(\|P\|^2 - r^2)$.                                                                                                                                                                                                                                                                                                                                       | Note that $1/c = \|P\|^2/r - r$, so there is no trouble if $\|P\| = r$; we just get first coordinate to be 0. Using the point representation $I_P$ from below, let's orient the sphere so that its normals point into the "positive side," where $Q(I_P, I_s) > 0$. The vector $I_s$ then represents a sphere with outward normals, while $-I_s$ represents one with inward normals.                                                                                                                                                                                                                 |
 | Plane p with unit normal $(x,y,z)$ through the (Euclidean) point $(sx,sy,sz)$   | $I_p = (-2s, 0, -x, -y, -z)$                                                                                                                                                                                                                                                                                                                                                                                                         | Note that $Q(I_p, I_p)$ is still $−1$. The parameter $s$ is the plane's distance from the origin. We orient planes using the same convention we use for spheres. For example, $(-2, 0, -1/\sqrt3, -1/\sqrt3, -1/\sqrt3)$ and $(2, 0, 1/\sqrt3, 1/\sqrt3, 1/\sqrt3)$ represent planes that coincide in space, which have their normals pointing away from and toward the origin, respectively.                                                                                                                                                                                                        |
 | Point P with Euclidean coordinates (x,y,z)                                      | $I_P = (\|P\|^2, 1, x, y, z)$                                                                                                                                                                                                                                                                                                                                                                                                        | Note $Q(I_P,I_P) = 0$. This gives us the freedom to choose a different normalization. For example, we could scale the representation shown here by $(\|P\|^2+1)^{-1}$, putting it on the sphere where the light cone intersects the plane where the first two coordinates sum to $1$.                                                                                                                                                                                                                                                                                                                |
 | ∞, the "point at infinity"                                                      | $I_\infty = (1,0,0,0,0)$                                                                                                                                                                                                                                                                                                                                                                                                             | The only solution to $Q(I,I) = 0$ not covered by the above case.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
 | P lies on sphere or plane given by I                                            | $Q(I_P, I) = 0$                                                                                                                                                                                                                                                                                                                                                                                                                      | Actually also works if $I$ is the coordinates of a point, in which case "lies on" simply means "coincides with".                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
 | Sphere/planes represented by I and J are tangent                                | If $I$ and $J$ have the same orientation where they touch, $Q(I,J) = -1$. If they have opposing orientations, $Q(I,J) = 1$.                                                                                                                                                                                                                                                                                                          | For example, the $xy$ plane with normal $-e_z$, represented by $(0,0,0,0,1)$, is tangent with matching orientation to the unit sphere centered at $(0,0,1)$ with outward normals, represented by $(0,1,0,0,1)$. Accordingly, their $Q$-product is $−1$.                                                                                                                                                                                                                                                                                                                                              |
-| Sphere/planes represented by I and J intersect (respectively, don't intersect) | $\|Q(I,J)\| \le (\text{resp. }>)\; 1$                                                                                                                                                                                                                                                                                                                                                                                                | Follows from the angle formula and the tangency condition, at least conceptually. One subtlety: parallel planes have $Q$-product $\pm 1$, because they intersect at infinity!                                                                                                                                                                                                                    |
