show/hide this revision's text 2 Corrected formatting in the title, removed incorrect description of p in the figure.

Mapping from $PSL(2,R)$ \text{PSL}(2,\mathbb{R})$ to transformations of the hyperboloid

Let $F$ map points in $\mathbb{R}^3$ to points on the unit disk, $\Delta$, in the $xz$-plane (identified with $\mathbb{C}$) by projecting through $\Delta$ along lines that intersect at $(0,-1,0)$. Let $H$ be the forward sheet of the hyperboloid as in the hyperboloid model.

  1. Given an element $p:z\to(az+b)/(cz+d)$ of $\text{PSL}(2,\mathbb{R})$ is there a transform transformation $g$ of $H$ such that $F \circ g \circ F^{-1} = p$ on $\Delta$?

  2. If the answer to (1) is "yes," then is there a known explicit formula for $g$ in terms of $a$, $b$, $c$, and $d$?

To clarify the situation, in the attached figure $p:z\to(z+1)/z$ and $g$ is a transformation of $H$ that nearly satisfies the condition from (1) on the green test curve. I found the $g$ in the figure by manually tweaking the elements of a 4x4 matrix until the red (projected) and green (correct) curves became visually close in the disk on the bottom right.

Figure showing transformations between the transformed hyperboloid and the unit disk

show/hide this revision's text 1

Mapping from $PSL(2,R)$ to transformations of the hyperboloid

Let $F$ map points in $\mathbb{R}^3$ to points on the unit disk, $\Delta$, in the $xz$-plane (identified with $\mathbb{C}$) by projecting through $\Delta$ along lines that intersect at $(0,-1,0)$. Let $H$ be the forward sheet of the hyperboloid as in the hyperboloid model.

  1. Given an element $p:z\to(az+b)/(cz+d)$ of $\text{PSL}(2,\mathbb{R})$ is there a transform $g$ of $H$ such that $F \circ g \circ F^{-1} = p$ on $\Delta$?

  2. If the answer to (1) is "yes," then is there a known explicit formula for $g$ in terms of $a$, $b$, $c$, and $d$?

To clarify the situation, in the attached figure $p:z\to(z+1)/z$ and $g$ is a transformation of $H$ that nearly satisfies the condition from (1) on the green test curve. I found the $g$ in the figure by manually tweaking the elements of a 4x4 matrix until the red (projected) and green (correct) curves became visually close in the disk on the bottom right.

Figure showing transformations between the transformed hyperboloid and the unit disk