Assuming a certain "purity" conjecture, we derive a formula for
the (complex) cohomology groups of the affine Springer fiber
corresponding to any unramified regular semisimple element. We use
this calculation to present a complex analog of the fundamental
lemma for function fields. We show that the "kappa" orbital
integral that arises in the fundamental lemma is equal to the
Lefschetz trace of the Frobenius acting on the étale cohomology
of a related variety.
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