## The geometry of Markoff numbers.(English)Zbl 0566.10024

This article offers a very fine exposition of the result of Andrew Haas to the effect that the Markoff quadratic irrationalities and simple closed geodesics on the punctured torus H/$$\Gamma$$ ’ are identified. In addition, the article contains a discussion of the analogue of the necessary dynamics for Haas’ theorem in the case of a torus, uniformized by the plane. Some interesting historical data are provided for this case, some of it concerning ancient astronomy. The article also discusses the author’s own work on the description of the exact connection between continued fractions and the tracing of geodesics on the modular surface, H/$$\Gamma$$ (1). The paper ends with the interesting speculation that perhaps there is also a nice structure for geodesics on a punctured torus that self-intersect precisely once.
One typo: The citations for references 6 (Haas’ paper) and 7 are reversed.
Reviewer: M.Sheingorn

### MSC:

 11J04 Homogeneous approximation to one number 37D40 Dynamical systems of geometric origin and hyperbolicity (geodesic and horocycle flows, etc.) 53D25 Geodesic flows in symplectic geometry and contact geometry 11F06 Structure of modular groups and generalizations; arithmetic groups 30F99 Riemann surfaces 20H10 Fuchsian groups and their generalizations (group-theoretic aspects)
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### References:

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