A note on cusp forms and representations of ${\mathrm{SL}}_2({\mathbb{F}}_p)$

Abstract

Cusp forms are certain holomorphic functions defined on the upper half-plane, and the space of cusp forms for the principal congruence subgroup $\Gamma (p)$, $p$ a prime, is acted by ${\mathrm{SL}}_2({\mathbb{F}}_p)$. Meanwhile, there is a finite field incarnation of the upper half-plane, the Deligne--Lusztig (or Drinfeld) curve, whose cohomology space is also acted by ${\mathrm{SL}}_2({\mathbb{F}}_p)$. In this note we compute the relation between these two spaces in the weight $2$ case.

shortened to 6 pages, and Lem~2.2 is upgraded