Continued fractions on Hecke triangle surfaces


The group of 2x2-integer matrices with determinant 1, i.e. SL_2(Z), is of fundamental importance in number theory. It is quite elementary to show that it is generated by the matrices T=[[1,1],[0,1]] and S=[[0,1],[-1,0]]. In fact, if one mods out scalar matrices, any matrix can be written in a unique (subject to some obvious condition) way as a "word" in S and T.

Such a word can be interpreted as a continued fraction, and vice-versa, a continued fraction (with a certain sign condition) gives rise to an element of SL_2(Z).

Essentially the Euclidean algorithm can be used to compute the continued fraction expansion of a rational number. This algorithm hence also gives a way to compute the expression as a word in S and T of any matrix in SL_2(Z). This algorithm also has a geometic interpretation.

In this project, we may also explore generalisations, such as (other) Hecke triangle surfaces, or surfaces arising from quaternion algebras.

The aim is to implement and visualise the interplay between continued fractions and geometry.

Schedule: To be determined.

Supervisors: Lassina Dembélé Gabor Wiese

Difficulty level: Any

Tools: Any computer language.


Results: [to be completed at the end of the project.]

FSTM -- University of Luxembourg