Weakly commensurable S-arithmetic subgroups in almost simple algebraic   groups of types B and C
  Let G and G' be absolutely almost simple algebraic groups of types B and C respectively, of rank at least 3, and defined over a number field K. We determine when G and G' have the same isomorphism or isogeny classes of maximal K-tori. This leads to the necessary and sufficient conditions for two Zariski-dense S-arithmetic subgroups of G and G' to be weakly commensurable. 
Skip Garibaldi and Andrei S. Rapinchuk
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