-
Character sheaves and characters of unipotent groups over finite fields
- American Journal of Mathematics
- Johns Hopkins University Press
- Volume 135, Number 3, June 2013
- pp. 663-719
- 10.1353/ajm.2013.0023
- Article
- Additional Information
Let $G_0$ be a connected unipotent group over a finite field ${\Bbb F}_q$,
and let $G=G_0\otimes_{{\Bbb F}_q}\overline{\Bbb F}_q$, equipped with the
Frobenius endomorphism ${\rm Fr}_q:G\rightarrow G$. For every character
sheaf $M$ on $G$ such that ${\rm Fr}_q^*M\cong M$, we prove that $M$ comes
from an irreducible perverse sheaf $M_0$ on $G_0$ such that $M_0$ is pure
of weight $0$ (as an $\ell$-adic complex) and for each integer $n\geq 1$
the ``trace of Frobenius'' function $t_{M_0\otimes_{\Bbb F}_q}}{\Bbb
F}_{q^n}}$ on $G_0({\Bbb F}_{q^n})$ takes values in ${\Bbb Q}^{ab}$, the
abelian closure of $\Bbb{Q}$. We further show that as $M$ ranges over all
${\rm Fr}_q^*$-invariant character sheaves on $G$, the functions $t_{M_0}$
form a basis for the space of all conjugation-invariant functions
$G_0({\Bbb F}_q)\rightarrow {\Bbb Q}^{ab}$, and are orthonormal with
respect to the standard {\it unnormalized} Hermitian inner product on this
space. The matrix relating this basis to the basis formed by the
irreducible characters is block-diagonal, with blocks corresponding to the
${\rm Fr}_q^*$-invariant $\Bbb{L}$-packets (of characters or,
equivalently, of character sheaves). We also formulate and prove a
suitable generalization of this result to the case where $G_0$ is a
possibly disconnected unipotent group over ${\Bbb F}_q$. (In general,
${\rm Fr}_q^*$-invariant character sheaves on $G$ are related to the
irreducible characters of the groups of ${\Bbb F}_q$-points of all pure
inner forms of $G_0$ over ${\Bbb F}_q$.)