##### Department of Mathematics,

University of California San Diego

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### RTG Colloquium in Algebra, Algebraic Geometry and Number Theory

## Alireza Golsefidy

#### UCSD

## Closure of orbits of the pure mapping class group on the character variety

##### Abstract:

For every surface S, the pure mapping class group G_S acts on the (SL_2)-character variety Ch_S of a fundamental group P of S. The character variety Ch_S is a scheme over the ring of integers. Classically this action on the real points Ch_S(R) of the character variety has been studied in the context of the Teichmuller theory and SL(2,R)-representations of P.

In a seminal work, Goldman studied this action on a subset of Ch_S(R) which comes from SU(2)-representations of P. In this case, Goldman showed that if S is of genus g>1 and zero punctures, then the action of G_S is *ergodic. *Previte and Xia studied this question from topological point of view, and when g>0, proved that the orbit closure is as large as algebraically possible.

Bourgain, Gamburd, and Sarnak studied this action on the F_p-points Ch_S(F_p) of the character variety with boundary trace equal to -2 where S is a puncture torus. They conjectured that in this case, this action has only two orbits, where one of the orbits has only one point. Recently, this conjecture was proved for large enough primes by Chen. When S is an n-punctured sphere, the finite orbits of this action on Ch_S(C) are connected to the algebraic solutions of Painleve differential equations.

I will report on my joint work with Natallie Tamam in this area.

### March 12, 2024

### 4:00 PM

APM 6402

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