Germ expansion for SL(2) in arbitrary characteristics

Abstract

Let $F$ be a local field of characteristic $p$ and $G$ be a connected reductive group over $F$. Recall that Shalika's germ expansion of orbital integrals of regular semi-simple elements near the identity, when it exists, is a sum indexed by the set of unipotent conjugacy classes in $G(F)$. Observe that if $G=SL(2)$ this set is always compact; it is finite if $p\ne 2$ while it is uncountable if $p=2$. As a consequence, Shalika's germ expansion for elliptic elements does not make sense if $p=2$. On the other hand the endoscopic expansion of elliptic orbital integrals always exists and yields a germ expansion equivalent if $p\ne 2$ (up to a Fourier transform) to Shalika's germ expansion but is new if $p=2$. A conjecture for arbitrary groups is stated.