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Aug 3, 2010 at 15:10 comment added AlgSoul to BCnrd: I was just too stupid at that time, being unable to figure out why it is enough to prove only for SL2(k)...your illustration is clear. Thanks!
Aug 3, 2010 at 13:59 comment added BCnrd Here's the proof that ${\rm{SL}}_2$ case (for which Kevin's argument works over any non-arch. local field $k$) implies case of any $k$-split conn'd reductive $G$. Let $G' \rightarrow D(G)$ be the $k$-split simply conn'd central cover of derived gp, so $G'(k) \rightarrow G(k)$ has normal image with abelian quotient. By Schur can replace $G$ with $G'$ to reduce to showing the only finite-dim. irred. adm. rep'n of $G(k)$ is trivial one when $G$ is simply conn'd. The cocharacter lattice of split max. $k$-torus is gen'td by coroots since $G$ is s.c., so $G(k)$ is gen'td by ${\rm{SL}}_2(k)$'s. QED
Aug 3, 2010 at 13:59 history edited Kevin Buzzard CC BY-SA 2.5
Questioner changed question slightly (removed assertion that only 1 rep existed) so I changed answer to reflect this.
Aug 3, 2010 at 13:35 vote accept AlgSoul
Aug 3, 2010 at 13:35 vote accept AlgSoul
Aug 3, 2010 at 13:35
Aug 3, 2010 at 13:30 history answered Kevin Buzzard CC BY-SA 2.5