This is a repost of a question I posted at MSE.
Mark L. Irons' paper The Curvature and Geodesics of the Torus gives a concise overview of the geodesics on the torus:
- There are five clear-cut families of geodesics.
- Most of the geodesics are "ergodic": aperiodic and covering either the entire surface - by spiraling endlessly around - or substantial parts of it.
- Some of the geodesics are "boring": the meridians, the inner and the outer equator
- A few of them are "æsthetically pleasing": returning to their starting point after just a few circuits.
Can the structure of geodesics on the torus change drastically when twisting the "hose" before gluing its ends?
alt text http://upload.wikimedia.org/wikipedia/commons/6/60/Torus_from_rectangle.gif
E.g., there might be no equators anymore because after twisting the (two) equators lost their "ends".