Department of Mathematics

Indian Institute of Science

Bangalore 560 012






Mr. Arpan Kabiraj
Affiliation : IISc, Bangalore

Subject Area






Department of Mathematics, Lecture Hall I




11.00 a.m.




May 04, 2016 (Wednesday)



"Goldman brackety : Center, geometric intersection number and length equivalent curves"


In 1980s Goldman introduced a Lie algebra structure on the free vector space generated by the free homotopy classes of oriented closed curves in any orientable surface $F$. This Lie bracket is known as the Goldman bracket and the Lie algebra is known as the Goldman Lie algebra. In this talk, we compute the center of the Goldman Lie algebra for any hyperbolic surface of finite type. We use hyperbolic geometry and geometric group theory to prove our theorems. We show that for any hyperbolic surface of finite type, the center of the Goldman Lie algebra is generated by closed curves which are either homotopically trivial or homotopic to boundary components or punctures. We use these results to identify the quotient of the Goldman Lie algebraof a non-closed surface by its center as a sub-algebra of the first Hochschild cohomology of the fundamental group. Using hyperbolic geometry, we prove a special case of a theorem of Chas, namely, the geometric intersection number between two simple closed geodesics is the same as the number of terms (counted with multiplicity) in the Goldman bracket between them. We also construct infinitely many pairs of length equivalent curves in any hyperbolic surface F of finite type. Our construction shows that given aself-intersecting geodesic x of F and any self-intersection point P of x,we get a sequence of such pairs.