Now the previous pattern I put in is pretty, whether one likes math or not. This one is for people who really like math, and in particular, Noneuclidean geometry:
Crocheting the Hyperbolic Plane
In a nutshell, to crochet a hyperbolic plane (well, a model of it) one makes a chain of modest length (say, oh, 15) and then keep doing row after row of single crochet, but increasing every certain fixed number of stitches.
For example, I did this, doing two sc in one sc every 6th stitch.
This makes a surface of "constant curvature" (like a sphere has constant curvature, but it curves "in", and not "out" like the hyperbolic plane). In any case, the link has a couple pictures of the crocheted models. People teaching hyperbolic geometry can use these models, and it's far easier than the usual paper models, as it can't tear.