Low-lying, fundamental, reciprocal geodesics

Low-lying, fundamental, reciprocal geodesics - Alex Kontorovich

Alex Kontorovich
Rutgers University; Member, School of Mathematics
March 24, 2016
Markoff numbers give rise to extremely low-lying reciprocal geodesics on the modular surface, but it is unknown whether infinitely many of these are fundamental, that is, the corresponding binary quadratic form has fundamental discriminant. In joint work with Jean Bourgain, we unconditionally produce infinitely many low-lying (though not "extremely" so), reciprocal geodesics on the modular surface, settling a question of Einsiedler-Lindenstrauss-Michel-Venkatesh. Some ingredients are expander graphs, sieving on thin orbits, and producing a level of distribution "beyond expansion". No prior knowledge of these topics will be assumed.