# Hyperbolic coordinates

You can uniquely specify any point in 2D Euclidian space using 2 numbers: the distance from the infinitely long X-axis, and the distance from the infinitely long Y-axis.

How do you uniquely specify a point in 2D hyperbolic space? Can you do it with just 2 numbers? Can you do it in a "uniform" way? How would that work?

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Do you know the upper half space or disc model of hyperbolic space? In these, you can of course specify a point with 2 coordinates.

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OK. But then how would you add or subtract coordinates? Clearly the usual Euclidian rules won't work... –  MathematicalOrchid Jun 1 '12 at 13:21
@MathematicalOrchid: How would you add and subtract points in the hyperbolic plane to begin with? It's not a vector space. –  Hans Lundmark Jun 1 '12 at 13:29
You did not ask about adding coordinates in your OP. If you need an analogy to the distance description, then this can be achieved -- eg in the disc model -- using the inteersection of the disc with the Euclidean $x$ and $y$ axis and then use geodesic distance. ($x$ and $y$ axis are geodesics in that model). –  user20266 Jun 1 '12 at 13:29
(Oh and if you think of adding coordinates as translations -- the group of isometries of hyperbolic space is well known and easy to describe). –  user20266 Jun 1 '12 at 13:33
Check out any introduction to hyperbolic geometry. Lectures on Hyperbolic Geometry by Benedetti and Petronio, Three Dimensional Geometry and Topology by Thurston are two books that come to mind. –  Neal Jun 1 '12 at 15:39