arXiv subjects
Publications and source records attributed to Mark Sheingorn.
We show that the set of real numbers of Lagrange value 3 has Hausdorff dimension zero by showing the appropriate generalization for each element of the Teichmueller space of the appropriate subgroup of the classical modular group.
We track the trajectories of individual horocycles on the modular surface. Our tracking is constructive, and we thus \emph{effectively} establish topological transitivity and even line-transitivity for the horocyclic flow. We also describe homotopy class jumps that occur under continuous deformation of horocycles.
This paper endeavors to track the trajectories of individual horocycles on \modsurf. It is far more common to study \emph{sets} of such trajectories, seeking some asymptotic behavior using an averaging process (see section \ref{previous}). Our work is only marginally related to these efforts. We begin by examining horocycles defined using the pencil of circles whose common point (in the words of the Nielsen-Fenchel manuscript \cite{wF}) is $\infty$. The orbits involved in this case are closed and long --- judged by arc length between two points compared to the hyperbolic distance between them. Using Ford circles of Farey sequences we find their lifts to the Standard Fundamental Region (SFR) and find points of these lifts making given angles with a horizontal. Next, we offer two algorithms, both involving continued fractions, of locating points whose angle with the horizontal is near any target angle and whose lifts are near any given point in the SFR. Next, we study the homotopy classes of horizontal horocycles as we descend to the real axis. We find these are stable during descent between encounters of the horizontal horocycle with elliptic fixed points. Such encounters change --- complicate --- the homotopy classes. We give these explicitly down to height $1/(2\sqrt{3})$. Finally we do an initial study of the open (infinite length) horocycle path with unit euclidean radius anchored at $ϕ-1$, where $ϕ$ is the Golden Mean. Enough information is adduced to suggest that this single doubly infinite path is transitive.
This paper studies certain horocyclic orbits on $Γ(1)\frontslash\mathcal{H}$. In the first instance we examine horocycles defined using the pencil of circles whose common point (in the words of the Nielsen-Fenchel manuscript is $\infty$. The orbits involved in this case are closed and long - judged by arc length between two points compared to the hyperbolic distance between them. We are concerned with tracking the paths of individual horocycles. Using Ford circles of Farey sequences we find lifts to the Standard Fundamental Region (SFR) and find points of these lifts making given angles with a horizontal. Next, we offer two methods, both involving continued fractions, of locating points with such angles whose lifts are near any given point in the SFR. This establishes in an effective manner a sort of transitivity, which necessarily involves infinitely many such horocycles. Next, we study the homotopy classes of horizontal horocycles as we descend to the real axis. We find these are stable during descent between encounters of the horizontal with elliptic fixed points. Such encounters change - complicate - the homotopy classes. We give these explicitly down to height $1/(2\sqrt{3})$. Finally we do an initial study of the open (infinite length) horocycle path with unit euclidean radius anchored at $ϕ-1$, where $ϕ$ is the Golden Mean. Enough information is adduced to suggest that this path is itself transitive. The methods resemble the Hardy-Littlewood Circle Method in a certain regard, albeit without the exponential sums.
We classify, in terms of topology of highest arcs, low height non-simple geodesics on the modular hyperbolic punctured sphere with three elliptic fixed points of order two. Of eight possible types, exactly one consists of geodesics that form a bigon about the cusp; we express all such geodesics in terms of Markoff triples. We also show that any geodesic joining two elliptic fixed points has locally isolated height.
We introduce a new method to establish McShane's Identity, based upon the fact that elliptic elements of order two in the Fuchsian group uniformizing the quotient of a fixed once-punctured hyperbolic torus act so as to exclude points as being highest points of geodesics.