+| Sphere/planes represented by I and J intersect (respectively, don't intersect)  | $\lvert Q(I,J)\rvert \le (\text{resp. }>)\; 1$                                                                                                                                                                                                                                                                                                                                                                                       | Follows from the angle formula and the tangency condition, at least conceptually. One subtlety: parallel planes have $Q$-product $\pm 1$, because they intersect at infinity!                                                                                                                                                                                                                                                                                                                                                                                                                        |
 | $P$ is center of sphere rep'd by $I$                                            | $Q(I, I_P) = -r/2$, where $1/r = 2Q(I_\infty, I)$ is the signed curvature of the sphere, and $I_P$ is normalized in the standard way, which is to set $Q(I_\infty, I_P) = 1/2$                                                                                                                                                                                                                                                       | This relationship is equivalent to both of the following. (1) The point $P$ has signed distance $-r$ from the sphere. (2) Inversion across the sphere maps $\infty$ to $P$.                                                                                                                                                                                                                                                                                                                                                                                                                          |
 | Distance between P and R is d                                                   | $Q(I_P, I_R) = d^2/2$                                                                                                                                                                                                                                                                                                                                                                                                                | If $P$ and $R$ are represented by non-normalized vectors $V_P$ and $V_R$, the relation becomes $Q(V_P, V_R) = 2\,Q(V_P, I_\infty)\,Q(V_R, I_\infty)\,d^2$. This version of the relation makes it easier to see why $d$ goes to infinity as $P$ or $R$ approaches the point at infinity.                                                                                                                                                                                                                                                                                                              |
-| Signed distance between point rep'd by $V$ and sphere/plane rep'd by $I$ is $d$| In general, $\frac{Q(I, V)}{2Q(I_\infty, V)} = Q(I_\infty, I)\,d^2 + d$. When $V$ is normalized in the usual way, this simplifies to $Q(I, V) = d^2/r + d$ for a sphere of radius $r$, and to $Q(I, V) = d$ for a plane.                                                                                                                                                                                                             | We can use a Euclidean motion, represented linearly by a Lorentz transformation that fixes $I_\infty$, to put the point on the $z$ axis and put the nearest point on the sphere/plane at the origin with its normal pointing in the positive $z$ direction. Then the sphere/plane is represented by $I = (0, 1/r, 0, 0, -1)$, and the point can be represented by any multiple of $I_P = (d^2, 1, 0, 0, d)$, giving $Q(I, I_P) = d^2/2r + d$. We turn this into a general expression by writing it in terms of Lorentz-invariant quantities and making it independent of the normalization of $I_P$. |
+| Signed distance between point rep'd by $V$ and sphere/plane rep'd by $I$ is $d$ | In general, $\frac{Q(I, V)}{2Q(I_\infty, V)} = Q(I_\infty, I)\,d^2 + d$. When $V$ is normalized in the usual way, this simplifies to $Q(I, V) = d^2/r + d$ for a sphere of radius $r$, and to $Q(I, V) = d$ for a plane.                                                                                                                                                                                                             | We can use a Euclidean motion, represented linearly by a Lorentz transformation that fixes $I_\infty$, to put the point on the $z$ axis and put the nearest point on the sphere/plane at the origin with its normal pointing in the positive $z$ direction. Then the sphere/plane is represented by $I = (0, 1/r, 0, 0, -1)$, and the point can be represented by any multiple of $I_P = (d^2, 1, 0, 0, d)$, giving $Q(I, I_P) = d^2/2r + d$. We turn this into a general expression by writing it in terms of Lorentz-invariant quantities and making it independent of the normalization of $I_P$. |
 | Distance between sphere/planes rep by I and J                                   | Note that for any two Euclidean-concentric spheres rep by $I$ and $J$ with radius $r$ and $s,$ $Q(I,J) = -\frac12\left(\frac rs  + \frac sr\right)$ depends only on the ratio of $r$ and $s$. So this can't give something that determines the Euclidean distance between the two spheres, which presumably grows as the two spheres are blown up proportionally. For another example, for any two parallel planes, $Q(I,J) = \pm1$. | Alex had said: $Q(I,J)=\cosh(d/2)^2$ maybe where d is distance in usual hyperbolic metric. Or maybe $\cosh(d)$. That may be right depending on what's meant by the hyperbolic metric there, but it seems like it won't determine a reasonable Euclidean distance between planes, which should differ between different pairs of parallel planes.                                                                                                                                                                                                                                                     |
 | Sphere centered on P through R                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                      | Probably just calculate distance etc.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
 | Plane rep'd by I goes through center of sphere rep'd by J                       | This is equivalent to the plane being perpendicular to the sphere: that is, $Q(I, J) = 0$.                                                                                                                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |